Steam Turbines / A Book of Instruction for the Adjustment and Operation of the Principal Types of this Class of Prime Movers

by Hubert E. Collins

The plate P rests upon ball bearings so that by simply bringing pressure to bear upon the hand-wheel, which is a part of the quill J, the spring and lock-nut may be held at rest and adjusted while the rest of the turbine remains unaffected. Another lever is mounted upon the yoke E on the pin shown at I, the other end of which is fastened to the piston of a dash-pot so as to dampen the governor against vibration. Under the yoke E will be noticed a small trigger M which is used to hold the governor in the full-load position when the turbine is at rest.

The throwing out of the weights elevates the sleeve E, carrying with it the collar C, which is spanned by the lever F upon the shaft H. The later turbines are provided with an improved form of governor operating on the same principle, but embodying several important features. First, the spindle sleeve is integral with the governor yoke, and the whole rotates about a vertical stationary spindle, so that two motions are encountered—a rotary motion and an up and down motion, according to the position taken by the governor. This spiral motion almost entirely eliminates the effect of friction of rest, and thereby enhances the sensitiveness of the governor. Second, the governor weights move outward on a parallel motion opposed directly by spring thrust, thus relieving the fulcrum entirely of spring thrust. Third, the lay shaft driving the governor oil pump and reciprocator is located underneath the main turbine shaft, so that the rotor may be readily removed without in the least disturbing the governor adjustment.

The Valve-Gear

The valve-gear is shown in section in Fig. [51], the main admission being shown at V1 at the right, and the secondary V2 at the left of the steam inlet. The pilot valve F receives a constant reciprocating motion from the eccentric upon the layshaft of the turbine through the lever F (Fig. [50]). These reciprocations run from 150 to 180 per minute. The space beneath the piston C is in communication with the large steam chest, where exists the initial pressure through the port A; the admission of steam to the piston C being controlled by a needle valve B. The pilot valve connects the port E, leading from the space beneath the piston to an exhaust port I.

When the pilot valve is closed, the pressures can accumulate beneath the piston C and raise the main admission valve from its seat. When the pilot valve opens, the pressure beneath the piston is relieved and it is seated by the helical spring above. If the fulcrum E (Fig. [50]) of the lever F were fixed the admission would be of an equal and fixed duration. But if the governor raises the fulcrum E, the pilot valve F (Fig. [51]) will be lowered, changing the relations of the openings with the working edges of the ports.

The seating of the main admission valve is cushioned by the dashpot, the piston of which is shown in section at G (Fig. [51]). The valve may be opened by hand by means of the lever K, to see if it is perfectly free.

The secondary valve is somewhat different in its action. Steam is admitted to both sides of its actuating piston through the needle valves M M, and the chamber from which this steam is taken is connected with the under side of the main admission valve, so that no steam can reach the actuating piston of the secondary valve until it has passed through the primary valve. When the pilot valve is closed, the pressures equalize above and below the piston N and the valve remains upon its seat. When the load upon the turbine exceeds its rated capacity, the pilot valve moves upward so as to connect the space above the piston with the exhaust L, relieving the pressure upon the upper side and allowing the greater pressure below to force the valve open, which admits steam to the secondary stage of the turbine.

It would do no good to admit more steam to the first stage, for at the rated capacity that stage is taking all the steam for which the blade area will afford a passage. The port connecting the upper side of the piston N with the exhaust may be permanently closed by means of the hand valve Q, to be found on the side of the secondary pilot valve chest, thus cutting the secondary valve entirely out of action. No dashpot is necessary on this valve, the compression of the steam in the chamber W by the fall of the piston being sufficient to avoid shock.