Part 8
The absolute freedom of all moving parts of the spraying mechanism should be one of the tester's first assurances. To facilitate this, it is customary to construct the parts, with the exception of the springs, of brass or some other non-corrosive metal. The spraying cone must be thoroughly clean in every channel, to insure a well-distributed stream of water. Nor is it less important that careful attention be given to the setting and operation of the relief valve, as will be seen later. The obvious object of such a valve is to prevent the internal condenser pressure ever being maintained much higher than the atmospheric pressure. A number of carefully designed rubber flap valves, or one large one, have been found to act successfully for this purpose, although a balanced valve of more substantial construction would appear to be more desirable.
Importance of Relief Valves
The question of relief valves in turbine installations is an important one, and it seems desirable at this point to draw attention to another necessary relief valve and its function, namely the turbine atmospheric valve. As generally understood, this is placed between the turbine and condenser, and, should the pressure in the latter, owing to any cause, rise above that of the atmosphere, it opens automatically and allows the exhaust steam to flow through it into the atmosphere, or into another condenser.
A general diagrammatic arrangement of a steam turbine, condenser, and exhaust piping is shown in Fig. 73. Connected to the exhaust pipe B, near to the condenser, is the automatic atmospheric valve D, from which leads the exhaust piping E to the atmosphere. The turbine relief valve is shown at F, and the condenser relief valve at G. The main exhaust valve between turbine and condenser is seen at H. We have here three separate relief valves: one, F, to prevent excessive pressure in the turbine: the second, D, an atmospheric valve opening a path to the air, and, in addition to preventing excessive pressure accumulating, also helping to keep the temperature of the condenser body and tubes low; the third, the condenser relief valve G, which in itself ought to be capable of exhausting all steam from the turbine, should occasion demand it.
Assuming a plant of this description to be operating favorably, the conditions would of necessity be as follows: The valves F, D, and G, all closed; the valve H open. Suppose that, owing to sudden loss of circulating water, the vacuum fell to zero. The condenser would at once fill with steam, a slight pressure would be set up, and whichever of the three valves happened to be set to blow off at the lowest pressure would do so. Now it is desirable that the first valve to open under such circumstances should be the atmospheric valve D. This being so, the condenser would remain full of steam at atmospheric pressure until the attendant had had time to close the main hand-or motor-operated exhaust valve H, which he would naturally do before attempting to regain the circulation of the condensing water. Again, assume the installation to be running under the initial conditions, with the atmospheric valve D and all remaining valves except H closed.
Suppose the vacuum again fell to zero from a similar cause, and, further, suppose the atmospheric valve D failed to operate automatically. The only valves now capable of passing the exhaust steam are the turbine and condenser relief valves F and G. Inasmuch as the pressures at exhaust in the turbine proper, on varying load, vary over a considerably greater range than the small fairly constant absolute pressures inside the condenser, it is obviously necessary to allow for this factor in the respective setting of these two relief valves. In other words, the obvious deduction is to set the turbine relief valve to blow off at a higher pressure than the condenser relief valve, even when considering the question with respect to condensing conditions only. In this second hypothetical case, then, with a closed and disabled atmospheric valve, the exhaust must take place through the condenser, until the turbine can be shut down, or the circulating water regained without the former course being found necessary.
There is one other remote case which may be assumed, namely, the simultaneous refusal of both atmospheric and condenser relief valves to open, upon the vacuum inside the condenser being entirely lost. The exhaust would then be blown through the turbine relief valve F, until the plant could be closed down.
Although the conditions just cited are highly improbable in actual practice, it can at once be seen that to insure the safety of the condenser, absolutely, the turbine relief valve must be set to open at a comparatively low pressure, say 40 pounds by gage, or thereabouts. To set it much lower than this would create a possibility of its leaking when the turbine was making a non-condensing run, and when the pressure at the turbine exhaust end is often above that of the atmosphere. From every point of view, therefore, it is advisable to make a minute examination of all relief valves in a system, and before a test to insure that these valves are all set to open at their correct relative pressures.
It must be admitted that the practice of placing a large relief valve upon a condenser in addition to the atmospheric exhausting valve is by no means common. The latter valve, where surface condensing is adopted, is often thought sufficient, working in conjunction with a quickly operated main exhaust valve. Similarly, with a barometric condenser as that illustrated in Fig. 72, the atmospheric exhaust valve D (seen in Fig. 73) is sometimes dispensed with. This course is, however, objectionable, for upon a loss of vacuum in the turbine, all exhaust steam must pass through the condenser body, or the entire plant be closed down until the vacuum is regained. The simple construction of the barometric condenser, however, is in such an event much to its advantage, and the passage of the hot steam right through it is not likely to seriously warp or strain any of its parts, as might probably happen in the case of a surface condenser.
The question of the advisability of thus adding to a plant can only be fairly decided when all conditions, operating and otherwise, are fully known. For example, if we assume a large turbine to be operating on a greatly varying load, and exhausting into a condenser, as that in Fig. 72, and, further, having an adequate stand-by to back it up, one's obvious recommendation would be to equip the installation with both a condenser relief valve and an atmospheric valve, in addition, of course, to the main exhaust valve, which is always placed between the atmospheric valve and condenser. There are still other considerations, such as water supply, condition of circulating water, style of pump, etc., which must all necessarily have an obvious bearing upon the settlement of this question; so that generalization is somewhat out of place, the final design in all cases depending solely upon general principles and local conditions.
Other Necessary Features of a Test
In connection with the condenser, of any type, and its auxiliaries, there remain a few necessary examinations and operations to be conducted, if it is desired to obtain the very best results during the test. It will be sufficient to just outline them, the method of procedure being well known, and the requirement of any strict routine being unnecessary. These include:
(1) A thorough examination of the air-pump, and, if possible, an equally careful examination of diagrams taken from it when running on full load. Also careful examination of the piping, and of any other connections between the air pump and condenser, or other auxiliaries. It will be well in this examination to note the general "lay" of the air pipes, length, hight to which they rise above condenser and air pump, facilities for drainage, etc., as this information may prove valuable in determining the course necessary to rectify deficiencies which may later be found to exist.
(2) In a surface condenser, inspection of the pumps delivering condensed steam to the measuring tanks or hot-well; inspection of piping between the condenser and the pump, and also between the pump and measuring tanks. If these pumps are of the centrifugal type it is essential to insure, for the purposes of a steam-consumption test, as much regularity of delivery as possible.
(3) In the case of a consumption test upon a turbine exhausting into a barometric condenser, and where the steam consumed is being measured by the evaporation in the boiler over the test period, time must be devoted to the feed-pipes between the feed-water measuring meter or tank and the boilers. Under conditions similar to those operating in a plant such as that shown in Fig. 72, the necessary boiler feed might be drawn from the hot-well, the remainder of the hot-well contents probably being pumped through water coolers, or towers, for circulating through the condenser. With the very best system, it is possible for a slight quantity of oil to leak into the exhaust steam, and thence to the hot-well. In its passage, say along wooden conduits, to the measuring tank or meter, this water would probably pass through a number of filters. The efficiency of these must be thoroughly insured. It is unusual, in those cases where a simple turbine steam-consumption test is being carried out, and not an efficiency test of a complete plant, to pass the measured feed-water through economizers. Should the latter course, owing to special conditions, become necessary, a careful examination of all economizer pipes would be necessary.
(4) The very careful examination of all thermometer pockets, steam- and temperature-gage holes, etc., as to cleanliness, non-accumulation of scale, etc.
Special Auxiliaries Necessary
Having outlined the points of interest and importance in connection with the more permanent features of a plant, we arrive at the preparation and fitting of those special auxiliaries necessary to carry on the test.
It is customary, when carrying out a first test, upon both prime mover and auxiliaries, to place every important stage in the expansion in communication with a gage, so that the various pressures may be recorded and later compared with the figures of actual requirement. To do this, in the case of the turbine, it is necessary to bore holes in the cover leading to the various expansion chambers, and into each of these holes to screw a short length of steam pipe, having preferably a loop in its length, to the other end of which the gage is attached. Fig. 74 illustrates, diagrammatically, a complete turbine installation, and shows the various points along the course taken by the steam at which it is desirable to place pressure gages. The figure does not show the high-pressure steam pipe, nor any of the turbine valves. With regard to these, it will be desirable to place a steam gage in the pipe, immediately before the main stop-valve, and another immediately after it. Any fall of pressure between the two sides of the valve can thus be detected. To illustrate this clearly, Fig. 75 is given, showing the valves of a turbine, and the position of the gages connected to them. The two gages E and F on either side of the main stop-valve A are also shown. The steam after passing through the valve, which, in the case of small turbines, is hand-operated, goes in turn through the automatic stop-valve B, the function of which is to automatically shut steam off should the turbine attain a predetermined speed above the normal, the steam strainer C, and finally through the governing valve D into the turbine. As shown, gages G and H are also fitted on either side of the strainer, and these, in conjunction with gages E and F, will enable any fall in pressure between the first two valves and the governing valve to be found. Up to the governing-valve inlet no throttling of the steam ought to take place under normal conditions, i.e., with all valves open, and consequently any fall in pressure between the steam inlet and this point must be the result of internal wire-drawing. By placing the gages as shown, the extent to which this wire-drawing affects the pressures obtainable can be discovered.
On varying and even on normal and steady full load, the steam is more or less reduced in pressure after passing through the governing valve D; a gage I must consequently be placed between the valve, preferably on the valve itself, and the turbine. Returning to Fig. 74, the gages shown are A, B, C, D, and E, connected to the first, second, third, fourth, and fifth expansions; also F in the turbine and exhaust space, where there are no blades, G in the exhaust pipe immediately before the main exhaust valve E (see Fig. 73), and H connected to the condenser. On condensing full load it is probable that A, B, and C will all register pressures above the atmosphere, while gages D, E, F, and G will register pressures below the atmosphere, being for this purpose vacuum gages. On the other hand, with a varying load, and consequently varying initial pressures, one or two of the gages may register pressure at one moment and vacuum at another. It will therefore be necessary to place at these points compound gages capable of registering both pressure and vacuum. With the pressures in the various stages constantly varying, however, a gage is not by any means the most reliable instrument for recording such variations. The constant swinging of the finger not only renders accurate reading at any particular moment both difficult and, to an extent, unreliable, but, in addition, the accompanying sudden changes of condition, both of temperature and pressure, occurring inside the gage tube, in a comparatively short time permanently warp this part, and thus altogether destroy the accuracy of the gage. It is well known that even with the best steel-tube gages, registering comparatively steady pressures, this warping of the tube inevitably takes place. The quicker deterioration of such gage tubes, when the gage is registering quickly changing pressures, can therefore readily be conceived, and for this reason alone it is desirable to have all gages, whatever the conditions under which they work, carefully tested and adjusted at short intervals. If it is desired to obtain reliable registration of the several pressures in the different expansions of a turbine running on a varying load, it would therefore seem advisable to obtain these by some type of external spring gage (an ordinary indicator has been found to serve well for this purpose) which the sudden internal variations in pressure and temperature cannot deleteriously affect.
In view of the great importance he must attach to his gage readings, the tester would do well to test and calibrate and adjust where necessary all the gages he intends using during a test. This he can do with a standard gage-testing outfit. By this means only can he have full confidence in the accuracy of his results.
In like manner it is his duty personally to supervise the connecting and arrangement of the gages, and the preliminary testing for leakage which can be carried out simultaneously with the vacuum test made upon the turbine casing.
Where Thermometers are Required
Equally important with the foregoing is the necessity of calibrating and testing of all thermometers used during a test. Where possible it is advisable to place new thermometers which have been previously tested at all points of high temperature. Briefly running them over, the points at which it is necessary to place thermometers in the entire system of the steam and condensing plant are as follows:
(1) A thermometer in the steam pipe on the boiler, where the pipe leaves the superheater.
(2) In the steam pipe immediately in front of the main stop-valve, near point E in Fig. 75.
(3) In the main governing valve body (see I, Fig. 75) on the inlet side.
(4) In the main governing valve body on the turbine side, which will register temperatures of steam after it has passed through the valve.
(5) In the steam-turbine high-pressure chamber, giving the temperature of the steam before it has passed through any blades.
(6) In the exhaust chamber, giving the temperature of steam on leaving the last row of blades.
(7) In the exhaust pipe near the condenser.
(8) In the condenser body.
(9) In the circulating-water inlet pipe close to the condenser.
(10) In the circulating-water outlet pipe close to the condenser.
(11) In the air-pump suction pipe close to the condenser.
(12) In the air-pump suction pipe close to the air pump.
It is not advisable to place at those vital points, the readings at which directly or indirectly affect the consumption, two thermometers, say one ordinary chemical thermometer and one thermometer of the gage type, thus eliminating the possibility of any doubt which might exist were only one thermometer placed there.
There is no apparent reason why one should attempt to take a series of temperature readings during a consumption test on varying load. The temperatures registered under a steady load test can be obtained with great reliability, but on a varying load, with constantly changing temperatures at all points, this is impossible. This is, of course, owing to the natural sluggishness of the temperature-recording instruments, of whatever class they belong to, in responding to changes of condition. As a matter of fact, the possibility of obtaining correctly the entire conditions in a system running under greatly varying loads is very doubtful indeed, and consequently great reliance cannot be placed upon figures obtained under such conditions.
A few simple calculations will reveal to the tester his special requirements in the direction of measuring tanks, piping, etc., for his steam consumption test. Thus, assuming the turbine to be tested to be of 3000 kilowatt capacity normal load, with a guaranteed steam consumption of, say, 14.5 pounds per kilowatt-hour, he calculates the total water rate per hour, which in this case would be 43,500 pounds, and designs his weighing or measuring tanks to cope with that amount, allowing, of course, a marginal tank volume for overload requirements.
VIII. TROUBLES WITH STEAM TURBINE AUXILIARIES[7]
[7] Contributed to _Power_ by Walter B. Gump.
The case about to be described concerns a steam plant in which there were seven cross-compound condensing Corliss engines, and two Curtis steam turbines. The latter were each of 1500-kilowatt capacity, and were connected to surface condensers, dry-vacuum pumps, centrifugal, hot-well and circulating pumps, respectively. In the illustration (Fig. 76), the original lay-out of piping is shown in full lines. Being originally a reciprocating plant it was difficult to make the allotted space for the turbines suitable for their proper installation. The trouble which followed was a perfectly natural result of the failure to meet the requirements of a turbine plant, and the description herein given is but one example of a great many where the executive head of a concern insists upon controlling the situation without regard to engineering advice or common sense.
Circulating Pump Fails to Meet Guarantee
Observing the plan view, it will be seen that the condensers for both turbines receive their supply of cooling water from the same supply pipe; that is, the pipes, both suction and discharge, leading to No. 1 condenser are simply branches from No. 2, which was installed first without consideration for a second unit. When No. 1 was installed there was a row of columns from the basement floor to the main floor extending in a plane which came directly in front of the condenser. The column P shown in the plan was so located as to prevent a direct connection between the centrifugal circulating pump and the condenser inlet. The centrifugal pump was direct-connected to a vertical high-speed engine, and the coupling is shown at E in the elevation.
Every possible plan was contemplated to accommodate the engine and pump without removing any of the columns, and the arrangement shown was finally adopted, leaving the column P in its former place by employing an S-connection from the pump to the condenser. It should be stated that the pump was purchased under a guarantee to deliver 6000 gallons per minute under a head of 50 feet, with an impeller velocity of 285 revolutions per minute. The vertical engine to which the pump was connected proved to be utterly unfit for running at a speed beyond 225 to 230 revolutions per minute, and in addition the S-bend would obviously reduce the capacity, even at the proper speed of the impeller.
Besides these factors there was another feature even more serious. It was found that when No. 2 unit was operating No. 1 could not get as great a quantity of circulating water as when No. 2 was shut down. This was because No. 2 was drawing most of the water, and No. 1 received only that which No. 2 could not pull from the suction pipe A. This will be clear from the fact that the suction and discharge pipes for No. 1 were only 16 inches, while those of No. 2 were 20 inches and 16 inches, respectively. The condenser for No. 2 had 1000 square feet less cooling surface than No. 1, which had 6000 square feet and was supplied with cooling water by means of two centrifugal pumps of smaller capacity than for No. 1 and arranged in parallel. These were each driven by an electric motor, and were termed "The Siamese Twins," due to the way in which they were connected.
The load factor of the plant ranged from 0.22 to 0.30, the load being almost entirely lighting, so that for the winter season the load factor reached the latter figure. The day load was, therefore, light and not sufficient to give one turbine more than from one-fourth to one-third its rated capacity. Under these conditions No. 1 unit was able to operate much more satisfactorily than when fully loaded, because of the fact that the cooling water was more effective. This was, of course, all used by No. 1 unit when No. 2 was not operating. At best, however, it was found that the vacuum could not be made to exceed 24 inches, and during the peak, with the two turbines running, the vacuum would often drop to 12 inches. A vacuum of 16 inches or 18 inches on the peak was considered good.
An Investigation
Severe criticism "rained" heavily upon the engineer in charge, and complaints were made in reference to the high oil consumption. An investigation on the company's part followed, and the firm which furnished the centrifugal pump and engine was next in order to receive complaints. Repeated efforts were made to increase the speed of the vertical engine to 285 revolutions per minute, but such a speed proved detrimental to the engine, and a lower speed of about 225 revolutions per minute had to be adopted.
A thorough test on the pump to ascertain its delivery at various speeds was the next move, and a notched weir, such as is shown in the elevation, was employed. The test was made on No. 2 cooling tower, not shown in the sketch, and showed that barely 3000 gallons per minute were being delivered to the cooling tower. While the firm furnishing the pump was willing to concede that the pump might not be doing all it should, attention was called to the fact that there might be some other conditions in connection with the system which were responsible for the losses. Notable among these was the hydraulic friction, and when this feature of the case was presented, the company did not seem at all anxious to investigate the matter further; obviously on account of facing a possible necessity for new piping or other apparatus which might cost something.