id="pgepubid00014">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[2]

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