1.5.2 Governor valve relay and control unit Type 2
Type 2 control units are fitted to similar machines to Type 1, but in this case run-up is carried out on the HP stop valves, with the governor valves partially open. The control units for HP stop and HP and IP governor valves are therefore similar. The IP stop valves require simple open/closed control only.
Figure 2.24 shows the control arrangements for a governing valve. Again, the input stage, comprising a servo-valve and primary ram, is positioned by a servo-valve amplifier using the LVDT connected to the primary ram as feedback. In this case, however, the positioning of the main ram is accomplished by mechanical and hydraulic means without a further electronic feedback loop. The primary ram provides the input onto a feedback linkage, the other end being connected to measure main ram position. A pilot valve is connected to the linkage so that, with the spool aligned with its ports, the main ram will be in equilibrium with the primary ram. A dump valve is interposed in the outlet port of the pilot valve supplying the main ram. In the normal operating condition, if the primary ram lifts, it will lift the pilot valve, allowing fluid to flow at high pressure to open the main ram. This will move downwards to open until the main ram position corresponds to the primary ram input and the pilot valve is restored to the null condition.
When protection fluid is present, the dump valve is held down against its seat against the opening spring. If the protection fluid pressure is lost, the dump valve is unseated by its spring and releases the fluid supply to the main ram to drain, thereby closing the valve rapidly. The dump valve lower seat is also closed by the spring force, thus cutting off the high pressure supply from the accumulator adjacent to the valve relay. The dump valve may also be operated by an electrical signal to the solenoid valve in the protection fluid line, thereby providing a back-up means of tripping individual valves. Energising the solenoid valve maintains the tripping pressure to the remainder of the valves, but locally releases the high pressure fluid above the dump valve to drain, causing the valve to trip.