that two weights or weighted levers in moving outwards from the engine
shaft, from the action of centrifugal force, move or rather shift the
eccentric across the shaft, reducing its throw, and therefore by
reducing the travel of the valve hasten the point of cut off and reduce
the power of the engine.
In the governor of the Buckeye engine, the centrifugal force may be
varied by increasing or diminishing the distance of the weights from the
pivots of the arms on which they swing.
[Illustration: Fig. 3382_a_.]
This is shown in Fig. 3382_a_, in which it is seen that the weights A
are adjustable along the arms _a_, _a_. The points of attachment _d_,
_d_ of the springs to the weight arms are also adjustable.
When reversing is done, by shifting the eccentric across the shaft, the
lead cannot be kept equal, but will, if the eccentric is swung from a
pivot that is on the line of centres, when the crank is on a dead
centre, be greater at the head end than at the crank end of the
cylinder. The discrepancy may, however, be equalized by swinging the
eccentric from a pivot that is not on the line of centres at a time the
crank is on a dead centre.
But this equalization will only exist at some one point in the
eccentric position, or in other words, if the eccentric is shifted
across the crank shaft, simply to reverse the engine, and not to vary
the point of cut-off, it will naturally be moved, in reversing the
engine across the shaft, to a given and constant amount, and in this
case, the pivot on which its hanger is hung may be so located with
reference to the line of centres and the crank (the latter being on a
dead centre when the point of suspension of the eccentric hanger is
found) that the lead is equal for both the backward and forward gears.
But if the eccentric is shifted across the shaft to vary the point of
cut off as well as to reverse the direction of engine revolution, the
lead cannot be kept equal.
It is better, in this case, to so locate the point of eccentric hanger
suspension as to let the lead be the most at the head end cylinder port,
because the piston travels fastest at that end of the cylinder, and
therefore requires more lead, in order to cushion the piston.
[Illustration: Fig. 3383.]
A construction for shifting the eccentric across the shaft is shown in
Fig. 3383, in which D, D is a disc, having at _b_ a pivot for the
eccentric hanger. The amount the throw line of the eccentric must be
shifted to reverse from full gear forwards to full gear backwards is
from the line _b_ _x_ to line _b_ _x´_, and the shifting is done by two
racks F and J, having teeth at an angle of 45° to their lengths. F is
fast to the eccentric, and J is carried in a sleeve that slides along
the shaft, and sliding it moves the eccentric across the shaft by reason
of the teeth of one rack being at a right angle to those of the other.
It is obvious that the eccentric may be moved around the shaft in place
of across it, the distance its throw line requires to be moved being the
same in either case.
To shift an eccentric so as to reverse the direction of engine
revolution, all that is necessary is to place the crank on either dead
centre and measure the amount of valve lead. Then loosen the eccentric
from the crank shaft, and while the crank is stationary, move it around
upon the shaft until it has opened the port full, and nearly closed it
again, leaving it open to the same amount as it was before the eccentric
was moved, or in other words, open to the amount of the lead.
[Illustration: Fig. 3384.]
Fig. 3384 represents a side elevation of a high speed wheel governor
engine, designed and constructed by the Straight Line Engine Company of
Syracuse, New York, the construction of the governor being shown in Fig.
3385, in which R is the eccentric rod, the eccentric being carried in a
lever strap pivoted at A, and connected at B to two links C and D, the
former of which connects to the spring E, and the latter to the weighted
lever F. The centrifugal force generated by the weighted end of F
endeavors to move the eccentric inwards, and thus reduce its throw,
which reduces the valve travel and hastens the point of cut off.
On the other hand, the tension of the spring E acts to move the
eccentric in the opposite direction, and maintain the full throw of the
eccentric and maximum point of cut off. These two forces are so
calculated in the design and proportion of the parts that under a
maximum load the engine will run at its proper speed, while, if the
load decreases, the action of F will hasten the point of cut off enough
to allow for the decreased engine load, and thus keep the engine still
going at the same speed.
[Illustration: Fig. 3385.]
Other novel and interesting details in the construction of this engine
are as follows:
The two arms forming the frame are cast with and run in straight lines
from the cylinder to the two main bearings, and rest upon these
self-adjusting points of support.
There are two fly wheels, both between the main bearings, and one of
which carries the governor so that the centre of the valve is brought in
line with the centre of the eccentric.
[Illustration: Fig. 3386.]
In order to simplify the explanation, the mechanism has been separated
into three separate sections. Figs. 3386 and 3387 show such of the
details of the parts between the cylinder and crank as are peculiar to
this engine. The cross head is of the slipper guide style, and the
illustration, Fig. 3386, shows the simple method adopted for adjusting
the guide to the proper height to maintain the alignment. Another
feature peculiar to the straight line not mentioned above, that of
making the cross head pin fast in the connecting rod, is used in this
engine also, but in a somewhat different form. As will be seen by Fig.
3387, the pin is made much larger, and this allows of its being made of
"steel casting" and cast hollow with cross bars at each end for
centring. These pins are held in the rod by a binding screw which
catches in a groove that is milled around one-fourth of its
circumference. After the pin is placed in the rod and the binding bolt
is inserted, the pin is prevented from working out endwise, and the
binding bolt prevents it from turning; but when the binding bolt is
slackened, the pin can be rotated one-fourth of a revolution. The scheme
is as follows: After running the engine for a while, the engineer is
instructed to slack the binding bolt, give the pin a quarter turn and
bind it fast. By repeating this, the pin can be kept more nearly round,
probably, than by any other plan. By referring again to Fig. 3386, it
will be seen that the plan for taking up the wear in the cross head pin
bearings is simply that of setting up the common half box, and the
endurance of the arrangement, with the hardened and ground steel pin
running in babbitt lined boxes of double the ordinary size and length,
must be satisfactory.
[Illustration: Fig. 3387.]
The drop oil cups for lubricating the cross head pin are located so as
to have the drop "picked" off just as the cross head completes its
stroke at the cylinder end, and while it is travelling at its slowest
speed. The oil, as it leaves the wearing surfaces of the pin, is
conveyed to the lower slide.
[Illustration: Fig. 3388.]
[Illustration: Fig. 3389.]
Figs. 3388 and 3389 show the parts that connect the eccentric with the
valve. The method of connecting the rod to the eccentric strap is
convenient. The lower joint in the eccentric strap is set up tight,
metal to metal, and the upper joint left open 1/8 of an inch.
STEAM FIRE ENGINE.
In a steam fire engine the prime requisites are rapidity of getting up
steam and efficiency with lightness, economy of fuel being a secondary
consideration.
Fig. 3390 is a general view of a steam fire engine constructed by the
Clapp & Jones Manufacturing Company.
Fig. 3390_a_ is a longitudinal section through the boiler and one steam
cylinder and pump.
The construction of the boiler is shown in Figs. 3390_a_ and 3391, the
former being a vertical section of the engine and boiler bearing the
steam pipe and exhaust pipe shown in place, and one of the draught tubes
shown in section, and the latter a vertical central section.
The outside shell is represented at _a´´_, _a´´_. This shell extends the
whole length of the boiler. The fire box sheet _b´´_, _b_ is less in
length, extending only to the lower tube sheet.
The lower tube sheet C´´ is perforated by all the tubes; the heavy lines
showing the coil tubes in fire box, the others are smoke tubes. The
upper tube sheet _d_ has holes only for the smoke tubes. The smoke or
draught tubes are shown at _e´´_, _e´´_, _e´´_; these also answer the
important purposes of drying and superheating the steam.
F´´, F´´, F´´ are the sectional coil tubes, the main feature of this
boiler. They are in the form of a spiral coil, the spiral bend being
enough to leave room for five others of the same size between, so that
there are six of these coils in each circular row. The number of rows is
determined by the size of the boiler and the amount of steam required.
Each coil is connected with the lower tube sheet by screw joints, all
right hand, that require no fibrous or elastic packing, an angle elbow
being used to get the short bend at the end. The tubes then make about
one turn around the fire box, and are joined to the side sheet of the
same, with the same union used at its upper end, which makes a joint
that never gets loose from any kind of work it may be subjected to.
These unions or couplings are made of different kinds of metal, and put
together so that no two pieces of iron come in contact to corrode and
stick together; and should it, from any cause whatever, become necessary
to take these coils out, it can be done, and the same tubes replaced
without destroying any part of them, or damaging any piece so that it
could not be used again.
G´´, G´´ is the ornamental dome or covering for the upper end; _g´´_,
_g´´_ is the smoke bonnet and pipes for concentrating the hot escaping
products of combustion for the purpose of making a draught of air
through the fuel. H´´ are grate bars, and I´´ fire door. J´´, J´´ is the
water line. The height has been determined by experiment, yet should be
varied a little to get the best drying effect of the coal. A coal that
makes a flame would call for a higher range of the water line, while
coal that produces heat without the flame would call for a lower range;
this the engineer will soon find. The working of the boiler is as
follows: The fire being started in the fire box, as soon as the water in
the coils begins to heat circulation commences from natural causes (nor
is it at any time necessary to use a hand pump or any other artificial
means for keeping it up), the heated water passing up in the steam drum,
and the colder water from the leg and drum taking its place, as is shown
by the arrows in the leg, till the whole is heated to the steam making
temperature. At this point steam pressure begins to show, which goes up
very fast, as the water is all so near the steam temperature. Of course,
it is better to carry the water at about the height shown, as a uniform
pressure of steam is easier maintained, which is always desirable; yet
the limit of safety is not reached till the water is nearly all out, or
so long as it is not below the connection of the coils in the leg; and
even at this point the only danger is in the damage to the coils from
the heat when there is no water to protect them.
[Illustration: _VOL. II._ =STEAM FIRE ENGINE.= _PLATE XXXV._
Fig. 3390_a_.]
[Illustration: Fig. 3390.]
[Illustration: Fig. 3391.]
In Fig. 3391_a_, one engine and pump is shown in side elevation, and the
other in section, the cranks being at a right angle, one to the other. A
yoke from the piston rod spans the crank, so that the steam and pump
pistons are in line and directly connected. From the lower end of this
yoke, a rod connects to the crank shaft upon which are the two fly
wheels and the eccentrics for the steam valves.
It will be seen in the longitudinal section, Fig. 3390_a_, that the
steam valve face is a segment of a circle and therefore answers, so far
as the distribution of the steam is concerned, to a simple D slide
valve, which exhausts through the pipes _m_, _p_. The steam pipe _n_
enters the bottom of the steam chest at _n´_.
The two main pumps _a_ are made in one piece, entirely of composition;
one of them is shown in section. The piston is a solid piece of brass,
as well as the cylinder in which it works, but are made of different
composition, one hard, the other soft, to prevent cutting. The valves
are of India rubber; the discharge valve is a ring, one for each end of
the pump, as shown at _b_, Fig. 3391_a_. One is shown open, while the
other is closed. They are held in place by grooved rings of brass; these
rings fit in grooves in the rubber, which, when they are put in the
pump, and their set screws are in, with their points in the grooves in
the brass rings spoken of above, the discharge valves are complete for
work.
The suction valves are shown at K on Fig. 3391_a_, and will be easily
understood. They are of a design for this special use and place, which
is around the pump cylinder in a circular chamber. The water ways
covered by these valves are long and narrow, one valve covering two of
these openings, they being held in place by two studs that go through
the centre part of the valve, a wire going through these studs, and
close to the back of the valve which keeps it up to the seat, the only
spring to either of these valves being the elasticity of the rubber. The
opening and connection D, D is the inlet to the pump, and where the
suction hose goes on, there being a pipe or chamber with branches for
the two air chambers, and at each end is a discharge gate and a
connection for the leading hose. The part _d_ is the feed pump for the
boiler supply, _e_ is the air chamber on the pipe that leads to the
boiler to ease off the shocks caused by the plunger striking the water,
when the pump does not fill. It is drawn broken off to show the upper
part of the pump barrel and stuffing box. The pipe _f_ is the feed water
pipe from the pump to boiler, shown from different points in Figs.
3390_a_ and 3391_a_. _g_ is what we call the suction pipe to the feed
pump. It connects to the main pump in the discharge part of it.
A piece of hose pipe connects to the boiler at a point just above the
water line, so that hot water or steam (according to the height of the
water in the boiler) may be applied to any part that may have become
frozen.
[Illustration: Fig. 3391_a_.]
Heaters are almost universally used in connection with steam fire
engines to keep the water hot, and in many cases to keep a few pounds
pressure to shorten the time of going to work should the fire be close
at hand. This boiler has an advantage for this kind of heating; the
circulation is so perfect and free that all the water in it is heated
alike; so when the fire is lighted the steam starts immediately up,
instead of having to wait till some cold water has been heated that had
not been reached by the very limited circulation in them, there being
some parts that the circulation produced by the heater does not reach,
leaving, of course, this water cold.
The arrows K´´ (Fig. 3391), show the direction of the circulation when
working with fire in the fire box; those marked L´´ show the direction
of it when on the heater which is directly opposite.
The outside pipe connected at about the water line is the outlet from
the heater, and the inlet to the boiler, which carries the heated water
over the crown sheet, where, as it gets cooler, it enters the coils,
descends into the leg, and from there to the pipe near the bottom of the
boiler; this pipe leads to the heater, so that the water is kept moving
just in proportion to the heat given it; any kind of a heater can be
used with the same result.
CHAPTER XLII.--MARINE ENGINES.
Marine engines are made in the following forms:
1. With a single or with two cylinders receiving live steam from the
boilers, and exhausting into the atmosphere. These are termed high
pressure engines, let the steam pressure be what it may. They are also,
and more properly, termed non-condensing engines.
[Illustration: Fig. 3392.]
In the small sizes, such as are used for launch engines, it is simply a
non-condensing engine, with a link motion for varying the point of cut
off as well as for reversing purposes. Fig. 3392 represents an engine of
this class constructed by Chas. P. Willard & Co.
The cylinder is what is called "inverted," meaning that it is above the
crank shaft.
The slide spindle or valve rod passes through a guide and connects
direct to the link block or die, as it is sometimes called.
The thrust block is provided in the bearing of the crank shaft, and
consists, as seen in the sectional view, of a series of collars on the
crank shaft bearing.
2. The addition to each high pressure cylinder of a low pressure
cylinder constitutes a compound engine, and if the engine has also a
condenser, it is a compound condensing engine, an example being shown in
Fig. 3392_a_, which represents an engine in which the link motions are
employed to vary the points of cut off of both cylinders, as well as to
reverse the engine. The engine being small, the power required to move
the links is small enough to permit of their operation by hand, by means
of the hand lever L, which is secured to its adjusted position on the
sector T by the small lever nut shown on the side of the lever. The
lever L operates a shaft D which shifts both link motions. The air and
circulating pumps are at the back of the condenser, being operated from
the beams B, B, each beam connecting to rods J which connect to rod _c_,
which drives the air and circulating pumps.
The steam from the high pressure cylinder exhausts into a receiver or
chamber between the two cylinders, and from which the low pressure
cylinder receives its steam.
[Illustration: Fig. 3392_a_.]
[Illustration: Fig. 3395.]
[Illustration: Fig. 3396.]
The exhaust from the low pressure cylinder passes into the condenser,
where it is condensed, leaving a partial vacuum on the exhaust side of
the low pressure piston.
Figs. 3393 and 3394 show the arrangement of the pumps on a pair of
compound engines for a dredger. The steam from the low pressure cylinder
passes into the body of the condenser with which the air pump is in
communication, as shown in the end elevation. At _a_ is the foot valve
of the condenser. The piston of the air pump has a similar valve, and at
_e_ is the delivery valve.
The circulating pump is shown in the back elevation (Fig. 3394), being a
piston pump which forces the water through the tubes of the condenser.
There are two principal methods of compounding, in one of which the two
cylinders are placed one above the other, with their axes in line, and
both pistons connecting to the same crank, while in the other the
cylinders are side by side, and each connects to its own crank, the two
cranks usually being at a right angle.
When one cylinder is placed above the other, as in Fig. 3395, R being
the high pressure and S the low pressure piston, no receiver is
employed, the steam passing direct from the high pressure cylinder
through the pipe P to the low pressure steam chest _c_. The high
pressure steam valve V and the low pressure valve V are on the same
stem, a cut off valve V´ being provided for the high pressure cylinder.
3. Triple expansion engines have three cylinders, a high pressure, an
intermediate, and a low pressure cylinder.
In a triple expansion engine the intermediate cylinder receives the
steam that is exhausted from the high pressure cylinder, and expands it
further. The low pressure cylinder receives its steam from the exhaust
of the intermediate cylinder, and exhausts into the condenser.
[Illustration: Fig. 3397.]
[Illustration: Fig. 3398.]
In the illustrations from Fig. 3396 to Fig. 3406 are represented the
triple expansion engines of the steamship _Matabele_, constructed by
Messrs. Hall, Russell & Company, of Aberdeen, Scotland. Fig. 3396 is a
cross sectional view of the vessel showing the engine and its
connections, and Fig. 3397 a similar view, showing the boilers. Fig.
3398 is a back elevation of the engine, showing the boilers also, and
Fig. 3399 a plan of the same. Fig. 3400 is a sectional view, and Fig.
3401 an end view of the boilers. Fig. 3402 is a plan, Fig. 3403 an end
elevation, and Fig. 3404 a front elevation, partly in section, of the
engines. H P is the high pressure cylinder, I C the intermediate
cylinder, and L P the low pressure cylinder. The high pressure cylinder
has a piston valve, the steam chest being shown at A. The intermediate
cylinder is provided with a double ported flat valve as shown at B, and
the low pressure cylinder is provided with a similar valve whose weight
is counterbalanced by the small piston at E; at F are the relief valves
for relieving the cylinders of water.
[Illustration: _VOL. II._ =COMPOUND MARINE ENGINE.= _PLATE XXXVI._
Fig. 3393.
Fig. 3394.]
[Illustration: Fig. 3399.]
[Illustration: Fig. 3400.]
[Illustration: Fig. 3401.]
[Illustration: Fig. 3402.]
Each steam valve is provided with a link motion that may be used for
varying the point of cut off (and therefore the expansion) as well as
for reversing purposes.
The link motions are all shifted from one shaft, which may be operated
by hand or by steam, the construction being as follows:
For shifting by hand, the wheel W is operated, its shaft having a worm
driving the worm wheel G, Fig. 3403, which operates rod H, and through
the lever J and rod K shifts the link L, one pair of eccentric rods
being shown at N and P.
The shaft of the wheel W is, however, a crank shaft, and at M is a small
engine, which may be connected or disconnected at will to shaft W. The
lever J operates a shaft R in Fig. 3404, which connects (by a rod
corresponding to rod K in Fig. 3403) to each link motion; hence all the
links reverse together, and the ratio expansion of one cylinder to the
other cannot be varied, or in other words, the point of cut off will be
alike for each cylinder, let the link motion be shifted to whatever
position it may.
The beam S, Fig. 3403, for working the air, circulating and feed pumps,
is driven from the cross head of the intermediate cylinder.
The boilers are of the Scotch pattern that is usually employed for high
pressures, as 160 or more lbs. per square inch, and have Fox corrugated
furnaces and stay tubes.
Each cylinder requires a starting valve (which is sometimes called an
auxiliary valve or a bye pass valve), which is used to warm the cylinder
before starting the engine, and also (when there is no vacuum in the
condenser) to admit high pressure steam when the high pressure piston is
on the dead centre, in which case, there being no vacuum and no
admission of steam to the low pressure cylinder, the engine would not
have sufficient power to start.
[Illustration: Fig. 3403.]
In some cases the high pressure cylinder has no starting valve, the
reversing gear being used to admit steam to one end or the other of the
high pressure piston, and the starting valve being used to admit enough
live steam to the low pressure cylinder to compensate for the absence of
the vacuum.
When the vacuum in the low pressure cylinder is maintained while the
engine is standing still, its starting valve obviously need not be used,
except for warming purposes, before starting the engine; as soon,
however, as the engine has started, the starting valve must be closed.
Each cylinder is provided with a relief valve, both at the top and at
the bottom, to relieve the cylinder from a heavy charge of water, such
as may occur if the boiler primes heavily.
Each cylinder is also provided with drain cocks, to permit of the escape
of the ordinary water of condensation in the cylinders when the engine
is started, and also for use if the boiler primes.
The low pressure relief valve also prevents the accumulation of too
great a pressure in the low pressure cylinder, which, from its large
diameter, is not strong enough to withstand high pressure.
The oiling apparatus for the cylinders is arranged as follows:
In some cases pumps, and in others automatic or self-feeding devices are
used. Oil is fed to the steam pipe of the high pressure cylinder, and
this lubricates both the valves and the cylinders, but in many cases it
is also fed to the steam chest, so as to afford more perfect lubrication
to the valve.
For the low pressure cylinder the oil is fed into the receiver, and
usually at a point near the slide valves.
Large marine cylinders are usually constructed with a separate lining,
which may be replaced when worn or otherwise required.
A surface condenser consists of a cast iron shell or chamber forming the
back of the engine frame. At each end of this chamber is a short
partition, so that the condenser is divided lengthways into what may be
called three compartments, of which the middle one is the longest and
contains a number of thin brass tubes about 5/8 or 3/4 inch in diameter,
the ends of these tubes being held in the plates or tube sheets forming
the partitions. The object of providing tubes of small diameter is to
obtain a large area of cooling surface.
The exhaust steam from the engine generally passes into the shell or
body of the condenser, filling the middle partition and surrounding the
tubes.
The condensing or circulating water passes through the tubes, and by
keeping them cool condenses the steam and forms a vacuum or partial
vacuum in the condenser, which, having open communication with the low
pressure cylinder, therefore gives a corresponding degree of vacuum on
the exhaust side of the low pressure piston.
In some designs, however, the steam passes through the tubes and the
circulating water fills the middle compartment of the condenser. As,
however, there is no pressure to counterbalance the weight of the water,
it is preferable to have the water inside the tubes, so that they are
subjected to a bursting pressure, in which case they may, for a given
strength, be made thinner, because the strength of the tube to resist
bursting is greater than its strength to resist collapsing, hence the
circulating water usually passes through the tubes. The chamber at the
ends of the condenser permits the water to distribute through all the
tubes.
In some cases the chamber at one end is divided horizontally into two
compartments, so that the water is compelled to pass through one half
and return through the other half of the tubes.
The water of condensation falls to the bottom of the condenser, from
which it is removed by the air pump, which delivers it to the hot well.
The hot well is situated on the side of, and extends above, the pump,
whose upper end it covers, thus water sealing the top of the air pump
and preventing air from passing into it through a leaky valve or bucket.
The top of the hot well is provided with a _vapor pipe_, which permits
the air and gases to pass overboard. This pipe emerges through the side
of the ship above the water line, and as there is no valve between the
hot well and the sea, no pressure can possibly accumulate in the hot
well.
The boiler feed is taken from the hot well either by the feed pump or by
injectors, as the case may be.
In case the boiler feed should stop working, however, the hot well is
provided with a pipe of large diameter, and called the overboard
discharge pipe, so that the water of condensation may not accumulate a
pressure in the hot well if the boiler feed ceases.
This overboard discharge pipe is provided with a weighted valve (placed
at the side of the ship), which is constructed after the manner of a
safety valve, relieving the hot well of pressure if the water
accumulates, and preventing the sea water from entering the hot well.
To prevent loss of fresh water, the exhaust steam from the various
engines and pumps (if any) about the ship passes to the condenser and is
pumped into the hot well.
In some cases, however, a separate and independent condenser is used for
the smaller engines about the ship.
An independent condenser is one whose air pump and circulating pump are
not worked from the main engine, and can therefore be operated when the
main engine is standing still.
If the main condenser is independent, it may be started so as to form a
vacuum before the main engine is started, and thus obviate the use of
the starting valve on the low pressure cylinder except to warm the
cylinder before starting.
Feed water for the boilers when the engine is standing is obtained by a
pipe from the bottom of the condenser, so that the water of condensation
of steam blown through the engine cylinders, and from the exhausts from
the smaller engines about the ship, may be pumped or forced direct from
the bottom of the condenser to the boiler.
This feed from the bottom of the condenser is necessary when the air
pump is not working, and the water of condensation is not pumped into
the hot well.
If the water thus obtained is not enough to keep the boilers supplied,
an auxiliary or salt water feed admits extra water from the circulating
water to the inside of the condenser to supply the deficiency.
This secondary suction pipe is provided with a valve because it must be
shut off before the engine is started.
All the drain pipes from the cylinder pass into the condenser so as to
save the fresh water.
The air pump is usually worked by a beam, receiving motion from the
cross head of the low pressure cylinder.
The circulating pump is usually worked by the same beam as the air pump,
or receives its motion from some other part of the main engine. In some
cases, however, an independent circulating pump is employed.
It receives its water from a pipe leading to the sea, which is provided
with an injection cock or Kingston valve, placed close to the side of
the ship and well below the sea level. This valve is used to shut off
the circulating water and prevent its flooding the ship in case of
accident to the condenser or circulating pump.
The circulating water, after passing through the condenser, discharges
overboard through the circulator discharge pipe.
This pipe is also provided with a valve placed close to the ship's side,
at or above the water level, so that the opening at the ship's side may
be closed, and sea water prevented from entering the ship in case of
breakage to the condenser, etc.
To enable a surface condenser to be used as a jet condenser in case of
accident to the circulating pump, a pipe leads from the injection cock
of the circulating supply pump into the bottom of the exhaust pipe or
column, where it enters the condenser.
This pipe is supplied with a spray or rose nozzle, which divides up the
injection water and causes it to condense the steam as it enters the
condenser.
An additional pipe is sometimes added to the suction side of the
circulating pump, for use in pumping out the bilge by means of the
circulating pump in case of emergency, and also for pumping out ballast
tanks when the vessel is provided with such tanks.
An air valve is sometimes fitted to a reciprocating double acting
circulating pump. It admits air to the water during the up stroke of the
pump, and closes on the down stroke. The air thus admitted acts as a
cushion to soften the shock of the water.
A snifting (or snifter valve, as it is sometimes called) is a valve
fitted to the condenser and that opens upwards to permit of the
discharge of the air and gases before the engine is started. It also
serves to prevent any water from leaky condenser tubes from filling the
condenser and flooding the engine cylinders. It is so loaded with dead
weight that it opens automatically when the water in the condenser has
reached a certain height and must be placed as low down on the condenser
as possible, so as to receive the weight of the full height of the water
in the condenser.
Condenser tubes are made water tight in the tube plates of the condenser
by wooden or sometimes paper ferrules, which fit the tube and drive into
the tube plate. In other cases, however, the tube ends project through
the plates, and a rubber washer is placed on the end of each tube. A
covering plate is then bolted over the whole of the tube ends, the holes
in the covering plate being parallel for a short distance, and then
reduced in diameter so as to form a shoulder. The rubber rings compress
and make a joint, and the shoulders prevent the condenser tubes from
working out endways from expansion and contraction. The tubes are
usually about 3/64 inch thick.
[Illustration: _VOL. II._ =TRIPLE EXPANSION MARINE ENGINE.= _PLATE
XXXVII._
Fig. 3404.]
A blow through valve is a valve attached to the casing or steam chest,
and connecting by a pipe to condenser to blow out the air and gases that
may have collected there when the engine is standing still, and that
also connects to the exhaust port of the high pressure cylinder, so as
to supply live steam to the low pressure cylinder in case the high
pressure cylinder should get disabled.
A bucket air pump is one in which there is a valve or valves in the pump
piston, hence the pump is single acting, drawing on the lower side of
the piston and delivering on the upper, hence the capacity of the pump
per engine revolution is equal to the diameter of the bucket multiplied
by the length of its stroke. The suction or foot valve is at the foot of
the pump, and the delivery valve at the head.
A piston air pump is double acting, since it draws on each side
alternately of the piston, one side delivering while the other is
drawing, hence two suction and two delivery valves are required.
A plunger air pump is one in which a plunger is used in place of a
piston, the delivery being due to the displacement of the plunger.
An air pump trunk is a hollow brass cylinder attached to or in one piece
with the piston or bucket of the air pump. The rod which drives the
piston passes through the trunk, and connects to a single eye at the
bottom of the trunk.
A trunk air pump is necessary when the pump rod is driven direct from
the crank shaft, and therefore has sufficient lateral motion to push the
pump piston sideways, which would cause friction and excessive wear to
the gland that keeps the trunk tight. The delivery capacity of the pump
is obviously diminished to an amount equal to the displacement of that
part of the plunger that passes through the gland and within the pump
bore, whereas in a piston pump the delivery capacity is only diminished
to an amount corresponding to the displacement of the pump piston rod.
A bucket pump may in some cases be worked without either a foot or a
head valve, since the bucket valve will answer for both in cases when
the delivery water cannot pass back into the pump on the down stroke of
the bucket.
It will, however, be more efficient with the addition of either of them,
and most efficient with both.
A bucket pump with a foot valve and no discharge valve would, however,
suffer more from a leaky gland than if it had a discharge valve and no
foot valve, because the air would, on the ascent of the bucket and the
closing of the bucket valve, pass to the suction side of the bucket and
impair the vacuum.
Let the delivery valves be where they may, the foot valve will always
have some water above it, and the pump bucket will dip into this water,
and on lifting produce a vacuum that will cause the pump to fill with
water. Notwithstanding that the gland may leak air on the other side of
the bucket, this air will in a single acting pump be expelled with the
water, but in a double acting pump it will impair the vacuum, and
therefore the suction, on the gland side of the piston.
Bucket air pumps are provided with a valve or pet cock on the top or
delivery side of the bucket and above the bucket, when the latter is at
the highest point of its stroke. This valve opens on the descent of the
bucket, admitting air to act as a cushion between the surface of the
water and the delivery valve, when the water is about to meet the
latter. It obviously reduces the effectiveness of the pump, and in a
double acting pump is inadmissible, because of its impairing the vacuum
and the suction.
This valve also enables the engineer to know whether the air pump is
working properly.
A pet cock is also supplied to the feed pumps for this same purpose.
A bilge injection is one in which the injection water is taken from the
bilge, which may be done when the ship makes more water than the bilge
pumps can get rid of.
The fittings necessary for a bilge injection are a cock or globe valve
placed on the side of the condenser, and at or near the foot of the
exhaust pipe, with a spray or rose inside that pipe. From the cock a
pipe, usually lead, leads to the bilge, having at its end a strainer or
strum, and care must be taken that this strum does not get choked and
let the condenser get hot from the exhaust steam not being condensed.
The water in the hot well of a surface condenser is usually kept at a
temperature of about 100° Fahrenheit. A higher temperature than 100°
Fahrenheit injures the rubber valves of the air pump, while lower
temperatures cool the engine cylinders too much and cause waste from
cylinder condensation. Moreover, it is obvious that, since the boiler
feed is taken from the hot well, it is desirable to keep it as hot as
the valves and as the desired degree of vacuum will permit.
An air vessel or air chamber is a vessel fitted to the delivery and
sometimes also to the suction side of a pump. Its office is to maintain
a steady flow of water through the pipes.
Thus, in the case of the delivery air chamber, when the pump piston is
travelling at a speed above its average for the stroke, the water
accumulates in the air chamber, and the air is more compressed, while,
when the pump is on the dead centre, or at the end of its stroke and the
delivery valve closes, the air compressed in the air chamber continues
the delivery or discharge, thus maintaining a more uniform flow.
Pumps sometimes have an air or vacuum chamber on the suction side, from
which the air is exhausted when the pump starts, leaving a vacuum which
causes a steady flow of water up the suction pipe.
Both these chambers are more effective as the speed of the pump
increases. The chamber on the delivery side is apt to lose its air,
which is gradually absorbed by the water, which should be let out when
the pump is standing still.
Feed escape valves or feed relief valves are fitted to the feed pumps,
so that in case all the feed water cannot pass into the boiler it may
pass back to the hot well.
The construction of a feed escape valve is as follows:
It is an ordinary mitre valve held to its seat by the compression of a
spiral spring, whose pressure upon the valve may be regulated by an
adjusting screw, whose end abuts upon a stem provided for the purpose.
In proportion as the valve is relieved of the pressure of this spring, a
greater proportion of the water delivered by the feed pump will pass
back into the hot well, hence the amount of boiler feed may be regulated
by the feed escape valve, which also acts as a safety valve, preventing
undue pressure in the feed pipe.
When no feed escape valve is employed, the delivery water from the feed
pump must pass unobstructed to the boiler, or the feed pipes may burst
from over pressure, and it follows that the feed check valve on the side
of the boiler must not be restrained in its amount of lift, hence it
must not have a lift adjusting screw.
The amount of the boiler feed must, in this case, be regulated from the
suction side of the pump, the suction pipe being fitted with a cock or
valve whose amount of opening may be adjusted so as to regulate the
amount of water drawn per pump stroke from the hot well.
If the feed valve on the suction side, or the escape valve on the
delivery side of the pump, as the case may be, is adjusted to permit of
a proper amount of boiler feed, and yet the feed is insufficient or
ceases altogether, it may occur from the following causes:
1st. From the suction valve sticking or being choked, or from the
delivery valve being choked and not seating itself, thus either letting
the suction water pass back into the hot well, or the delivery water
pass back into the pump.
2d. Through leaks in the joints of the pump or of the suction pipe.
3d. From the water in the hot well being too hot.
4th. Through the spring of the escape valve having become disarranged.
5th. If two or more boilers are connected, and one has less pressure in
it than the other, it may take most of the feed water, or the water of
the other may empty itself into it.
Bilge Injection. The injection water for a common or jet condenser may
be obtained in one of two ways: first, direct from the sea, which is
that for ordinary use; and secondly from the bilge, which is resorted to
to assist the bilge pump in cases of emergency.
The necessary fittings for a bilge injection are, a pipe leading from
the condenser to the bilge, with a cock at the condenser end and a
strainer at the bilge end.
This pipe should be fitted with a check valve, which opens by lifting
upwards so that no water can pass down it into the bilge, or otherwise,
if the main and bilge injections should happen to be left open
together, the water from the main injection might pass down into the
bilge. This check valve should be so constructed that its amount of lift
can be regulated and as much of the bilge water used for injection as
the circumstances may require.
In the case of surface condensers, the bilge water is drawn off by the
circulating pump and used to supplement the main circulating water. The
pipe from the bilge in this case leads to the suction side of the
circulating pump, and requires a strainer at the bilge end, a cock at
the circulating pump, and a check valve.
A ship's side air pump discharge valve is an ordinary dead weight mitre
valve that opens to let the water pass out into the sea, but seats
itself and closes if the water attempts to pass inwards. It differs from
a common stop valve in being weighted, and therefore self-acting. It
requires to be lifted before starting the engine, as such valves are
liable to stick in their seats.
The course of the main injection water of a jet condenser is as follows:
From the rose plate or strainer, through the injection valve and pipe to
the condenser, where it mingles with the exhaust steam and from which it
is pumped with the products of condensation into the hot well. From the
hot well it passes mainly overboard through the Kingston valve, but that
part of it used for the boiler feed passes through the suction pipe and
valve into the pump, and thence through the delivery valve, pipe and
check valve into the boiler.
The course of the main circulating water of a surface condenser is
through the Kingston valve (on the ship's side or bottom), and the
circulator inlet pipe, either direct to the condenser, from which it is
_drawn_ by the circulating pump, or else it passes through it, and is
_forced_ through the condenser. It circulates through the condenser
twice or thrice according to the construction, and is forced overboard
by the action of the circulating pump, passing through a valve on the
ship's side or bottom.
The advantages of surface condensation are, first, that the feed water
is obtained at a higher temperature than if injection water was fed to
the boiler. Second, the feed water is purer, and therefore less water
requires to be blown out of the boiler in order to keep it clean. Third,
the boiler does not scale so much, hence its heating surface is
maintained more efficient; and fourth, the boiler suffers less from
expansion and expansion strains when hot feed water is used.
Surface condensers foul from the grease with which the cylinders are
lubricated and from the salt in the injection water. The condenser is
cleaned by the admission of soda with the exhaust steam and by washing
out.
A condensing engine has the following cocks and valves on the skin of
the ship in the engine room: The main Kingston valve for the injection,
or circulating water, the main delivery valve from the condenser, the
bilge delivery valves, and the water service cocks for keeping the main
bearings of the engine cool with streams of cold water.
A donkey engine is a small engine used to feed the boiler, and has the
following connections: A steam pipe from the boiler to drive the donkey
engine; and exhaust pipe into the condenser; a suction pipe from the hot
well or from the sea, as the case may be; and a delivery pipe to the
boiler; a suction pipe from the bilge, so that the donkey pump can
assist in pumping the bilge out; a suction pipe to the condenser, to
circulate the water when the main engines are stopped, and thus maintain
the vacuum; and a suction pipe from the water ballast tanks, to pump
them out when necessary.
The pipes that lead from, or go to, the sea are: Boiler blow off pipe,
sea injection or circulator pipe, condenser discharge pipe, and, in some
cases, donkey feed suction pipe.
The parts of an engine that are generally made of wrought iron are those
in which strength with a minimum of weight and size is desired; for
example, the piston rod, cross head, connecting rod, crank shaft, crank,
eccentric rods, link motion, valve spindle pump rods, and all studs,
bolts, and nuts.
The parts generally made of cast iron are those where strength and
rigidity are required, and which are difficult to forge, while weight or
size is of lesser importance, such as the bed plate, cylinders, pistons,
condensers, and pumps.
The parts sometimes made of steel are those subject to great wear, and
for which strength with a minimum of size is necessary, as piston
springs, piston rods, connecting rods, cranks, crank pins, and valve
rods.
The parts generally made of brass are those subject to abrasion or
corrosion, as the connecting rod brasses, the bearings for the crank
shaft, the pump plungers or pistons, and their rods, linings for the
pump barrels or bores, the bores of the glands, the condenser tubes, and
all cocks and valves.
White metal or babbitt metal is sometimes used in place of, or in
connection with, brasses, serving as an anti-abrasion surface. It is
easily renewed, as it is cast into its place, but will melt and run out
at a temperature of about 600° Fahrenheit.
Muntz metal is used where iron or steel would suffer greatly from
corrosion when in contact with salt water. It can be forged.
The difference in the composition of cast iron and steel has never been
determined; the difference lies in the percentage of carbon they contain
and the structure of the metal. Cast iron will not weld.
Cast iron is brittle, of granular structure, and always breaks short,
having a very low elastic limit.
Wrought iron is tough and fibrous, will weld but will not harden, and is
stronger than cast iron.
Steel is stronger than wrought iron, and will weld and harden and
temper. The breaking strain of wrought iron varies from about 42,000 to
60,000 lbs. per square inch of section.
Steel is tempered by first being heated red hot and suddenly cooled
(usually by plunging it into cold water), which hardens it. The surface
is then brightened, and on being reheated the tempering colors appear,
beginning at a pale yellow, and deepening into red, brown, purple, and
blue, the latter gradually fading away as the metal is re-heated to a
red heat. The higher the temperature to which the hardened steel is
reheated the softer or lower it is tempered.
These colors merely indicate the temperature to which the piece is
reheated, since they will appear on steel not hardened and upon iron.
Case hardening is a process that converts the surface of wrought iron
into steel, which is accomplished by placing them in a box filled with
bone dust, animal charcoal, or leather hoofs, etc. The box is sealed
with clay, heated red hot for about 12 hours, and the pieces are
quenched in water.
The parts usually case hardened are the link motion, and other light
working parts that are of wrought iron.
The forgeable metals used in engine work are wrought iron, steel,
copper, and Muntz metal. The brittle or short metals are cast iron and
brass.
Welding is the joining of two pieces solidly together. Wrought iron,
steel, and Muntz metal can be welded.
All the metals used in the construction of marine engines expand by
heat, and this is allowed for in adjusting the lengths of the eccentric
rods, or of the valve spindles when setting the valve lead. In the case
of two marine boilers being connected together, the steam pipe is fitted
with an expansion joint, one pipe end having an enlarged bore to receive
the other. The joint is made by packing, which is squeezed up by a
gland, whose bore fits on the outside of the pipe which moves through
the gland bore, from the expansion and contraction.
The piston of a marine engine steam cylinder is a disc of cast iron,
into which the piston rod is secured. Its body is cored out to lighten
it. Around its circumference is a recess to receive the packing ring or
rings, each of which is split across so that it may be expanded (to fit
the bore of the cylinder) by means of the packing or of the springs. The
split is closed in the centre by a tongue piece let into the ring, and
fastened to one end of the ring.
To hold the piston rings or ring in place, a junk ring is employed,
being an annular ring bolted to the piston. The piston rings are set out
to fit the cylinder bore by suitable springs. The round plugs seen on
the piston face merely fill the holes used to support the core in the
mould and to extract it from the finished casting.
Cylinder drain cocks sometimes have a check valve upon them, so that
while the water may pass out of the cylinder the air cannot pass in and
destroy or impair the vacuum.
Cylinder escape or relief valves are provided at the top and at the
bottom of the cylinders, and consist of a spring loaded valve with an
adjusting screw to regulate the pressure at which they shall act. They
are most needed when the boiler primes heavily, and the water might
knock out the cylinder heads or covers. They should be enclosed in a
case with a pipe to lead the water away, thus preventing it from flying
out and scalding the engineer.
A link motion is a valve gear by which the engine may be reversed
(caused to run in either direction), or which may be used to vary the
point of cut off. The advantage of the link motion is its simplicity and
durability.
A link motion for a marine engine is usually of the Stephenson type, and
consists of two eccentrics or eccentric sheaves fixed upon the crank
shaft, and so set as to give more lead at the bottom than the top ports,
because the wear of the journals, brasses, and pins gradually increases
the lead at the upper, and correspondingly diminishes that at the lower
port. In addition to this, however, more lead is required at the bottom
port, to counterbalance the weight of the piston at the end of its
descending stroke. The eccentric hoops or straps drive the rods which
connect to the ends of the link.
The link may be a curved, solid, or a slotted bar, and in either case
has fitted to it a block or die which connects to the valve spindle.
The link is pivoted at its centre to a swinging arm or suspension
link,[58] and by this arm may be moved endways to bring the required end
of the link beneath the valve rod or spindle. From the positions in
which the eccentrics are set, one end of the link operates the valve to
go ahead, while the other end operates it to go astern; hence all that
is necessary (so far as the link motion is concerned) to reverse the
engine is to move the link endwise to the requisite amount, which, for
full gear, is so that the block is at or near the end of the link.
[58] See page 383 for the construction of a link motion.
In proportion as the link block is (by moving the link endways) brought
nearer to the middle of the link, the valve travel is reduced and the
point of cut off is hastened, thus increasing the expansion.
When the link block is in the middle of the link, the latter is in mid
gear, and the valve only opens the ports to the amount of the lead, and
the link action is the same, whether the engine moves backwards or
forwards.
The motion of the link is as follows:
The two ends are vibrated by the eccentrics from the central pin of the
link hanger (or suspension link) as a centre of motion, while at the
same time this end of the link hanger swings in an arc of which its
other end is the centre of motion.
In small engines the link is sometimes used for varying the expansion as
well as for reversing the direction of engine revolution.
In large engines it is used for reversing only, a separate expansion
valve being used for varying the point of cut off.
In small engines the link is moved endwise for forward or backward gear
by a simple arrangement of hand levers. In large engines these levers
are supplemented by a worm and worm gear, and in still larger engines a
steam reversing gear is used for shifting the links from forward to
backward gear, or vice versa.
When there is no link motion, a Joy valve gear, a Marshall valve gear,
or a loose eccentric may be used. A loose eccentric is one that can be
moved around the shaft to reverse the engine. It may be moved around the
shaft by mechanical means, or the eccentric rods may be disconnected,
and the valve worked by hand, to cause the engine to run in the required
direction, until a pin fast in the shaft meets a lug on the eccentric
and drives it, there being two such lugs or shoulders spaced the
requisite distance apart on the eccentric. This plan is obviously only
suitable for small engines.
A separate expansion valve is a valve employed to effect the cut off and
vary the expansion. It does not affect either the admission or exhaust
of the steam to the cylinder.
It is used because by its means an early point of cut off and high rate
of expansion may be obtained with a fixed point of exhaust, a fixed
amount of compression, and a fixed amount of lead, whereas with the link
motion alone the exhaust occurs earlier in the stroke, and the
compression and the lead increase as the link is moved from full gear
towards mid gear. The expansion valve should, when the engine is to be
started, be set for the latest point of cut off. The eccentric for the
expansion valve is set opposite to the crank, in order that its action
may be the same, whether the engine runs backward or forward.
The small cylinders on top of the steam chests are for the purpose of
guiding the upper ends of the valve spindles, and are fitted with
pistons having steam beneath, the space above being in communication
with the condenser. The steam pressure on the piston supports the weight
of the valves and valve gear.
The friction of a slide valve may be relieved or reduced by excluding
the steam from its back, which is done by various means, such as by a
ring cast on its back and working steam tight against a plate held
independently of the valve. The interior of the ring should be open to
the exhaust.
The friction of a slide valve is caused by the steam pressing it to its
seat, the amount of this pressure varying with the fit of the valve to
its seat, and its position over the ports, or, in other words, upon how
much of the valve area has steam pressing on one side only.
The travel of the eccentric rod is the distance it moves measured on a
straight line. It is equal to twice the throw of the eccentric.
The throw of an eccentric is the distance between the axis of its bore
and the centre or axis from which its circumference was turned in the
lathe.
Double beat valves are composed of two discs or mitre valves, one above
the other on the same stem, so that as the steam presses on the opposite
faces of the two discs the valve is balanced. The objection to their use
as safety valves is, that they are balanced and would not lift unless
the area of the upper disc was made larger than that of the lower one,
in which the objection would remain that the two discs do not expand
equally, hence they are apt to leak. They are sometimes used instead of
slide valves, but are objectionable because a separate admission and
exhaust valve is required at each end of the cylinder, and because at
quick speeds of revolution they fall to their seats with a shock or blow
which wears out both the valve and the seat. When a high piston speed is
obtained by great length of piston stroke, and not by high rotative
speed, their use is less objectionable.
Expansion joints are joints which permit the parts they connect to
expand and contract without straining them. They are necessary on the
steam pipe connecting one boiler to another, and on the main steam pipe
from the boilers to the engine. The working surfaces require to be of
brass, so that they will not corrode.
They require the collar on the internal pipe of the joint (on which the
gland fits) to be permanently fixed by soldering or brazing, and check
nuts on the studs, so that the internal pipe shall not be blown out from
the steam pressure.
This pipe is also sometimes fitted with chains or stops, in case the
studs should break, or the nuts or collar strip.
An oil cup is either a cavity cast in the piece or a cup shaped vessel
or hollow cylinder screwed in. It contains a pipe extending up about
three-fourths of its height, and through this pipe the oil is fed to the
surface required to be lubricated. A hinged lid or, in some cases, a
screwed cap covers the oil cup to exclude dust, etc.
The syphon or worsted consists of a number of threads of worsted or lamp
wick of equal lengths; a piece of lead or copper wire is laid across the
middle of the worsted, the copper wire is doubled and twisted and is
then pushed down the tube, carrying the doubled end of the worsted with
it. The upper ends of the wire are bent over the end of the tube so as
to hold the worsted, whose lower end should pass down below the level of
the bottom of the oil cup. The oil feeds (on the syphon principle)
through the medium of the wick or worsted, which should not fit the tube
tight but quite easily, its upper ends hanging over the top of the tube
to the bottom of the cup.
The worsted may be cleaned with scalding water, or by water thrown upon
it from the boiler.
Tallow cups for high pressure cylinders must have two cocks, so that
after the cup is filled the top cock may be closed and the bottom one
then opened. The top cock prevents the tallow or oil from being blown
out of the cock by the steam. For the low pressure cylinder a cup with a
single cock will answer, as the cock may be opened when the vacuum is at
that end of the cylinder, and the air will force the oil or tallow in.
A steam lubricator or impermeator is an automatic oil feeding device
placed on the steam pipe of the high pressure cylinder. Steam
lubricators are made in various forms, some having a positive feed by a
pumping arrangement, while in others the oil floats upon water in the
body of the lubricator to which steam is admitted; the condensation of
the steam increases the quantity of water and causes the floating oil to
overflow and feed through a pipe leading into the steam pipe or steam
chest, as the case may be. Cooling the impermeator causes more rapid
condensation, and increases the amount of oil fed to the steam.
Cylinder escape or relief valves do not let all the water out of the
cylinder because of the clearance,[59] hence the amount of water left in
will equal the amount of clearance.
[59] See page 372, on clearance.
The small cylinders on top of the steam chest are for the purpose of
guiding the upper ends of the valve spindles, and are fitted with
pistons having steam beneath, the upper end being in communication with
the condenser.
The effort of the piston to rise supports the weight of the valves and
valve gear.
The valves of a marine engine that are worked by hand are, the stop
valves for letting on steam from the boiler, the safety valve, which is
lifted to see that it is in proper working order, the Kingston valve for
letting in the circulating water, the blow through or starting valve for
warming the cylinders and starting the engines. The valve for adjusting
the rate of boiler feed has its lift adjusting screw operated by hand.
The slide valve may also be operated by hand before the engine is
started, or it may be operated by a steam reversing gear. The expansion
valves are also set by hand to regulate the point of cut off or amount
of expansion. The valves that are operated automatically, or from the
motion of the parts, are the slide and expansion valves, the suction and
delivery and check valves of all pumps, the air pump bucket valves, the
snifting valves, and the ship's side overboard discharge valves. When
the engine is stopped and the steam shut off, close the dampers to check
the draught and open the drain cocks on the high pressure cylinders.
If the engine is soon to start and the pressure in the boiler is at the
blowing off point, start the boiler feed, if the height of the water in
the boiler will permit it, and this is a good time to clean the fires.
If the engine is to stop for any length of time, shut off the
impermeator and the injection supply.
A vacuum gauge is an instrument for measuring the total or absolute
pressure, or pressure above a perfect vacuum, and it is used to indicate
the degree of vacuum that exists in the condenser, which, when the
various joints about the cylinder and condenser are tight, averages
about 27 inches of mercury when the temperature in the hot well is about
100° Fahrenheit.
In round numbers a column of mercury 32 inches high equals the weight of
the atmosphere,[60] hence taking the weight of the atmosphere at sea
level to be 15 lbs. per square inch, then each two inches of mercury
represents an atmospheric pressure of 2 lbs. Suppose then that a bent U
shaped tube, each leg of which is 30 inches high, is half filled with
mercury, and that one end is in communication with the condenser, and
the other end is open to the atmosphere, and if there was a perfect
vacuum in the condenser, the pressure of the atmosphere in the open leg
would force all the mercury into the leg that communicated with the
condenser, hence there would be a column of 30 inches of mercury in one
leg, and air in the other.
[60] See "Barometer," Chapter XL.
If there was in the condenser a pressure of 1-1/2 pounds per square inch
above a perfect vacuum, the mercury would stand 27 inches high in one
leg, and 3 inches in the other, and so on, hence from the height of the
column of mercury above its natural level the degree of vacuum in the
condenser may be known. But the pressure of the atmosphere varies with
its temperature, and the weight of mercury also varies with its
temperature.
To find the total pressure in the condenser, therefore, we subtract
height of the column of mercury given by the condenser from the height
of the column in the barometer, and divide the remainder by 2.
_Examples._--The barometer stands at 29.5 and the vacuum gauge at 26,
what is the absolute pressure in the condenser?
Here,
29.5 - 26 = 3.5 ÷ 2 = 1.75
Answer, 1-75/100 lbs. per square inch.
A dial vacuum gauge of the Bourdon construction is similar to the
Bourdon steam gauge, that is used upon the boiler, except that the
inside of the elliptical tube is in communication with the condenser and
the atmospheric pressure bends the tube into a curve of smaller radius
(instead of to a larger one, as in the case of the steam gauge).
Obviously, therefore, the zero of the dial vacuum gauge is atmospheric
pressure.
Suppose the dial vacuum gauge shows 10 lbs., the steam gauge 120 lbs.,
and the barometer 15 lbs., and we may find the total pressure or
pressure above vacuum of the steam in the boiler is as follows:
One-half Pressure by steam gauge = 60 lbs.
A perfect vacuum = 15 lbs.
--
Total pressure supposing condenser had a perfect vacuum = 75 lbs.
To make the correction necessary because there is not a perfect vacuum
in the condenser, we then proceed as follows:
Barometer 30 inches of mercury = 15 lbs. per sq. in.
Dial vacuum gauge = 10 " " " "
--
Actual pressure in condenser = 5 " " " "
Then
Total pressure supposing condenser had a perfect vacuum = 75
Actual pressure in condenser = 5
--
Actual pressure of the steam = 70
Racing means a sudden acceleration of the engine speed, and occurs when
the propeller is not fully immersed in the sea, as by reason of the
pitching of the ship. Racing augments the strain on the working gear of
the pumps, and is likely to lead to accident. It is obviated by the use
of a governor or by partly shutting off the steam by hand.
A marine governor is a device for controlling the engine speed, by
reducing the supply of steam to the engine cylinder whenever the engine
begins to race. The governor is driven by band or rope on the crank
shaft. Governors are made in various forms; thus, in one the shaft has a
fly wheel and a friction clutch, one half of which is fast on the
governor shaft, while between it and the other is a spiral spring which
connects the two halves. If the speed accelerates, the sliding half of
the clutch is moved along the governor shaft, and by means of links it
closes the throttle valve of the main steam pipe, thus wire drawing the
steam, reducing its pressure and thereby controlling the engine speed.
A common paddle wheel has a cast iron centre into which the wrought iron
arms are set and secured by wrought iron bolts and nuts.
The bolts have hook heads to grip the back of the arm, and receive a nut
and plate to secure the paddles.
Paddle wheels are sometimes provided with cast iron floats to act as
counterweights to some unbalanced part of the engine. They are mostly
required on side lever engines having a single crank; they are placed
nearly opposite to the crank, but not quite, so that they may prevent it
from stopping on the centre, and be difficult to start again.
Paddle wheels for engines having a single crank sometimes have their
floats of varying breadths, so as to keep the speed of revolution as
uniform as possible. This is accomplished by making some of the floats
wider than the others. The broadest floats are in action when the crank
is at its points of greatest power, and the narrowest at the time the
engine is on a dead centre, hence there are four general graduations of
breadth in the circumference of the wheel.
A radial paddle wheel is one in which the floats are fixed to the paddle
arms, and their ends are in a line radiating from the centre of the
paddle shaft.
A feathering paddle float is pivoted at the centre of its ends, and so
arranged that by a mechanical movement it will remain vertical when in
the water, notwithstanding the circular path it revolves in.
The object of feathering is to cause the thrust of the float to be as
nearly as possible in a horizontal line, and therefore more nearly
parallel to the line of the ship's motion, and thus utilize more of the
paddle power to drive the ship.
The eccentric for feathering the floats is fixed to the ship's side, and
sometimes carries a plummer block or pillow block for the paddle shaft
bearing. The centre of the eccentric sheave or wheel is placed ahead of
and level with the paddle shaft axis. The working surfaces of a
feathering wheel are of brass, and the bushes of the paddle arms of
lignum vitæ.
The surfaces are lubricated by the water, but sometimes oil lubrication
is provided for the eccentric sheave.
A disconnecting paddle engine is one in which the paddles may be driven
separately or together. This is effected at the inner port bearing by a
clutch wheel, which slides endways on the shaft and is driven by
feathers seated in the shaft. This clutch wheel is operated by a lever
so as to engage or disengage with the crank pin, which is fast in the
outer crank.
Disconnecting paddle engines are always fitted with loose eccentrics,
such engines being used for steam tugs and ferry boats, where quickness
of turning and of reversing is of great importance.
The thread of a screw propeller is its length measured along the outer
edge of the blade.
The angle of the thread is its angle to the axial line of the propeller
shaft.
The length of the thread is the length of the outer or circumferential
edge of the blade.
The area is the surface of one side of the blade.
The diameter is the distance apart of the two points on the edges that
are diametrically opposite and furthest apart.
The pitch of a propeller is its degree of spirality, and is represented
by the distance it would move forward if the water was a solid. It is
measured by drawing a line representing the axis of the propeller shaft,
and at a right angle to it a line representing in its length the
circumference of the circle described by the tips of the blades; from
the point of intersection of these two right angle lines a diagonal line
is drawn representing the angle the blade at its outer edge stands at
the propeller shaft axis. The greatest distance between the diagonal
line and the line representing the propeller circumference is the pitch
of the propeller.
A left handed propeller has a left hand thread or spiral, and revolves
from left to right to move the ship ahead.
A right hand propeller has its blades inclined in the opposite
direction, and of course revolves in the opposite direction to a left
hand one.
The slip of a propeller is the difference between the distance the ship
is moved by the propeller and the distance it would move if the water
was solid. Slip is usually expressed in the percentage that the distance
the ship actually travels bears to the distance she would have travelled
if there had been no slip. From 10 to 20 per cent. is lost in slip.
A screw of increasing pitch is one in which the angle of the face of the
propeller blade to the axis of the shaft increases as the thread recedes
from the shaft, or from the centre to the circumference of the blade, or
in both directions.
In a uniform pitch the angle of the blade to the propeller axis is the
same at all distances from the axis.
An example of a screw of uniform pitch would be a piece of angle iron
wound around a parallel shaft. If wound on a tape shaft, the largest
diameter being nearest to the ship's stern, it would have an increasing
pitch. If wound around a parabola, the pitch would vary at every point
in its diameter and thread.
A thrust bearing is a journal bearing provided with a number of
corrugations or collars fitting with corresponding corrugations or
recesses in the thrust block, the area thus provided serving to resist
the end thrust placed by the propeller upon the shaft.
It must be freely lubricated by ways leading to each collar or
corrugation, and so situated that it is accessible for examination. It
is sometimes at the end of the first length of shaft aft of the engine.
A stern tube is a sleeve enveloping the aft end of the propeller shaft
to protect it from the sea water, which would corrode it. At the aft end
of the stern tube is a gland and stuffing box. At the inner end, which
extends to the aft bulkhead, it has a flange which is bolted to the
bulkhead.
The bearing area of the shaft and stern tube are lined with brass (about
half an inch thick) to prevent their oxidation from the action of the
sea water.
A lignum vitæ bearing is a wooden bearing generally fitted to the outer
end of the stern tube in propeller engines, or to the outer ends of the
paddle shaft of paddle engines. It consists of strips of lignum vitæ
dovetailed into the bearing or bush, and running lengthways of it. These
strips are prevented from working out by a check plate at each end of
the bearing.
Screw propellers may be fastened to their shafts in several ways, as by
a key or feather sunk in the shaft, and projecting into a keyway in the
propeller bore, and a nut on the end of the shaft with a safety pin
outside the nut, or by a key passing through the boss of the propeller,
and a safety pin or plate upon the key.
The principal pipes of a marine engine and boiler, and the parts they
connect, are, the main steam pipe, connecting the stop valve on the
superheater to the steam chest of the engine cylinders; the waste steam
pipe from the safety valve to the open air; the blow-off pipe,
connecting the blow-off cocks on the bottom of the boiler with the
blow-off Kingston cock on the ship's side; cylinder jacket pipe from the
stop cock on the boiler to the steam jacket.
The circulating suction pipe, connecting the main Kingston valve with
the bottom of the circulating pump; the circulating delivery pipe,
connecting the discharge compartment of the condenser with the main
delivery valve on the ship's skin; the air pump suction, connecting the
body of the condenser with the suction side or bottom of the air pump;
the main exhaust pipe, connecting the exhaust passage of the low
pressure cylinder with the condenser; the feed water suction pipe,
connecting the donkey feed pipe with the hot well; the feed water
delivery pipe, connecting the donkey feed pump with the check valve on
the boiler; the bilge suction pipe, connecting a strum box in the bilge
with the bilge pump; a suction pipe from the strum in the bilge to the
donkey pump; the bilge pump delivery pipe, connecting the bilge pumps
with bilge delivery valves on the ship's side.
A mud box is a rectangular box usually placed in the engine room, and
serving to clear the bilge water from foreign substances, as small
pieces of wood, coal, etc.; the construction is as follows: It is on the
suction side of the bilge pumps, and is provided with a hinged lid that
affords access to clean it out, and that must obviously close air tight,
or the bilge pumps will not draw. The box is divided into two
compartments by a loose division plate that stands vertical, and is
perforated so as to act as a strainer.
The steam from the boiler passes through the superheater, main stop cock
or valve, main steam pipe, separator, regulating and throttle valve,
steam chest, steam port, steam passage into cylinder, returns through
steam passage and port, exhaust cavity of valve into either the
condenser or the low pressure cylinder, as the case may be, finally
exhausting into condenser, whence the water of condensation is pumped by
the air pump into the hot well. In the case of a jet condenser part only
of the condensed steam goes back to the boiler, the rest going into the
sea through the injection discharge pipe.
A steam jacket[61] is an outer casing to a steam cylinder, the space
between it and the cylinder being filled with steam direct from the
boiler, with the object of preventing condensation of the steam in the
engine cylinder.
[61] See page 374 on steam jackets.
A drain cock is supplied to the bottom of the jacket to pass off
condensed water. Steam jackets should be lagged or felted to prevent
condensation.
The parts of an engine that require to be felted or lagged are the
cylinders and the steam pipes; the boilers also should be felted or
otherwise covered to prevent loss of heat by radiation, and the uptake
protected by means of thin plates, kept, by means of distance pieces and
bolts, at a distance of two or three inches from the plates of the
uptake.
Various non conducting substances are employed to prevent radiation, as,
for example, felt, mineral wool, asbestos, and various kinds of cement.
The pieces of the engine through which the steam pressure is received
and transmitted are as follows:
The piston, piston rod, cross head, cross head gudgeon, connecting rod,
crank pin, crank shaft and couplings to the propeller shaft.
Trunk engines are generally used in war vessels where it is required to
have the engines below the water line. The trunk passes through the
cylinder and the piston is upon the trunk, the connecting rod passes
down into the trunk and connects direct to the piston. A stuffing box
and gland in each cylinder cover keeps the trunk steam tight. The trunk
forms a guide to the piston in place of the ordinary cross head and
guides, and thus saves the room required by those parts.
The cylinders for a right handed propeller should be on the starboard
side of the vessel, so that the pressure on the piston, when the engine
is going ahead, shall be in a direction to lift the trunk in the
cylinder, and thus act to relieve the gland and cylinder bore of the
weight of the trunk and piston.
An oscillating engine is one in which the cylinder is mounted on
bearings called trunnions, so that the cylinder can swing and keep its
bore and the piston pointing to the crank at all parts of the engine
revolution. This enables the connecting rod and slide bars to be
dispensed with. The trunnions are hollow, one containing the steam and
the other the exhaust passage.
Oscillating engines are used for paddle steamers, because their
construction permits of a good length of piston stroke, while still
keeping the engine low down in the vessel.
The valve motion for an oscillating engine consists of an ordinary
eccentric gear or motion, with the addition of various mechanical
arrangements to accommodate the valve gear to the vibrating motion of
the valve chest.
The stuffing box of an oscillating engine is made deeper than usual
because the gland bore has more strain on it, and extra wearing surface
is therefore required to prevent its wearing oval.
Geared engines are those with gear wheels to increase the revolutions of
the shaft above those of the engine, and thus obtain a high propeller
speed without a high piston speed.
The pressure that propels a vessel is taken by the thrust block in a
screw propeller engine.
The pressure that drives a paddle steamer is applied to the hull at the
shaft bearings and their holding beams, and to the bed plates. The
amount of fuel required per horse power per hour, by modern compound
engines, is from about 1-1/2 to 3 lbs., and by common condensing engines
from 3 to 5 lbs. per horse power per hour.
The unit or measure of a horse power is the amount of power required to
lift 33,000 lbs. one foot high in a minute.[62]
[62] See page 407, Vol. II.
Nominal horse power is a term used to represent the commercial rating or
power of an engine, and is usually based upon the area of the piston. It
gives no measure of the engine power, however, because it does not take
the piston speed into account.[63]
[63] See page 374, Vol. II.
In a surface condensing engine the duty of the air pump is to merely
pump the condensed steam and vapor from the condenser to the hot well,
whereas in a jet condensing engine it has to also take the condensing
water from the condenser, hence an air pump for a surface condenser may
be made smaller than that for a jet condenser. As the air pump works
against the pressure of the atmosphere, therefore the smaller it is the
less of the engine power is absorbed in working it.
The injection cocks are regulated for opening by rods having handles
attached. If the injection cocks are not open wide enough, the condenser
will get hot and impair the vacuum, while if opened too wide, the water
in the hot well will be cold and the boiler feed will be cold. These
cocks should be so regulated as to keep the temperature in the hot well
at about 100° Fahrenheit.
The parts of a marine engine that are exposed to danger in a cold
climate are all pipes through which cold water circulates, and are
liable to freeze.
The precautions necessary to prevent freezing in cold climates are to
cover all pipes liable to freeze, to keep the water circulating through
them, or to let it out of them if necessary, as in the case of the
engine standing.
A marine engine may fail to start, or may be prevented from starting by
the following causes:
1st. The H. P. slide valve may be off, or away from its seat, thus
admitting the steam to both sides of the piston at the same time.
2d. The engineer may have forgotten to disengage the hand turning gear
from the crank shaft.
3d. The propeller may be fouled with a piece of timber, or by a chain or
rope (these causes sometimes occurring when the ship is in port), or
there may be something wrong with the outer bearing of the propeller
shaft.
4th. In the case of a propeller fitted with a banjo frame (for the
purpose of raising the propeller) the propeller may be locked.
5th. An obstruction, as a block of wood, in the crank pit may prevent
the crank from turning.
6th. The slide valve nut may have slackened back, thus loosening the
slide valve.
7th. The slide valve spindle may have broken.
8th. When an engine has no auxiliary or starting, but an _impulse_ valve
that merely lets a puff of steam into the receiver, this impulse valve
may leak, and if the escape or relief valve on the receiver is too much
loaded, it may gag the H. P. piston by giving it high pressure steam on
both sides, and this may throw the valve off its seat. Similarly, if the
engine has an auxiliary or starting valve, and it leaks, high pressure
steam may be admitted to both sides of the L. P. piston, thus gagging it
and causing its slide valve to throw back and away from its seat.
9th. The cylinders may be choked with water, and the drain cocks choked
up.
10th. The crank shaft bearings may be screwed up too tightly.
11th. The air or the circulating pump may be choked with water, either
the air pump overflow valve or the circulating discharge valve being
secured down.[64]
[64] The air pump overflow valve should never be permanently fastened
down. More engines have been broken down from this than from almost
any other neglectful cause, because, from great leaks in the condenser
tubes and engines standing for a length of time, a larger quantity of
water may require to be got rid of during the first few strokes of the
pump than can pass through the small air or vapor pipe, which is
usually fitted from the hot well either into the bilge or else
overboard. Unless the valve in this overflow pipe is heavy enough of
itself (which is very rarely the case), it should be loaded by a
spring or weight, so that when the puff of the air pump causes it to
lift, and the vessel is rolling, sea water may not pass into the hot
well. To avoid this, some engineers erroneously fasten this valve
down. An experienced engineer states that in his experience five
engines have been broken down from this cause alone.
12th. From the engines being allowed to stand a long time in one
position, and the glands being too tightly packed. An engine should be
turned a little daily when not in use.
13th. From the piston rings being set out too tight to the cylinder
bore.
14th. From the throttle or stop valve being shut, as from its spindle
being broken.
15th. From the eccentric sheave, or wheel, having shifted on the shaft,
some eccentrics having a key that is not sunk in the sheave, which is
done so that the eccentric may shift rather than break if it should
seize in its strap.
16th. From the H. P. piston leaking badly, or its ring being broken,
which will permit the cylinder to fill with steam and the slide valve to
unseat.
17th. If the engine has been overhauled, the forward eccentric may have
been connected to the wrong end of the link, thus giving an improper
motion to the slide valve.
18th. The expansion may be set to cut off too early in the stroke.
19th. From the air pump rod, or from the circulating pump rod being
broken, or from the valves being broken.
20th. From the cylinder casing or the receiver being cracked so as to
admit steam to both sides of the piston at the same time.
A defective vacuum, or loss of vacuum, may occur from the following
causes:
1st. From the glands of the low pressure cylinder leaking.
2d. From the pet cock of the air pump being left open.
3d. From the joints of the connections about the condenser leaking.[65]
[65] To discover a leak about a condenser, pass an exposed light, as a
candle, about the joints, etc., and where there is a leak the flame
will be drawn in towards the condenser.
4th. From the condenser being cracked, and therefore leaky.
5th. From the injection cock or valve being closed.
6th. From the condenser tubes being foul for lack of being cleaned. From
the L. P. cylinder escape valves or cylinder cocks being leaky, and
therefore letting in air.
7th. From the slide valve and piston of the L. P. cylinder leaking.
8th. From the air pump valve being leaky or broken. From the circulating
pump being defective, as from having leaky valves.
9th. From the Kingston injection valve not being properly opened, or
from its outside orifice being choked.
10th. The bilge injection may be so connected with the air pump or
condenser as to impair the vacuum when its valve is accidentally stuck
and its stop cock is left open.[66]
[66] It is obvious that a defective vacuum may or may not prevent an
engine from starting, according to the degree of defectiveness.
The principal causes of heating are:
1st. The bearing caps being screwed down too tight.
2d. The bearings being left uncovered, thus allowing the brick dust used
for cleaning the machinery, the dirt from coaling the ship, or the sand
used for cleaning the decks, to get into the bearing.
3d. The oil grooves in the brasses being worn out or too shallow, or the
brasses not being cleared at the sides.
4th. Improper fitting of the distance pieces or fit strips between the
brasses.
5th. Bad oil or too light an oil.
6th. If the brasses are too slack and thump or pound, the back of the
brass may be stretched by pening, causing the sides of the brass to
close in upon and bind the crank journal or crank pin, and this will
cause heating.
For other information concerning the engine see as follows:
Page.
Angularity of connecting rod 375
The slide valve 376
Double ported and griddle valves 377
Balanced valves 377
Piston valves 378
Separate cut off valves 378
Reversing gears 383
Finding the working results of a slide valve 376
Condensing engines 442, 444
Calculations on the mechanical powers 405
The unit of power 407
Calculating horse power 407
Calculations of safety valves 409
Heat, water, and steam 410
The expansion of steam 411
The conversion of heat into work 411
The indicator 413
Indicator diagrams 414, 421
The barometer 415
Calculating the horse power from indicator diagrams 419
Finding the steam of water consumption from an indicator 421
Figs. 3405 and 3406 represent a triple expansion marine engine, the
construction being as follows:
The high pressure cylinder has a piston valve and the intermediate and
low pressure cylinders flat valves. Each cylinder has a link motion, and
all three link motions are shifted from the same shaft, which is moved
by a steam reversing gear. At _a_, Fig. 3405, are the eccentrics for the
link B, for the high pressure cylinder; _b´_, _b´_ are those for link
B´, for the intermediate cylinder; and _c´_ _c´_ are those for the link
C´, for the low pressure cylinder. From each link are rods E, Fig. 3406,
connected to arms on the shaft F _f_, to an arm on which is connected
the rod G, from the worm wheel H, whose actuating worm I is on a crank
shaft operated by the small steam cylinder J. The slide spindles D work
in guides, and their cross heads C span the edges of the links, gibs
being provided to take up the wear.
The gear for turning the engine when there is no steam in the main
boilers is constructed as follows:
On the shaft of the wheel _m_, Fig. 3405, is a worm _n_ operating a worm
wheel _p_, on whose shaft is a worm which operates the large worm wheel
shown on the main crank shaft.
Figs. 3407 and 3408 represent the compound engines of the steamship
_Poplar_, concerning which _The Engineer_ (from which the engravings are
taken) says:
"Both the cylinders of these engines are fitted with piston valves,
placed at the back of the cylinders and worked by the single eccentric
valve gear, which has been so largely adopted and so successfully
carried out by this firm in triple expansion as well as compound
engines. It will be noticed that whilst this valve gear permits of the
cylinders being close together, it allows of the crank shaft being made
in two similar pieces, and affords exceptionally long main and crank pin
bearings, of the former of which there are only three, instead of the
usual four. In the case of the _Poplar_ the cylinders are 29 in. and 55
in. in diameter and 33 in. stroke, and the crank pins are 11 in. long,
whilst the centre main bearing, which does duty for both the engines, is
23-3/4 in. in length, each of the outer bearings being 18 in. in length,
the diameter of the crank shaft being 9-1/2 in. Another very interesting
feature about these compact little engines is the design of the front
framework. Instead of the ordinary upright columns in front of each
engine there is an arrangement which gives exceptional stiffness to the
whole structure whilst affording the fullest possible accessibility to
the main working parts, and which has the appearance of an arch, from
the shoulders of which there are branches worked up to receive the feet
of the cylinders, thus accommodating the close centres and providing for
the support of the reversing wheel without in the least obstructing the
gear below. The condenser is divided horizontally through the centre on
a plan strongly advocated by the builders, the whole of the base of the
engines being cast in one piece and made level on the under side, so as
to enable it to receive support from, and be bolted to, the engine
seating immediately beneath the crank shaft, as well as round the
margin."
[Illustration: Fig. 3405.]
[Illustration: Fig. 3406.]
[Illustration: Fig. 3407.]
[Illustration: Fig. 3408.]
CHAPTER XLIII.--MARINE BOILERS.
Boilers for marine engines are, in England, made of special qualities of
plate, the best being termed Yorkshire, and a nearly equal grade,
Staffordshire. The plates for the shell, the furnace bottoms and the
gusset stays are made of Staffordshire, while the tube plates, furnace
tops, and such parts as require to be flanged and are subject to more
intense heat, are made of Yorkshire plate, which has more ductility.
In the United States the grades of iron used for boilers are C H No. 1
S, or charcoal hammered No. 1 shell iron, for the shell, and C H No. 1
F, or charcoal No. 1 flange iron, which is used for the furnaces and
such parts as require flanging.
In both countries steel is also used for boilers, except for the tubes,
for which it is not entirely reliable if very high pressures are to be
used.
Both the iron and steel plates are tested for tensile strength and
ductility.
The breaking strain is that which is sufficient to cause rupture, while
the _proof strain_ is that which the metal is required to withstand with
safety.
The safe working strain, or working pressure, W P, is the strain under
which it is considered safe to work the boiler.
The strength of a boiler of a given diameter and thickness of plate
varies according to the construction of the riveted seams or joints.
Boiler stays or braces are rods, ribs, or plates for supporting the
weaker parts of the boiler. Thus the tube plates may be stayed by rods
passing through both plates and screwed into them, or nuts and washers
may be used on the stays one on each side of each tube plate.
Gusset stays are iron plates which are riveted to [T] irons or in some
cases to [L] irons, which are riveted on the surfaces to be stayed.
Stay tubes are thick tubes (usually about 3/16 inch thick), which screw
into the tube sheets and are riveted over at the ends. A superior
construction, however, is to provide nuts and washers to the ends of the
stay tubes, one on each side of each tube plate.
Boiler stays are usually made of such diameters that when new they will
sustain a tensile strain of not more than 5,000 lbs. per square inch of
cross section, this being the rule of the Board of Trade.
Stays are sometimes screwed into the tube plates and then riveted over
at the outside ends. A better method, however, is to let the ends of the
stays receive a nut on each side of each tube plate.
Boiler tubes are secured in their tube plates by being _expanded_ in.
This may be done by driving in a taper steel mandrel, and then clinching
them over, or by using a tube expander. There are two principal kinds of
tube expanders, in one of which small rolls travel around the bore of
the tube and expand it, while in the other a number of segments, held
together by a spring, are forced outwards by a mandrel driven in by
hammer blows.
Too much expanding is apt to weaken the tube close to the inside face of
the tube sheet.
Boiler tubes leak first at the end which receives the greatest heat from
the fire, the leakage being caused by the expansion and contraction of
the tube, which is obviously hotter than the water which causes the tube
to expand more than the boiler shell. The remedy is to re-mandrel or
expand the tube.
The scale that forms on the face of the tube sheet keeps the water away
from contact with the plate, which with an undue thickness of scale will
crack between the tube holes.
A tube that is split or that cannot be made steam tight by being
re-mandrelled or expanded is plugged up at each end by means of either
wooden or iron plugs. The best plan, however, is to use iron discs
having a stepped diameter, so that one end will fit the bore of the
tube, and the other will form a shoulder that will cover the end of the
tube.
[Illustration: Fig. 3409.]
Each disc has a hole through its centre, so that a wrought iron rod or
bolt may be passed through the hole and receive a nut at each end.
Beneath the flange of each disc, a grummet of spun yarn and white lead
is placed, so as to make a steam tight joint when the nuts are screwed
home. This stays the tube plates as well as stopping the leaky tube.
If wooden plugs are used, they are made a driving fit in the tube bore,
and driven through until they have passed the split, and a second wooden
plug is driven tightly from the same end of the tube.
The up take of a marine boiler is the casing or passage way through
which the heat and gases pass after leaving the boiler. A dry up take is
one which is outside of the boiler, as in Fig. 3409, which represents an
outside view of a boiler such as shown in Figs. 3410 and 3411.
A wet up take is one which passes through the boiler, and therefore has
fire on one side and steam on the other. It is therefore under a
collapsing pressure.
The furnace of a marine boiler extends from the fire door to the
combustion chamber (_i. e._, the box in which the heat of the furnace
passes before returning through the tubes).
The superheater of a marine boiler is a cylindrical vessel receiving the
steam from the boiler, and delivering it to the main steam pipe, whence
the steam is delivered to the engines, etc. When it has no connection
with the up take, it may, however, be more properly termed a steam
driver, since it serves to separate the steam from entrained water, and
does not superheat the steam.
In some cases, however, the superheater takes the form of a spherical
ended cylinder standing in the up take.
The receiver of a marine boiler is a drum or cylinder that receives the
steam from the boiler and from which the steam passes through the steam
pipe to the engine. The receiver is by some called the _steam chest_ of
the boiler.
The fittings essential for a marine boiler are: The safety valves; the
test cocks (or gauge cocks, as they are sometimes termed); the water
gauge glass; the stop valves; the check valve for the boiler feed pipe,
and the valves for letting on steam to the main engine and such other
engine or engines as may take steam from the main boiler; the scum
cocks; the blow off cocks; and a small cock to enable the drawing of
water from the boiler to test its degree of saltness.
There are two kinds of safety valves, the dead weight and the spring
loaded.
A dead weight safety valve is one in which the valve is held to its seat
by dead weight, the objection to which is, that when the vessel rolls
the effect of the weight or weights upon the valve is diminished; hence
under heavy rolling the steam may blow off at a less pressure than the
valve is set for.
A lock up safety valve is a dead weight safety valve, the top of whose
spindle is provided with a cast iron cap or bonnet with two handles on.
This cap is keyed to the spindle, and the keyway is so disposed that no
extra weight can be added to the valve, while at the same time the valve
can be lifted from its seat and turned around.
A spring loaded safety valve is one in which the valve is held down by
the pressure of a spiral spring, and this pressure will obviously not
vary, no matter how much the ship rolls.
In proportion as the valve lifts and the spring compresses, its
resistance increases, and this tends to impair the accuracy of the
valve. This, however, is offset from the fact that when the valve rises
from its seat it presents a greater area for the steam to act against.
The area of safety valve required by the English Board of Trade is about
1/2 square inch of valve area per square foot of fire grate area.[67]
[67] See page 409, Vol. II., for safety valve calculations.
There are three test cocks, which are sometimes placed in a diagonal row
on the front of the boiler, and sometimes on the fitting for the gauge
glass. The top test cock shows highest level to which the water should
rise in the boiler, and the lowest one the lowest level, the middle cock
indicating the average. There is usually a vertical distance of about 4
inches between the test cocks, which gives a permissible range of 8
inches in the level of the water in the boiler.
Test cocks are prevented from choking with scale by passing a wire
through the cock and clear into the boiler, a plug being provided,
which, when removed by unscrewing, permits the insertion of the wire.
This cleaning must obviously be performed when there is no steam on the
boiler.
A gauge glass is a glass tube whose bore is open to the boiler. It is
fitted at each end to a brass socket that is screwed into the boiler,
each socket having a cock that permits communication between the gauge
glass and the boiler to be shut off in case the glass should break. The
bottom socket is also fitted with a cock, which, on being opened,
permits the water and steam to blow through the gauge glass and clean it
of scum or dirt.
The gauge glass must be plainly in sight, and placed at such a height
that when the desired quantity of water is in the boiler it will half
fill the gauge glass.
Glass water gauges, instead of attaching to the boiler, are sometimes
fitted to a fitting that connects to the top and bottom of the boiler,
with the object of attaining, for the gauge glass, water free from the
scum and impurities which collect at and near the surface of the water
in the boiler. This fitting should have cocks in each pipe leading to
the boiler, so that in case the gauge glass breaks, steam can be shut
off from the boiler.
In some cases the test cocks are also attached to this fitting, and in
this case the construction should be such that shutting off
communication between the gauge glass and the boiler will not at the
same time shut off communication between the test cocks and the boiler.
When the boiler is priming or steaming very fast, the gauge glass may
show a false water level, hence reading should be compared with that of
the test cocks.
If the water gets too low, the first parts of the boiler to be injured
will be the top of the flame box, or the combustion chamber, and the top
row of tubes, because they are the first surfaces that the water will
fall below and leave exposed to the heat without having water on the
other side.[68]
[68] See page 370, Vol. II., on low water in boilers.
The pressure in the boiler is shown by a steam gauge, pressure gauge, or
dial gauge as it is promiscuously called.
A Bourdon dial gauge or pressure gauge consists of a dial casing,
containing a hollow thin brass hoop, oval in cross section, which
receives steam from the boiler.
This hoop is fixed at one end, while the other end is closed and free to
move. The free end is connected by a small link to a toothed sector,
which gears or engages with a small pinion fast upon the spindle of the
pointer or needle. When the steam is admitted into the hoop, it
straightens out or expands in diameter to an amount that is
proportionate to the amount of the pressure within the hoop, and thus
causes the needle or index pointer to revolve, and denote from the
markings or readings of a dial plate the amount of pressure within the
hoop.
If the pressure within the hoop is released, it will move to its normal
or zero position. In the course of time, however, the hoop is apt to get
a slight permanent set and not indicate correctly. It may, however, be
approximately tested for accuracy by testing its readings with that of
the safety valve.
The working parts of the gauge, and its casings also, are made of brass,
so that they shall not corrode, and to prevent the heat of the steam
from permeating the gauge and impairing the action from expanding the
parts, a small quantity of water interposes between the gauge and the
steam, the construction being as follows:
Outside the gauge casing the steam pipe is bent into a loop forming an
inverted syphon which is to contain the water.
At the lowest point in the bend of the syphon a small cock is inserted,
which lets the water out of the leg of the syphon nearest to the boiler,
because water in that leg would, from its weight, cause the gauge to
show a pressure higher than that in the boiler.
The pressure shown by a steam gauge is that above atmosphere,[69] and
not that above vacuum.
[69] See page 367, Vol. II., for remarks on total pressure and
pressure by gauge.
The stop valve of a marine boiler is a valve that is opened to let the
steam into the main steam pipe.
A blow off cock is a cock employed to blow off, or let all, or a part
of, the water out of a boiler. There are generally two, one on the
bottom of the boiler, and the other at the ship's side, so that if the
pipe was to break or get damaged, the cock at the vessel's side can be
closed to keep the sea water out, while that on the boiler may be closed
to keep the water and steam in the boiler. These two ends cannot
obviously be obtained if one blow off cock only was used.
Blow off cocks are opened and closed by a spanner or key that is
removable from the cock, and to prevent the possibility of taking off
the spanner or key, before the blow off cock is closed, a spanner guard
is employed.
A spanner guard is a cap having a lug or tongue, which projects into the
hole in the spanner guard, through which the spanner or key must pass
before it can fit on the head of the blow off cock, and the key or
spanner has a corresponding recess, so that the spanner or key can only
be put on or taken off when the cock is closed.
Blowing off a boiler is emptying it entirely, as for examining the whole
interior of the boiler.
Blowing down a boiler is letting out a portion of the water, so as to
carry off the loose scale, mud, or sludge that may accumulate on the
bottom of the boiler. The mud or sludge would form into scale if allowed
to remain.
A scum cock is a cock employed to blow off a portion of the surface
water in a boiler, and thus remove the scum, salt, and impurities which
float or are thrown up to the surface.
Two scum cocks are employed, one on the side of the boiler, and one on
the side of the ship. These two cocks are connected by a pipe. That on
the boiler is placed a little below the working level, which is supposed
to be (and is kept as nearly as possible) about 9 inches above the top
row of tubes.
Sluice valves are doors sliding, water tight, in ways at the entrance to
the bulkheads on both sides of the ship. They should be worked from
above, in order that they may be shut when the depth of water in the
bulk heads might prevent them from being worked from below. These valves
should be operated occasionally to ensure that they slide easily and are
in working order.
Scale in marine boilers using salt water is composed of sulphate of
lime. It is most objectionable on the furnace tops, on the sides and
tops of the combustion chamber, on the tubes and on the tube plates. It
may be prevented to some extent from forming by a rapid circulation of
the water in the boiler, by blowing down the boiler through the scum
cocks, by the suspension in the boiler of zinc plates in contact with
iron ones, by impregnating the water with chemical antidotes, which
maintain the impurities in the form of mud or sludge, and by purifying
the feed water. If surface condensers are used, scaling is obviously
diminished by feeding as little salt water as possible, which may be
done by not getting up a steam pressure high enough to cause the safety
valve to blow off, and by preserving the water from the exhausts of the
donkey or other engines about the ship.
A thin coating of scale, as say 1/32 inch thick, may serve as a
protection against the chemical action of water that would act to
corrode the surfaces, as in the case of harbors receiving the waste
waters from chemical works or other impure waters. A thick coating of
scale causes the plates to burn on the side receiving the furnace heat,
and causes blisters to rise, while at the same time it decreases the
value of the heating surface.
Scale on the tubes causes them to expand more, and therefore leak in the
tube sheets.
This extra expansion sometimes breaks away the scale at the neck of the
tube in the tube sheet and gives access to the water there, and the
chemical action of water will in some cases cause the tube to be eaten
through close to the tube plate.
Scale is removed mechanically by chisels, scrapers and chipping hammers,
which are applied to all the surfaces that can be got at from the inside
of the boiler (the man hole affording access to the boiler). After the
scale has thus as far as possible been removed, it is washed out of the
boiler. The efficiency with which scale may be removed from the tube
sheets and tubes depends, to a great extent, upon the facilities the
arrangement of the rows of tubes affords in giving access to the scaling
chisels.
The salinometer. Salt water is heavier than fresh water, hence the
amount of saltiness of water may be known from its density or weight. A
salinometer is an instrument that determines from the density of the
water the amount of salt contained in the water. It consists of a
graduated stem at whose extremity is a weighted bulb which partially
sinks the tube in the water; the depth to which the bulb sinks shows the
density of the water.
The reading of a salinometer is taken at the water level, and is read on
the tube, which is graduated as follows: The mark furthest from the bulb
or highest up the stem is marked O, and if the zero line is level with
the surface of the valve in which the salinometer floats, it indicates
fresh water. If salt be added to the fresh water, the salinometer will
rise in the water, and when the water contains 1 lb. of salt to 32 lbs.
of water (which is the average degree of saltiness of sea water), the
line marked 1/32 on the salinometer tube will be level with the surface
of the water. If the saltiness of the water be increased, the
salinometer will rise in the water until, at 2 lbs. of salt to 32 lbs.
of water, a line (on the tube) marked 2/32 will be level with the
surface of the water. The space between the 1/32 and 2/32 is divided
into halves and quarters.
As the density of the water varies with its temperature, therefore the
readings on the salinometer must agree with some specific temperature,
which is usually 200° Fahrenheit, and the reading of the salinometer is
correct only when the water is at that temperature. If, however, the
water varies a few degrees from the standard of temperature for which
the salinometer is marked, a correction of the reading may be made by
adding 1/8 of 1/32 for each 10 degrees, that the water is hotter, or
subtracting the same for each 10 degrees that it is cooler than the
temperature at which the salinometer is correct.
The density or specific gravity of ordinary sea water is 1.027 (that of
distilled water being unity or 1), and it contains about 4 oz. of salt
per imperial gallon.
Tallow is sometimes forced into a boiler fed with salt water to stop
priming, by means of a syringe that is screwed into a tallow cock
provided upon the boiler below the water level. If the boiler is fed
with fresh water, tallow is apt to cause priming.
Angle irons are used in boiler construction to be riveted to plates that
require supporting or strengthening, or for gusset stays to be riveted
to. Flanged plates are used in the construction of the furnaces, flame,
boxes or combustion chambers, boiler ends and tube plates or tube
sheets.
Division plates are fitted in some boilers to prevent the water from
passing from one side of the boiler to the other when the vessel rolls
heavily. This prevents some of the tubes from being left uncovered by
water, and thereby getting injured from undue heat.
These division plates are neither steam nor water tight, and stand fore
and aft of the ship. Similar division plates are sometimes used,
however, to prevent the tops of the combustion boxes from getting
overheated from the motion of the ship leaving them uncovered with
water, their location being subserved to this end and varying with the
position of the boiler.
The superheater of a marine boiler is provided with a safety valve, and
sometimes with a pressure gauge to enable the comparing of the steam
pressure with that in the boiler, and should also be provided with a
gauge glass, to show when heavy priming is going on.
The main stop valve is upon the superheater, as is also the blast pipe.
Priming is a lifting, into the steam space of the boiler, of a part of
the water, and may arise from heavy firing, from the safety valve
blowing off, from too little steam space, and from other causes.
Priming[70] often occurs when the boiler feed is changed from salt water
to fresh water, or from fresh to salt water.
[70] See page 370 Vol. II., on priming.
A separator or interceptor is a device fitted to either the superheater
or to the steam receiver, for separating entrained water from the steam.
It consists of a rectangular box or chamber with a partition plate
extending from the top half down into the box.
The entering steam strikes the face of the partition plate against which
the water collects, and from which it drops to the bottom of the box,
while the steam passes under the partition and out at the other side to
the engine.
The draught of a boiler is caused by the heat expanding the air and
lightening it, thus causing it to ascend. It can be checked by stopping
the exit of heated air up the funnel by means of a damper, by checking
the flow of cold air into the furnace, by closing the dampers, by
opening the furnace doors and letting cold air in the furnaces above the
fires.[71]
[71] See page 368, Vol. II.
A blast pipe is a small pipe leading from the superheater to the funnel,
and provided with a stop cock.
It is used for letting a jet of steam up the funnel to promote the
draught.
Flame seen at the top of the funnel is caused by the combustion of gases
that would have been consumed in the furnace had there been sufficient
air or sufficient room for complete combustion. It may be caused in a
variety of ways, as insufficient openings between the fire bars, too
narrow a space between the bridge wall and the boiler, or too deep a
fire upon the bars. It is detrimental, because it obviously wastes fuel.
Dampers are used to regulate the draught in the furnace; they are fitted
to the ash-pits or to the funnel, and should be fitted to both, because
closing a damper in the funnel sets up a certain amount of pressure in
the furnace by holding the heat, whereas dampers at the ash pit doors
and none in the funnel lets the heat out and prevents cold air from
getting in to promote combustion.
When there are no dampers the furnace doors are open instead, to check
the draught; this is, however, highly injurious to the boilers.
The most rapid wasting of the plates of a marine boiler occurs alongside
the fire bars, on the furnace tops, at the back of the flame box or
combustion chamber, and in those plates generally that receive the most
intense heat, and especially when they are heavily coated with scale and
are not covered with water.
The scale that forms on the face of the tube sheet keeps the water away
from contact with the plate, which, with an undue thickness of scale,
will crack between the tube holes.
A tube that is split or that cannot be made steam tight by being
re-mandrelled or expanded is plugged up at each end by means of either
wooden or iron plugs. The best plan, however, is to use iron discs
having a stepped diameter, so that one end will fit the bore of the
tube, and the other will form a shoulder that will cover the end of the
tube.
Each disc has a hole through its centre, so that a wrought iron rod or
bolt may be passed through the hole and receive a nut at each end.
Beneath the flange of each disc, a grummet of spun yarn and white lead
is placed, so as to make a steam tight joint when the nuts are screwed
home. This stays the tube plates as well as stopping the leaky tube.
If wooden plugs are used, they are made a driving fit in the tube bore,
and driven through until they have passed the split, and a second wooden
plug is driven tightly from the same end of the tube.
Black smoke is an evidence of incomplete or imperfect combustion, and
may be, to a great extent, prevented by careful firing, as by feeding
gradually and evenly, by the admission of the proper quantity of air, or
by a jet of steam admitted above the dead plates.
The furnace bars are ordinarily of cast iron about 1-1/4 inches thick at
the top, tapered towards the bottom, and with an air space of from 1/2
to 3/4 inch between them.
They require less air space for Welsh than for Newcastle coal, as the
latter is the flaming or gaseous coal, and burns the fastest.
The quantity of coal burned in marine boiler furnaces is about 15 lbs.
per square foot of fire grate area per hour; hence the quantity burnt
per day with common average engines with 4 furnaces, 3 feet wide and 5
feet long, may be found by multiplying the area of the 4 furnaces (60
feet) by the number of lbs. (15) burned per foot of grate per hour,
which will give the total lbs. weight burned per hour, which, divided by
112 lbs., will give the hundredweight burned per hour, and this,
multiplied by the number of hours reckoned as constituting a day, gives
the fuel consumption per day, based upon 15 lbs. coal per square foot of
fire grate area.
The number of tons of steam coal burnt per day to drive an ordinary
steamer of 40 feet beam 10 knots an hour by steam alone (or without
sail), will depend upon the kind of engine used. Experience teaches us
that with average vessels, the beam squared equals the consumption of
coal for 40 days, in the case of an ordinary jet condenser engine; 50
days with a surface condensing engine; and 60 days with a compound
engine; hence, in the present example, assuming the engine to be jet
condensing, we may calculate the fuel consumption per day, for a vessel
40 feet beam giving 10 knots an hour, as follows:
The beam squared gives 1600 (40 × 40 = 1600), which divided by 40 (40
days) gives 40 tons per day. For surface condensing the 1600 would be
divided by 50, giving 32 tons per day; and for a compound engine the
1600 would be divided by 60, giving 26 tons 13-1/3 cwt. per day.
It is obvious, however, that calculations of this kind, in which the
ratio of expansion is not stated, are the merest approximations.
The number of tons of steam coal that will be burnt per day with a pair
of average surface condensing engines having cylinders 50 inches in
diameter will be, under average conditions, 16 tons per day, the
calculations being based upon the common assumption that the diameter of
one cylinder squared and divided by 100 gives the consumption of fuel in
tons per day for condensing engines not compounded; thus, 40 × 40 = 1600
÷ 100 = 16 tons of coal burned per day.
Here again, the ratio of expansion not being specified, the calculation
has no real practical value.
If at sea and short of coal, bear in mind that the consumption of fuel
per mile run is greater for fast than for slow speeds; hence the
following points should be attended to:
Reduce the speed of the ship to say half the usual. Regulate the fire so
as to keep up full boiler pressure without blowing off. This will allow
the expansion or cut off valve to be set to cut off early in the stroke,
and thus save steam. If, under these conditions, the steam should
sometimes blow off at the safety valve, cover up part of the fire grate
area.
Use a thin, rather than a thick, fire, but be careful that it is not so
thin as to let currents of cold air pass through.
TO RELIEVE THE BOILER IN CASE OF EMERGENCY.--Suppose an engine breaks
down at a time when the fires are heavy and going full, that the steam
gauge shows blowing off pressure, but that the safety valve is stuck, or
from some cause or other is prevented from blowing off, and cannot be
eased or lifted, and the following is the course to be pursued:
1st. Close the ash pit dampers and open the smoke box door and fire
door. If there are no ash pit doors, close the damper in the up take and
open the fire and smoke box doors.
2d. Start the donkey engine to feed cold water into the boilers.
3d. Start the steam winches, and any other small engines that take steam
from the main boilers.
4th. Slacken the escape valves, and open the drain cocks of the
cylinders and receivers, and steam will blow through the H.P. cylinder
escape valve and drain cock at once. The H. P. slide valve may then be
worked by hand, back and forth, to let steam pass into the receiver and
blow through its escape valves and drain cock.
5th. Open the scum or brine cocks and keep them open, also open all
gauge or test cocks, etc., about the boiler.[72]
[72] It is not safe to draw the fire at a time when the pressure is at
a dangerous point, especially if heavy, as disturbing it may
temporarily increase the combustion and the danger of explosion.
[Illustration: Fig. 3410.]