possible, to keep the water from backing up from the condenser into the
turbine. If the condenser must be located above the turbine, then the
pipe should be carried first downward and then upward in the U form, in
the manner of the familiar "entrainer," which will be found effectively
to prevent water getting back when the turbine is operating.
Condensers
As has been previously pointed out, the successful and satisfactory
operation of the turbine depends very largely on the condenser. With the
reciprocating engine, if the condenser will give 25 inches vacuum, it is
considered fairly good, and it is allowed to run along by itself until
the vacuum drops to somewhere below 20 inches, when it is completely
gone over, and in many cases practically rebuilt and the vacuum brought
back to the original 25 inches. It has been seen that this sort of
practice will never do in the case of the turbine condenser and, unless
the vacuum can be regularly maintained at 27 or 28 inches, the condenser
is not doing as well as it ought to do, or it is not of the proper type,
unless perhaps the temperature and the quantity of cooling water
available render a higher vacuum unattainable.
On account of the great purity of the condensed steam from the turbine
and its peculiar availability for boiler feed (there being no oil of any
kind mixed with it to injure the boilers), the surface condenser is very
desirable in connection with the turbine. It further recommends itself
by reason of the high vacuum obtainable.
Where a condenser system capable of the highest vacuum is installed,
the need of utilizing it to its utmost capacity can hardly be emphasized
too strongly. A high vacuum will, of course, mean special care and
attention, and continual vigilance for air leaks in the exhaust piping,
which will, however, be fully paid for by the great increase in economy.
It must not be inferred that a high vacuum is essential to successful
operation of this type of turbine, for excellent performance both in the
matter of steam consumption and operation is obtained with inferior
vacuum. The choice of a condenser, however, is a matter of special
engineering, and is hardly within the province of this article.
Oils
There are several oils on the market that are suitable for the purpose
of the turbine oiling system, but great care must be exercised in their
selection. In the first place, the oil must be pure mineral,
unadulterated with either animal or vegetable oils, and must have been
washed free from acid. Certain brands of oil require the use of
sulphuric acid in their manufacture and are very apt to contain varying
degrees of free acid in the finished product. A sample from one lot may
have almost no acid, while that from another lot may contain a dangerous
amount.
Mineral oils that have been adulterated, when heated up, will partially
decompose, forming acid. These oils may be very good lubricants when
first put into use, but after awhile they lose all their good qualities
and become very harmful to the machine by eating the journals in which
they are used. These oils must be very carefully avoided in the turbine,
as the cheapness of their first cost will in no way pay for the damage
they may do. A very good and simple way to test for such adulterations
is to take up a quantity of the oil in a test tube with a solution of
borax and water. If there is any animal or vegetable adulterant present
it will appear as a white milk-like emulsion which will separate out
when allowed to stand. The pure mineral oil will appear at the top as a
clear liquid and the excess of the borax solution at the bottom, the
emulsion being in between. A number of oils also contains a considerable
amount of paraffin which is deposited in the oil-cooling coil,
preventing the oil from being cooled properly, and in the pipes and
bearings, choking the oil passages and preventing the proper circulation
of the oil and cushioning effect in the bearing tubes. This is not
entirely a prohibitive drawback, the chief objection being that it
necessitates quite frequently cleaning the cooling coil, and the oil
piping and bearings.
Some high-class mineral oils of high viscosity are inclined to emulsify
with water, which emulsion appears as a jelly-like substance. It might
be added that high-grade oils having a high viscosity might not be the
most suitable for turbine use.
Since the consumption of oil in a turbine is so very small, being
practically due only to leakage or spilling, the price paid for it
should therefore be of secondary importance, the prime consideration
being its suitability for the purpose.
In some cases a central gravity system will be employed, instead of the
oil system furnished with the turbine, which, of course, will be a
special consideration.
For large installations a central gravity oiling system has much to
recommend it, but as it performs such an important function in the power
plant, and its failure would be the cause of so much damage, every
detail in connection with it should be most carefully thought out, and
designed with a view that under no combination of circumstances would it
be possible for the system to become inoperative. One of the great
advantages of such a system is that it can be designed to contain very
large quantities of oil in the settling tanks; thus the oil will have
quite a long rest between the times of its being used in the turbine,
which seems to be very helpful in extending the life of the oil. Where
the oil can have a long rest for settling, an inferior grade of oil may
be used, providing, however, that it is absolutely free of acid.
V. PROPER METHOD OF TESTING A STEAM TURBINE[3]
[3] Contributed to _Power_ by Thomas Franklin.
The condensing arrangements of a turbine are perhaps mainly instrumental
in determining the method of test. The condensed steam alone, issuing
from a turbine having, for example, a barometric or jet condenser,
cannot be directly measured or weighed, unless by meter, and these at
present are not sufficiently accurate to warrant their use for test
purposes, if anything more than approximate results are desired. The
steam consumed can, in such a case, only be arrived at by measuring the
amount of condensing water (which ultimately mingles with the condensed
steam), and subtracting this quantity from the condenser's total
outflow. Consequently, in the case of turbines equipped with barometric
or jet condensers, it is often thought sufficient to rely upon the
measurement taken of the boiler feed, and the boiler's initial and final
contents. Turbines equipped with surface-condensing plants offer better
facilities for accurate steam-consumption calculations than those plants
in which the condensed exhaust steam and the circulating water come into
actual contact, it being necessary with this type simply to pump the
condensed steam into a weighing or measuring tank.
In the case of a single-flow turbine of the Parsons type, the covers
should be taken off and every row of blades carefully examined for
deposits, mechanical irregularities, deflection from the true radial and
vertical positions, etc. The blade clearances also should be gaged all
around the circumference, to insure this clearance being an average
working minimum. On no account should a test be proceeded with when any
doubt exists as to the clearance dimensions.
[Illustration: FIG. 60]
The dummy rings of a turbine, namely, those rings which prevent
excessive leakage past the balancing pistons at the high-pressure end,
should have especial attention before a test. A diagrammatic sketch of a
turbine cylinder and spindle is shown in Fig. 60, for the benefit of
those unfamiliar with the subject. In this A is the cylinder or casing,
B the spindle or rotor, and C the blades. The balancing pistons, D, E,
and F, the pressure upon which counterbalances the axial thrust upon the
three-bladed stages, are grooved, the brass dummy rings G G in the
cylinder being alined within a few thousandths of an inch of the grooved
walls, as indicated. After these rings have been turned (the turning
being done after the rings have been calked in the cylinder), it is
necessary to insure that each ring is perfectly bedded to its respective
grooved wall so that when running the several small clearances between
the groove walls and rings are equal. A capital method of thus bedding
the dummy rings is to grind them down with a flour of emery or
carborundum, while the turbine spindle is slowly revolving under steam.
Under these conditions the operation is performed under a high
temperature, and any slight permanent warp the rings may take is thus
accounted for. The turbine thrust-block, which maintains the spindle in
correct position relatively to the spindle, may also be ground with
advantage in a similar manner.
The dummy rings are shown on a large scale in Fig. 61, and their
preliminary inspection may be made in the following manner:
The spindle has been set and the dummy rings C are consequently within a
few thousandths of an inch of the walls _d_ of the spindle dummy grooves
D. The clearances allowed can be gaged by a feeler placed between a ring
and the groove wall. Before a test the spindle should be turned slowly
around, the feelers being kept in position. By this means any mechanical
flaws or irregularities in the groove walls may be detected.
[Illustration: FIG. 61]
It has sometimes been found that the groove walls, under the combined
action of superheated steam and friction, in cases where actual running
contact has occurred, have worn very considerably, the wear taking the
form of a rapid crumbling away. It is possible, however, that such
deterioration may be due solely to the quality of the steel from which
the spindle is forged. Good low-percentage carbon-annealed steel ought
to withstand considerable friction; at all events the wear under any
conditions should be uniform. If the surfaces of both rings and grooves
be found in bad condition, they should be re-ground, if not sufficiently
worn to warrant skimming up with a tool.
As the question of dummy leakage is of very considerable importance
during a test, it may not be inadvisable to describe the manner of
setting the spindle and cylinder relatively to one another to insure
minimum leakage, and the methods of noting their conduct during a
prolonged run. In Fig. 62, showing the spindle, B is the thrust (made in
halves), the rings O of which fit into the grooved thrust-rings C in the
spindle. Two lugs D are cast on each half of the thrust-block. The
inside faces of these lugs are machined, and in them fit the ball ends
of the levers E, the latter being fulcrumed at F in the thrust-bearing
cover. The screws G, working in bushes, also fit into the thrust-bearing
cover, and are capable of pushing against the ends of the levers E and
thus adjusting the separate halves of the block in opposite directions.
[Illustration: FIG. 62]
The top half of the turbine cylinder having been lifted off, the spindle
is set relatively to the bottom half by means of the lower thrust-block
screw G. This screw is then locked in position and the top half of the
cover then lowered into place. With this method great care must
necessarily be exercised when lowering the top cover; otherwise the
brass dummy rings may be damaged.
A safer method is to set the dummy rings in the center of the grooves of
the spindle, and then to lower the cover, with less possibility of
contact. There being usually plenty of side clearance between the blades
of a turbine, it may be deemed quite safe to lock the thrust-block in
its position, by screwing the screws G up lightly, and then to turn on
steam and begin running slowly.
Next, the spindle may be very carefully and gradually worked in the
required direction, namely, in that direction which will tend to bring
the dummy rings and groove walls into contact, until actual but very
light contact takes place. The slightest noise made by the rubbing parts
inside the turbine can be detected by placing one end of a metal rod
onto the casing in vicinity of the dummy pistons, and letting the other
end press hard against the ear. Contact between the dummy rings and
spindle being thus demonstrated, the spindle must be moved back by the
screws, but only by the slightest amount possible. The merest fraction
of a turn is enough to break the contact, which is all that is required.
In performing this operation it is important, during the axial movements
of the spindle, to adjust the halves of the thrust-block so that there
can exist no possible play which would leave the spindle free to move
axially and probably vibrate badly.
After ascertaining the condition of the dummy rings, attention might
next be turned to the thrust-block, which must not on any account be
tightened up too much. It is sufficient to say that the actual
requirements are such as will enable a very thin film of oil to
circulate between each wall of the spindle thrust-grooves and the brass
thrust-blocks ring. In other words, there should be no actual pressure,
irrespective of that exerted by the spindle when running, upon the
thrust-block rings, due to the separate halves having been nipped too
tightly. The results upon a test of considerable friction between the
spindle and thrust-rings are obvious.
The considerations outlined regarding balancing pistons and dummy rings
can be dispensed with in connection with impulse turbines of the De
Laval and Rateau types, and also with double-flow turbines of a type
which does not possess any dummies. The same general considerations
respecting blade conditions and thrust-blocks are applicable, especially
to the latter type. With pure so-called impulse turbines, where the
blade clearances are comparatively large, the preliminary blade
inspection should be devoted to the mechanical condition of the blade
edges and passages. As the steam velocities of these types are usually
higher, the importance of minimizing the skin friction and eliminating
the possibility of eddies is great.
Although steam leakage through the valves of a turbine may not
materially affect its steam consumption, unless it be the leakage
through the overload valve during a run on normal full load, a thorough
examination of all valves is advocated for many reasons. In a turbine
the main steam-inlet valve is usually operated automatically from the
governor; and whether it be of the pulsating type, admitting the steam
in blasts, or of the non-pulsating throttling type, it is equally
essential to obtain the least possible friction between all moving and
stationary parts. Similar remarks apply to the main governor, and any
sensitive transmitting mechanism connecting it with any of the turbine
valves. If a safety or "runaway" governor is possessed by the machine to
be tested, this should invariably be tried under the requisite
conditions before proceeding farther. The object of this governor being
automatically to shut off all steam from the turbine, should the latter
through any cause rise above the normal speed, it is often set to
operate at about 12 to 15 per cent. above the normal. Thus, a turbine
revolving at about 3000 revolutions per minute would be closed down at,
say, 3500, which would be within the limit of "safe" speed.
Importance of Oiling System and Water Service
The oil question, being important, should be solved in the early stages
previously, if possible, to any official or unofficial consumption
tests. Whether the oil be supplied to the turbine bearings by a
self-contained system having the oil stored in the turbine bedplate or
by gravity from a separate oil source, does not affect the question in
its present aspect. The necessary points to investigate are four in
number, and may be headed as follows:
(a) Examination of pipes and partitions for oil leakage.
(b) Determination of volume of oil flowing through each bearing per unit
of time.
(c) Examination for signs of water in oil.
(d) Determination of temperature rise between inlet and outlet of oil
bearings.
The turbine supplied with oil by the gravity or any other separate
system holds an advantage over the ordinary self-contained machine,
inasmuch as the oil pipes conveying oil into and from the bearings can
be easily approached and, if necessary, repaired. On the other hand, the
machine possessing its own oil tank, cooling chamber and pump is
somewhat at a disadvantage in this respect, as a part of the system is
necessarily hidden from view, and, further, it is not easily accessible.
The leakage taking place in any system, if there be any, must, however,
be detected and stopped.
Fig. 63 is given to illustrate a danger peculiar to the self-contained
oil system, in which the oil and oil-cooling chambers are situated
adjacently in the turbine bedplate. One end of the bedplate only is
shown; B is a cast-iron partition dividing the oil chamber C from the
oil-cooling chamber D. Castings of this kind have sometimes a tendency
to sponginess and the trouble consequent upon this weakness would take
the form of leakage between the two chambers. Of course this is only a
special case, and the conditions named are hardly likely to exist in
every similarly designed plant. The capacity of oil, and especially of
hot oil, to percolate through the most minute pores is well known.
Consequently, in advocating extreme caution when dealing with oil
leakage, no apology is needed.
[Illustration: FIG. 63]
It may be stated without fear of contradiction that the oil in a
self-contained system, namely, a system in which the oil, stored in a
reservoir near or underneath the turbine, passes only through that one
turbine's bearings, and immediately back to the storage compartment,
deteriorates more rapidly than when circulating around an "entire"
system, such as the gravity or other analogous system. In the latter,
the oil tanks are usually placed a considerable distance from the
turbine or turbines, with the oil-cooling arrangements in fairly close
proximity. The total length of the oil circuit is thus considerably
increased, incidentally increasing the relative cooling capacity of the
whole plant, and thereby reducing the loss of oil by vaporization.
The amount of oil passing through the bearings can be ascertained
accurately by measurement. With a system such as the gravity it is only
necessary to run the turbine up to speed, turn on the oil, and then,
over a period, calculate the volume of oil used by measuring the fall of
level in the storage tank and multiplying by its known cross-sectional
area. In those cases where the return oil, after passing through the
bearings, is delivered back into the same tank from which it is
extracted, it is of course necessary, during the period of test, to
divert this return into a separate temporary receptacle. Where the
system possesses two tanks, one delivery and one return (a superior
arrangement), this additional work is unnecessary. The same method can
be applied to individual turbines pumping their own oil from a tank in
the bedplate; the return oil, as previously described, being temporarily
prevented from running back to the supply.
The causes of excessive oil consumption by bearings are many. There is
an economical mean velocity at which the oil must flow along the
revolving spindle; also an economical mean pressure, the latter
diminishing from the center of the bearing toward the ends. The aim of
the economist must therefore be in the direction of adjusting these
quantities correctly in relation to a minimum supply of oil per bearing;
and the principal factors capable of variation to attain certain
requirements are the several bearing clearances measured as annular
orifices, and the bearing diameters.
It is not always an easy matter to detect the presence of water in an
oil system, and this difficulty is increased in large circuits, as the
water, when the oil is not flowing, generally filters to the lowest
members and pipes of the system, where it cannot usually be seen. A
considerable quantity of water in any system, however, indicates its
presence by small globular deposits on bearings and spindles, and in the
worst cases the water can clearly be seen in a small sample tapped from
the oil mains. There is only one effective method of ridding the oil of
this water, and this is by allowing the whole mass of oil in the system
to remain quiescent for a few days, after which the water, which falls
to the lowest parts, can be drained off. A simple method of clearing out
the system is to pump all the oil the whole circuit contains through the
filters, and thence to a tank from which all water can be taken off. One
of the ordinary supply tanks used in the gravity system will serve this
purpose, should a temporary tank not be at hand. If necessary, the
headers and auxiliary pipes of the system can be cleaned out before
circulating the oil again, but as this is rather a large undertaking, it
need only be resorted to in serious cases.
[Illustration: FIG. 64]
It is seldom possible to discover the correct and permanent temperature
rise of the circulating oil in a turbine within the limited time usually
alloted for a test. After a continuous run of one hundred hours it is
possible that the temperature at the bearing outlets may be lower than
it was after the machine had run for, say, only twenty hours. As a
matter of fact an oil-temperature curve plotted from periodical readings
taken over a continuous run of considerable length usually reaches a
maximum early, afterward falling to a temperature about which the
fluctuations are only slight during the remainder of the run. Fig. 64
illustrates an oil-temperature curve plotted from readings taken over a
period of twenty-four hours. In this case the oil system was of the
gravity description, the capacity of the turbine being about 6000
kilowatts. The bearings were of the ordinary white-metal spherical type.
Over extended runs of hundreds and even thousands of hours, the above
deductions may be scarcely applicable. Running without break for so
long, a small turbine circulating its own lubricant would possibly
require a renewal of the oil before the run was completed, in the main
owing to excessive temperature rise and consequent deterioration of the
quality of the oil. Under these conditions the probabilities are that
several temperature fluctuations might occur before the final maximum,
and more or less constant, temperature was reached. In this connection,
however, the results obtained are to a very large extent determined by
the general mechanical design and construction of the oiling system and
turbine. A reference to Fig. 63 again reveals at once a weakness in that
design, namely, the unnecessarily close proximity in which the oil and
water tanks are placed.
[Illustration: FIG. 65]
A design of thermometer cup suitable for oil thermometers is given in
Fig. 65 in which A is an end view of the turbine bedplate, B is a
turbine bearing and C and D are the inlet and outlet pipes,
respectively. The thermometer fittings, which are placed as near the
bearing as is practicable, are made in the form of an angular tee
fitting, the oil pipes being screwed into its ends. The construction of
the oil cup and tee piece is shown in the detail at the left where A is
the steel tee piece, into which is screwed the brass thermometer cup B.
The hollow bottom portion of this cup is less than 1/16 of an inch in
thickness. The top portion of the bored hole is enlarged as shown, and
into this, around the thermometer, is placed a non-conducting material.
The cup itself is generally filled with a thin oil of good conductance.