Part 3

The Mechanical Valve-Gear

Fig. 17 shows plainly the operation of the mechanical valve-gear. The valves are located in the steam chests, which are bolted to the top of the casing directly over the first sets of expansion nozzles. The chests, two in number, are on opposite sides of the machine. The valve-stems extend upward through ordinary stuffing-boxes, and are attached to the notched cross-heads by means of a threaded end which is prevented from screwing in or out by a compression nut on the lower end of the cross-head. Each cross-head is actuated by a pair of reciprocating pawls, or dogs (shown more plainly in the enlarged view, Fig. 18), one of which opens the valve and the other closes it. The several pairs of pawls are hung on a common shaft which receives a rocking motion from a crank driven from a worm and worm-wheel by the turbine shaft. The cross-heads have notches milled in the side in which the pawls engage to open or close the valve, this engagement being determined by what are called shield-plates, A (Fig. 18), which are controlled by the governor. These plates are set, one a little ahead of the other, to obtain successive opening or closing of the valves. When more steam is required the shield plate allows the proper pawl to fall into its notch in the cross-head and lift the valve from its seat. If less steam is wanted the shield-plate rises and allows the lower pawl to close the valve on the down stroke.

The valves, as can easily be seen, are very simple affairs, the steam pressure in the steam chest holding the valve either open or shut until it is moved by the pawl on the rock-shaft. The amount of travel on the rock-shaft is fixed by the design, but the proportionate travel above and below the horizontal is controlled by the length of the connecting-rods from the crank to the rock-shaft. There are besides the mechanical valve-gear the electric and hydraulic, but these will be left for a future article.

The Governor

The speed of the machine is controlled by the automatic opening and closing of the admission valves under the control of a governor (Fig. 19), of the spring-weighted type attached directly to the top end of the turbine shaft. The action of the governor depends on the balance of force exerted by the spring, and the centrifugal effort of the rectangular-shaped weights at the lower end; the moving weights acting through the knife-edge suspension tend to pull down the lever against the resistance of the heavy helical spring. The governor is provided with an auxiliary spring on the outside of the governor dome for varying the speed while synchronizing. The tension of the auxiliary spring is regulated by a small motor wired to the switchboard. This spring should be used only to correct slight changes in speed. Any marked change should be corrected by the use of the large hexagonal nut in the upper plate of the governor frame. This nut is screwed down to increase the speed, and upward to decrease it.

The Stage Valves

Fig. 20 represents one of the several designs of stage valve, sometimes called the overload valve, the office of which is to prevent too high pressure in the first stage in case of a sudden overload, and to transfer a part of the steam to a special set of expanding nozzles over the second-stage wheel. This valve is balanced by a spring of adjustable tension, and is, or can be, set to open and close within a very small predetermined range of first-stage pressure. The valve is intended to open and close instantly, and to supply or cut off steam from the second stage, without affecting the speed regulation or economy of operation. If any leaking occurs past the valve it is taken care of by a drip-pipe to the third stage.

The steam which passes through the automatic stage valves and is admitted to the extra set of nozzles above the second-stage wheel acts upon this wheel just the same as the steam which passes through the regular second-stage nozzles; i.e., all the steam which goes through the machine tends to hasten its speed, or, more accurately, does work and maintains the speed of the machine.

II. SETTING THE VALVES OF THE CURTIS TURBINE

Contributed to Power by F. L. Johnson.

Under some conditions of service the stage valve in the Curtis turbine will not do what it is designed to do. It is usually attached to the machine in such manner that it will operate with, or a little behind, in the matter of time, the sixth valve. The machine is intended to carry full load with only the first bank of five valves in operation, with proper steam pressure and vacuum. If the steam pressure is under 150 pounds, or the vacuum is less than 28 inches, the sixth valve may operate at or near full load, and also open the stage valve and allow steam to pass to the second-stage nozzles at a much higher rate of speed than the steam which has already done some work in the first-stage wheel. The tendency is to accelerate unduly the speed of the machine. This is corrected by the governor, but the correction is usually carried too far and the machine slows down. With the stage valve in operation, at a critical point the regulation is uncertain and irregular, and its use has to be abandoned. The excess first-stage pressure will then be taken care of by the relief valve, which is an ordinary spring safety valve (not pop) which allows the steam to blow into the atmosphere.

The mechanical valve-gear does not often get out of order, but sometimes the unexpected happens. The shop man may not have properly set up the nuts on the valve-stems; or may have fitted the distance bushings between the shield plates too closely; the superheat of the steam may distort the steam chest slightly and produce friction that will interfere with the regulation. If any of the valve-stems should become loose in the cross-heads they may screw themselves either in or out. If screwed out too far, the valve-stem becomes too long and the pawl in descending will, after the valve is seated, continue downward until it has broken something. If screwed in, the cross-head will be too low for the upper pawl to engage and the valve will not be opened. This second condition is not dangerous, but should be corrected. The valve-stems should be made the right length, and all check-nuts set up firmly. If for any purpose it becomes necessary to "set the valves" on a 1500-kilowatt mechanical gear, the operator should proceed in the following manner.

Setting the Valves of a 1500-Kilowatt Curtis Turbine

We will consider what is known as the "mechanical" valve-gear, with two sets of valves, one set of five valves being located on each side of the machine.

In setting the valves we should first "throw out" all pawls to avoid breakage in case the rods are not already of proper length, holding the pawls out by slipping the ends of the pawl springs over the points of the pawls, as seen in Fig. 21. Then turn the machine slowly by hand until the pawls on one set of valves are at their highest point of travel, then with the valves wide open adjust the drive-rods, i.e., the rods extending from the crank to the rock-shaft, so that there is 1/32 of an inch clearance (shown dotted in Fig. 17, Chap. I) at the point of opening of the pawls when they are "in." (See Fig. 22.) Then set up the check-nuts on the drive-rod. Turn the machine slowly, until the pawls are at their lowest point of travel. Then, with the valves closed, adjust each valve-stem to give 1/32 of an inch clearance at the point of closing of the pawls when they are "in," securely locking the check-nut as each valve is set. Repeat this operation on the other side of the machine and we are ready to adjust the governor-rods. (Valves cannot be set on both sides of the machine at the same time, as the pawls will not be in the same relative position, due to the angularity of the drive-rods.)

Next, with the turbine running, and the synchronizing spring in mid-position, adjust the governor-rods so that the turbine will run at the normal speed of 900 revolutions per minute when working on the fifth valve, and carrying full load. The governor-rods for the other side of the turbine (controlling valves Nos. 6 to 10) should be so adjusted that the speed change between the fifth and sixth valves will not be more than three or four revolutions per minute.

The valves of these turbines are all set during the shop test and the rods trammed with an 8-inch tram. Governors are adjusted for a speed range of 2 per cent. between no load and full load (1500 kilowatt), or 4 per cent. between the mean speeds of the first and tenth valves (no load to full overload capacity).

The rods which connect the governor with the valve-gear have ordinary brass ends or heads and are adjusted by right-and-left threads and secured by lock-nuts. They are free fits on the pins which pass through the heads, and no friction is likely to occur which will interfere with the regulation, but too close work on the shield-plate bushings, or a slight warping of the steam chest, will often produce friction which will seriously impair the regulation. If it is noticed that the shield-plate shaft has any tendency to oscillate in unison with the rock-shaft which carries the pawls, it is a sure indication that the shield-plates are not as free as they should be, and should be attended to. The governor-rod should be disconnected, the pawls thrown out and the pawl strings hooked over the ends.

The plates should then be rocked up and down by hand and the friction at different points noted. The horizontal rod at the back of the valve-gear may be loosened and the amount of end play of each individual shield-plate noticed and compared with the bushings on the horizontal rod at the back which binds the shield-plates together. If the plates separately are found to be perfectly free they may be each one pushed hard over to the right or left and wedged; then each bushing tried in the space between the tail-pieces of the plates. It will probably be found that the bushings are not of the right length, due to the alteration of the form of the steam chest by heat. It will generally be found also that the bushings are too short, and that the length can be corrected by very thin washers of sheet metal. It has been found in some instances that the thin bands coming with sectional pipe covering were of the right thickness.

After the length of the bushings is corrected the shield-plates may be assembled, made fast and tested by rocking them up and down, searching for signs of sticking. If none occurs, the work has been correctly done, and there will be no trouble from poor regulation due to friction of the shield-plates.

The Baffler

The water which goes to the step-bearing passes through a baffler, the latest type of which is shown by Fig. 23. It is a device for restricting the flow of water or oil to the step- and guide-bearing. The amount of water necessary to float the machine and lubricate the guide-bearing having been determined by calculation and experiment, the plug is set at that point which will give the desired flow. The plug is a square-threaded worm, the length of which and the distance which it enters the barrel of the baffler determining the amount of flow. The greater the number of turns which the water must pass through in the worm the less will flow against the step-pressure.

The engineers who have settled upon the flow and the pressure decided that a flow of from 4-1/2 to 5-1/2 gallons per minute and a step-pressure of from 425 to 450 pounds is correct. These factors are so dependent upon each other and upon the conditions of the step-bearing itself that they are sometimes difficult to realize in every-day work; nor is it necessary. If the machine turns freely with a lower pressure than that prescribed by the engineers, there is no reason for raising this pressure; and there is only one way of doing it without reducing the area of the step-bearing, and that is by obstructing the flow of water in the step-bearing itself.

A very common method used is that of grinding. The machine is run at about one-third speed and the step-water shut off for 15 or 20 seconds. This causes grooves and ridges on the faces of the step-bearing blocks, due to their grinding on each other, which obstruct the flow of water between the faces and thus raises the pressure. It seems a brutal way of getting a scientific result, if the result desired can be called scientific. The grooving and cutting of the step-blocks will not do any harm, and in fact they will aid in keeping the revolving parts of the machine turning about its mechanical center.

The operating engineer will be very slow to see the utility of the baffler, and when he learns, as he will sometime, that the turbine will operate equally well with a plug out as with it in the baffler, he will be inclined to remove the baffler. It is true that with one machine operating on its own pump it is possible to run without the baffler, and it is also possible that in some particular case two machines having identical step-bearing pressures might be so operated. The baffler, however, serves a very important function, as described more fully as follows: It tends to steady the flow from the pump, to maintain a constant oil film as the pressure varies with the load, and when several machines are operating on the same step-bearing system it is the only means which fixes the flow to the different machines and prevents one machine from robbing the others. Therefore, even if an engineer felt inclined to remove the baffler he would be most liable to regret taking such a step.

If the water supply should fail from any cause and the step-bearing blocks rub together, no great amount of damage will result. The machine will stop if operated long under these conditions, for if steam pressure is maintained the machine will continue in operation until the buckets come into contact, and if the step-blocks are not welded together the machine may be started as soon as the water is obtained. If vibration occurs it will probably be due to the rough treatment of the step-blocks, and may be cured by homeopathic repeat-doses of grinding, say about 15 seconds each. If the step-blocks are welded a new pair should be substituted and the damaged ones refaced.

Some few experimental steps of spherical form, called "saucer" steps, have been installed with success (see Fig. 24). They seem to aid the lower guide-bearing in keeping the machine rotating about the mechanical center and reduce the wear on the guide-bearing. In some instances, too, cast-iron bushings have been substituted for bronze, with marked success. There seems to be much less wear between cast-iron and babbitt metal than between bronze and babbitt metal. The matter is really worth a thorough investigation.

III. ALLIS-CHALMERS COMPANY STEAM TURBINE