It must be borne in mind that the simple fact that a boiler is of the
water-tube design does not as a necessity indicate that it is a good or
safe boiler.
Safety--Many of the water-tube boilers on the
market are as lacking as are fire-tube boilers in the positive
circulation which, as has been demonstrated by Mr. Babcock's lecture, is
so necessary in the requirements of the perfect steam boiler. In boilers
using water-leg construction, there is danger of defective circulation,
leaks are common, and unsuspected corrosion may be going on in portions
of the boiler that cannot be inspected. Stresses due to unequal
expansion of the metal cannot be well avoided but they may be minimized
by maintaining at the same temperature all pressure parts of the boiler.
The result is to be secured only by means of a well defined circulation.
The main feature to which the Babcock & Wilcox boiler owes its safety is
the construction made possible by the use of headers, by which the water
in each vertical row of tubes is separated from that in the adjacent
rows. This construction results in the very efficient circulation
produced through the breaking up of the steam and water in the front
headers, the effect of these headers in producing such a positive
circulation having been clearly demonstrated in Mr. Babcock's lecture.
The use of a number of sections, thus composed of headers and tubes, has
a distinct advantage over the use of a common chamber at the outlet ends
of the tubes. In the former case the circulation of water in one
vertical row of tubes cannot interfere with that in the other rows,
while in the latter construction there will be downward as well as
upward currents and such downward currents tend to neutralize any good
effect there might be through the diminution of the density of the water
column by the steam.
Further, the circulation results directly from the design of the boiler
and requires no assistance from "retarders", check valves and the like,
within the boiler. All such mechanical devices in the interior of a
boiler serve only to complicate the design and should not be used.
This positive and efficient circulation assures that all portions of the
pressure parts of the Babcock & Wilcox boiler will be at approximately
the same temperature and in this way strains resulting from unequal
temperatures are obviated.
Where the water throughout the boiler is at the temperature of the steam
contained, a condition to be secured only by proper circulation, danger
from internal pitting is minimized, or at least limited only to effects
of the water fed the boiler. Where the water in any portion of the
boiler is lower than the temperature of the steam corresponding to the
pressure carried, whether the fact that such lower temperatures exist as
a result of lack of circulation, or because of intentional design,
internal pitting or corrosion will almost invariably result.
Dr. Thurston has already been quoted to the effect that the admitted
safety of a water-tube boiler is the result of the division of its
contents into small portions. In boilers using a water-leg construction,
while the danger from explosion will be largely limited to the tubes,
there is the danger, however, that such legs may explode due to the
deterioration of their stays, and such an explosion might be almost as
disastrous as that of a shell boiler. The headers in a Babcock & Wilcox
boiler are practically free from any danger of explosion. Were such an
explosion to occur, it would still be localized to a much larger extent
than in the case of a water-leg boiler and the header construction thus
almost absolutely localizes any danger from such a cause.
Staybolts are admittedly an undesirable element of construction in any
boiler. They are wholly objectionable and the only reason for the
presence of staybolts in a boiler is to enable a cheaper form of
construction to be used than if they were eliminated.
In boilers utilizing in their design flat-stayed surfaces, or staybolt
construction under pressure, corrosion and wear and tear in service
tends to weaken some single part subject to continual strain, the result
being an increased strain on other parts greatly in excess of that for
which an allowance can be made by any reasonable factor of safety. Where
the construction is such that the weakening of a single part will
produce a marked decrease in the safety and reliability of the whole, it
follows of necessity, that there will be a corresponding decrease in the
working pressure which may be safely carried.
In water-leg boilers, the use of such flat-stayed surfaces under
pressure presents difficulties that are practically unsurmountable. Such
surfaces exposed to the heat of the fire are subject to unequal
expansion, distortion, leakage and corrosion, or in general, to many of
the objections that have already been advanced against the fire-tube
boilers in the consideration of water-tube boilers as a class in
comparison with fire-tube boilers.
[Illustration: McAlpin Hotel, New York City, Operating 2360 Horse Power
of Babcock & Wilcox Boilers]
Aside from the difficulties that may arise in actual service due to the
failure of staybolts, or in general, due to the use of flat-stayed
surfaces, constructional features are encountered in the actual
manufacture of such boilers that make it difficult if not impossible to
produce a first-class mechanical job. It is practically impossible in
the building of such a boiler to so design and place the staybolts that
all will be under equal strain. Such unequal strains, resulting from
constructional difficulties, will be greatly multiplied when such a
boiler is placed in service. Much of the riveting in boilers of this
design must of necessity be hand work, which is never the equal of
machine riveting. The use of water-leg construction ordinarily requires
the flanging of large plates, which is difficult, and because of the
number of heats necessary and the continual working of the material, may
lead to the weakening of such plates.
In vertical or semi-vertical water-tube boilers utilizing flat-stayed
surfaces under pressure, these surfaces are ordinarily so located as to
offer a convenient lodging place for flue dust, which fuses into a hard
mass, is difficult of removal and under which corrosion may be going on
with no possibility of detection.
Where stayed surfaces or water legs are features in the design of a
water-tube boiler, the factor of safety of such parts must be most
carefully considered. In such parts too, is the determination of the
factor most difficult, and because of the "rule-of-thumb" determination
frequently necessary, the factor of safety becomes in reality a factor
of ignorance. As opposed to such indeterminate factors of safety, in the
Babcock & Wilcox boiler, when the factor of safety for the drum or drums
has been determined, and such a factor may be determined accurately, the
factors for all other portions of the pressure parts are greatly in
excess of that of the drum. All Babcock & Wilcox boilers are built with
a factor of safety of at least five, and inasmuch as the factor of the
safety of the tubes and headers is greatly in excess of this figure, it
applies specifically to the drum or drums. This factor represents a
greater degree of safety than a considerably higher factor applied to a
boiler in which the shell or any riveted portion is acted upon directly
by the fire, or the same factor applied to a boiler utilizing
flat-stayed surface construction, where the accurate determination of
the limiting factor of safety is difficult, if not impossible.
That the factor of safety of stayed surfaces is questionable may perhaps
be best realized from a consideration of the severe requirements as to
such factor called for by the rules and regulations of the Board of
Supervising Inspectors, U. S. Government.
In view of the above, the absence of any stayed surfaces in the Babcock
& Wilcox boiler is obviously a distinguishing advantage where safety is
a factor. It is of interest to note, in the article on the evolution of
the Babcock & Wilcox boiler, that staybolt construction was used in
several designs, found unsatisfactory and unsafe, and discarded.
Another feature in the design of the Babcock & Wilcox boiler tending
toward added safety is its manner of suspension. This has been indicated
in the previous chapter and is of such nature that all of the pressure
parts are free to expand or contract under variations of temperature
without in any way interfering with any part of the boiler setting. The
sectional nature of the boiler allows a flexibility under varying
temperature changes that practically obviates internal strain.
In boilers utilizing water-leg construction, on the other hand, the
construction is rigid, giving rise to serious internal strains and the
method of support ordinarily made necessary by the boiler design is not
only unmechanical but frequently dangerous, due to the fact that proper
provision is not made for expansion and contraction under temperature
variations.
Boilers utilizing water-leg construction are not ordinarily provided
with mud drums. This is a serious defect in that it allows impurities
and sediment to collect in a portion of the boiler not easily inspected,
and corrosion may result.
Economy--That the water-tube boiler as a class lends itself more readily
than does the fire-tube boiler to a variation in the relation of grate
surface, heating surface and combustion space has been already pointed
out. In economy again, the construction made possible by the use of
headers in Babcock & Wilcox boilers appears as a distinct advantage.
Because of this construction, there is a flexibility possible, in an
unlimited variety of heights and widths that will satisfactorily meet
the special requirements of the fuel to be burned in individual cases.
An extended experience in the design of furnaces best suited for a wide
variety of fuels has made The Babcock & Wilcox Co. leaders in the field
of economy. Furnaces have been built and are in successful operation for
burning anthracite and bituminous coals, lignite, crude oil, gas-house
tar, wood, sawdust and shavings, bagasse, tan bark, natural gas, blast
furnace gas, by-product coke oven gas and for the utilization of waste
heat from commercial processes. The great number of Babcock & Wilcox
boilers now in satisfactory operation under such a wide range of fuel
conditions constitutes an unimpeachable testimonial to the ability to
meet all of the many conditions of service.
The limitations in the draft area of fire-tube boilers as affecting
economy have been pointed out. That a greater draft area is possible in
water-tube boilers does not of necessity indicate that proper advantage
of this fact is taken in all boilers of the water-tube class. In the
Babcock & Wilcox boiler, the large draft area taken in connection with
the effective baffling allows the gases to be brought into intimate
contact with all portions of the heating surfaces and renders such
surfaces highly efficient.
In certain designs of water-tube boilers the baffling is such as to
render ineffective certain portions of the heating surface, due to the
tendency of soot and dirt to collect on or behind baffles, in this way
causing the interposition of a layer of non-conducting material between
the hot gases and the heating surfaces.
In Babcock & Wilcox boilers the standard baffle arrangement is such as
to allow the installation of a superheater without in any way altering
the path of the gases from furnace to stack, or requiring a change in
the boiler design. In certain water-tube boilers the baffle arrangement
is such that if a superheater is to be installed a complete change in
the ordinary baffle design is necessary. Frequently to insure
sufficiently hot gas striking the heating surfaces, a portion is
by-passed directly from the furnace to the superheater chamber without
passing over any of the boiler heating surfaces. Any such arrangement
will lead to a decrease in economy and the use of boilers requiring it
should be avoided.
Capacity--Babcock & Wilcox boilers are run successfully in every-day
practice at higher ratings than any other boilers in practical service.
The capacities thus obtainable are due directly to the efficient
circulation already pointed out. Inasmuch as the construction utilizing
headers has a direct bearing in producing such circulation, it is also
connected with the high capacities obtainable with this apparatus.
Where intelligently handled and kept properly cleaned, Babcock & Wilcox
boilers are operated in many plants at from 200 to 225 per cent of their
rated evaporative capacity and it is not unusual for them to be operated
at 300 per cent of such rated capacity during periods of peak load.
Dry Steam--In the list of the requirements of the perfect steam boiler,
the necessity that dry steam be generated has been pointed out. The
Babcock & Wilcox boiler will deliver dry steam under higher capacities
and poorer conditions of feed water than any other boiler now
manufactured. Certain boilers will, when operated at ordinary ratings,
handle poor feed water and deliver steam in which the moisture content
is not objectionable. When these same boilers are driven at high
overloads, there will be a direct tendency to prime and the percentage
of moisture in the steam delivered will be high. This tendency is the
result of the lack of proper circulation and once more there is seen the
advantage of the headers of the Babcock & Wilcox boiler, resulting as it
does in the securing of a positive circulation.
In the design of the Babcock & Wilcox boiler sufficient space is
provided between the steam outlet and the disengaging point to insure
the steam passing from the boiler in a dry state without entraining or
again picking up any particles of water in its passage even at high
rates of evaporation. Ample time is given for a complete separation of
steam from the water at the disengaging surface before the steam is
carried from the boiler. These two features, which are additional causes
for the ability of the Babcock & Wilcox boiler to deliver dry steam,
result from the proper proportioning of the steam and water space of the
boiler. From the history of the development of the boiler, it is evident
that the cubical capacity per horse power of the steam and water space
has been adopted after numerous experiments.
That the "dry pipe" serves in no way the generally understood function
of such device has been pointed out. As stated, the function of the "dry
pipe" in a Babcock & Wilcox boiler is simply that of a collecting pipe
and this statement holds true regardless of the rate of operation of the
boiler.
In certain boilers, "superheating surface" is provided to "dry the
steam," or to remove the moisture due to priming or foaming. Such
surface is invariably a source of trouble unless the steam is initially
dry and a boiler which will deliver dry steam is obviously to be
preferred to one in which surface must be supplied especially for such
purpose. Where superheaters are installed with Babcock & Wilcox boilers,
they are in every sense of the word superheaters and not driers, the
steam being delivered to them in a dry state.
The question has been raised in connection with the cross drum design of
the Babcock & Wilcox boiler as to its ability to deliver dry steam.
Experience has shown the absolute lack of basis for any such objection.
The Babcock & Wilcox Company at its Bayonne Works some time ago made a
series of experiments to see in what manner the steam generated was
separated from the water either in the drum or in its passage to the
drum. Glass peepholes were installed in each end of a drum in a boiler
of the marine design, at the point midway between that at which the
horizontal circulating tubes entered the drum and the drum baffle plate.
By holding a light at one of these peepholes the action in the drum was
clearly seen through the other. It was found that with the boiler
operated under three-quarter inch ashpit pressure, which, with the fuel
used would be equivalent to approximately 185 per cent of rating for
stationary boiler practice, that each tube was delivering with great
velocity a stream of solid water, which filled the tube for half its
cross sectional area. There was no spray or mist accompanying such
delivery, clearly indicating that the steam had entirely separated from
the water in its passage through the horizontal circulating tubes, which
in the boiler in question were but 50 inches long.
[Illustration: Northwest Station of the Commonwealth Edison Co.,
Chicago, Ill. This Installation Consists of 11,360 Horse Power of
Babcock & Wilcox Boilers and Superheaters, Equipped with Babcock &
Wilcox Chain Grate Stokers]
These experiments proved conclusively that the size of the steam drums
in the cross drum design has no appreciable effect in determining the
amount of liberating surface, and that sufficient liberating surface is
provided in the circulating tubes alone. If further proof of the ability
of this design of boiler to deliver dry steam is required, such proof is
perhaps best seen in the continued use of the Babcock & Wilcox marine
boiler, in which the cross drum is used exclusively, and with which
rates of evaporation are obtained far in excess of those secured in
ordinary practice.
Quick Steaming--The advantages of water-tube boilers as a class over
fire-tube boilers in ability to raise steam quickly have been indicated.
Due to the constant and thorough circulation resulting from the
sectional nature of the Babcock & Wilcox boiler, steam may be raised
more rapidly than in practically any other water-tube design.
In starting up a cold Babcock & Wilcox boiler with either coal or oil
fuel, where a proper furnace arrangement is supplied, steam may be
raised to a pressure of 200 pounds in less than half an hour. With a
Babcock & Wilcox boiler in a test where forced draft was available,
steam was raised from an initial temperature of the boiler and its
contained water of 72 degrees to a pressure of 200 pounds, in 12½
minutes after lighting the fire. The boiler also responds quickly in
starting from banked fires, especially where forced draft is available.
In Babcock & Wilcox boilers the water is divided into many small streams
which circulate without undue frictional resistance in thin envelopes
passing through the hottest part of the furnace, the steam being carried
rapidly to the disengaging surface. There is no part of the boiler
exposed to the heat of the fire that is not in contact with water
internally, and as a result there is no danger of overheating on
starting up quickly nor can leaks occur from unequal expansion such as
might be the case where an attempt is made to raise steam rapidly in
boilers using water leg construction.
Storage Capacity for Steam and Water--Where sufficient steam and water
capacity are not provided in a boiler, its action will be irregular, the
steam pressure varying over wide limits and the water level being
subject to frequent and rapid fluctuation.
Owing to the small relative weight of steam, water capacity is of
greater importance in this respect than steam space. With a gauge
pressure of 180 pounds per square inch, 8 cubic feet of steam, which is
equivalent to one-half cubic foot of water space, are required to supply
one boiler horse power for one minute and if no heat be supplied to the
boiler during such an interval, the pressure will drop to 150 pounds per
square inch. The volume of steam space, therefore, may be over rated,
but if this be too small, the steam passing off will carry water with it
in the form of spray. Too great a water space results in slow steaming
and waste of fuel in starting up; while too much steam space adds to the
radiating surface and increases the losses from that cause.
That the steam and water space of the Babcock & Wilcox boiler are the
result of numerous experiments has previously been pointed out.
Accessibility--Cleaning. That water-tube boilers are more accessible as
a class than are fire-tube boilers has been indicated. All water-tube
boilers, however, are not equally accessible. In certain designs, due to
the arrangement of baffling used it is practically impossible to remove
all deposits of soot and dirt. Frequently, in order to cheapen the
product, sufficient cleaning and access doors are not supplied as part
of the boiler equipment. The tendency of soot to collect on the crown
sheets of certain vertical water-tube boilers has been noted. Such
deposits are difficult to remove and if corrosion goes on beneath such a
covering the sheet may crack and an explosion result.
[Illustration: Rear View--Longitudinal Drum Vertical Header Boiler,
Showing Access Doors to Rear Headers]
It is almost impossible to thoroughly clean water legs internally, and
in such places also is there a tendency to unsuspected corrosion under
deposits that cannot be removed.
In Babcock & Wilcox boilers every portion of the interior of the heating
surfaces can be reached and kept clean, while any soot deposited on the
exterior surfaces can be blown off while the boiler is under pressure.
Inspection--The accessibility which makes possible the thorough cleaning
of all portions of the Babcock & Wilcox boiler also provides a means for
a thorough inspection.
Drums are accessible for internal inspection by the removal of the
manhole plates. Front headers may be inspected through large doors
furnished for the purpose. Rear headers in the inclined header designs
may be inspected from the chamber formed by such headers and the rear
wall of the boiler. In the vertical header designs rear tube doors are
furnished, as has been stated. In certain designs of water-tube boilers
in order to assure accessibility for inspection of the rear ends of the
tubes, the rear portion of the boiler is exposed to the atmosphere with
resulting excessive radiation losses. In other designs the means of
access to the rear ends of the tubes are of a makeshift and
unworkmanlike character.
By the removal of handhole plates, all tubes in a Babcock & Wilcox
boiler may be inspected for their full length either for the presence of
scale or for suspected corrosion.
Repairs--In Babcock & Wilcox boilers the possession of great strength,
the elimination of stresses due to uneven temperatures and of the
resulting danger of leaks and corrosion, the protection of the drums
from the intense heat of the fire, and the decreased liability of the
scale forming matter to lodge on the hottest tube surfaces, all tend to
minimize the necessity for repairs. The tubes of the Babcock & Wilcox
boiler are practically the only part which may need renewal and these
only at infrequent intervals When necessary, such renewals may be made
cheaply and quickly. A small stock of tubes, 4 inches in diameter, of
sufficient length for the boiler used, is all that need be carried to
make renewals.
Repairs in water-leg boilers are difficult at best and frequently
unsatisfactory when completed. When staybolt replacements are necessary,
in order to get at the inner sheet of the water leg, several tubes must
in some cases be cut out. Not infrequently a replacement of an entire
water leg is necessary and this is difficult and requires a lengthy
shutdown. With the Babcock & Wilcox boiler, on the other hand, even if
it is necessary to replace a section, this may be done in a few hours
after the boiler is cool.
In the case of certain staybolt failures the working pressure of a
repaired boiler utilizing such construction will frequently be lowered
by the insurance companies when the boiler is again placed in service.
The sectional nature of the Babcock & Wilcox boiler enables it to
maintain its original working pressure over long periods of time, almost
regardless of the nature of any repair that may be required.
[Illustration: 1456 Horse-power Installation of Babcock & Wilcox Boilers
at the Raritan Woolen Mills, Raritan, N. J. The First of These Boilers
were Installed in 1878 and 1881 and are still Operated at 80 Pounds
Pressure]
Durability--Babcock & Wilcox boilers are being operated in every-day
service with entirely satisfactory results and under the same steam
pressure as that for which they were originally sold that have been
operated from thirty to thirty-five years. It is interesting to note in
considering the life of a boiler that the length of life of a Babcock &
Wilcox boiler must be taken as the criterion of what length of life is
possible. This is due to the fact that there are Babcock & Wilcox
boilers in operation to-day that have been in service from a time that
antedates by a considerable margin that at which the manufacturer of any
other water-tube boiler now on the market was started.
Probably the very best evidence of the value of the Babcock & Wilcox
boiler as a steam generator and of the reliability of the apparatus, is
seen in the sales of the company. Since the company was formed, there
have been sold throughout the world over 9,900,000 horse power.
A feature that cannot be overlooked in the consideration of the
advantages of the Babcock & Wilcox boiler is the fact that as a part of
the organization back of the boiler, there is a body of engineers of
recognized ability, ready at all times to assist its customers in every
possible way.
[Illustration: 2400 Horse-power Installation of Babcock & Wilcox Boilers
in the Union Station Power House of the Pennsylvania Railroad Co.,
Pittsburgh, Pa. This Company has a Total of 28,500 Horse Power of
Babcock & Wilcox Boilers Installed]