1.5.1 Governor valve relay and control unit Type 1
Having covered the requirements, the Type 1 system will be examined in detail. This is used on large reheat turbine-generators having four steam inlets to both HP and IP cylinders, each inlet being fitted with series-connected emergency stop and governor valves. Since all control for both the run-up and loading phases is carried out by the governor valves, the stop valves must be fully open whenever the turbine is needed for operation.
The simple form of open/closed control required for the stop valves will be described separately.
Figure 2.22 shows a diagram of the governor valve relay and control unit. The input stage, comprising the servo-valve and actuator piston controlled by the electronic system, is identical to Fig 2.18. The heart of the unit is the relay plunger, rigidly connected to the actuator piston and sliding in the relay plunger sleeve. In the normal position of the sleeve, as shown in the diagram, the relay plunger is moved upwards by the actuator piston when the steam valve is required to open. This admits power fluid above the relay power piston to move it downwards against the force of the valve spring (not shown), and hence open the valve until such time as the sweep-resolver position signal is in balance with the valve position demand. This returns the servo-amplifier output to zero with the actuator piston and its transducer restored to the null position. Closing is effected in a similar manner, with the relay plunger being moved downwards to allow fluid to be drained out of the upper power piston chamber.
Tripping is accomplished by the relay plunger sleeve which, in the running condition, is held downwards (in the position shown) against its internal spring by the presence of high pressure tripping fluid in chamber F. If a trip occurs, tripping fluid pressure is lost and the relay plunger sleeve moves upwards due to the spring force. For any position of the actuator piston, this will cause the power piston to be drained.
Figure 2.23 shows a diagram of the emergency stop valve relay unit. Examining the operation of the relay plunger only; in the tripped condition, this is held on its bottom stop so that power fluid entering chamber В through port A is prevented from gaining access to port F. When high pressure tripping fluid is applied through port С to chamber D, the relay plunger will lift, thereby allowing high pressure power fluid through port F to lift the relay power piston.
The dump piston acts as a parallel means of ensuring that the valve is closed if tripping pressure is lost. In the normal running condition it will be held against its top seat by high pressure tripping fluid in chamber E, thus sealing the power piston fluid from port H apart from a small warming flow.
The orifice arrangement permits warming flows to the lower end of the dump piston. In the event of a loss of tripping pressure, the dump piston will unseat and allow fluid to be drained rapidly from the power piston into chamber H.
The dump piston has clearances one order higher than the relay plunger and other servo components. The probability of its seizure in the event of widespread fluid contamination is therefore remote.
The orifices fitted to the systems of both the governor and stop valves ensure appropriate rates of valve travel in normal operation and prevent excessive rates of valve travel when on-load testing.
The arrangements of trip and test valves shown permit on-load testing of the valves in all their functioning modes.