1.5 Governor valve relays - part 4
These requirements are best met by a single-acting power piston opened by hydraulic fluid pressure and closed by the closing spring. Once the steam force and spring force are known, it is a straightforward design task to calculate the size of power piston required. Again, a large margin is provided to allow for friction and, where appropriate, the weight of the valve and linkage.
The valve closing rate of 200 ms for full stroke (typical superheat unit) is set by the need to control overspeed in the event of a load rejection. The capability of opening at the same rate is designed into the valve, as it is needed in some applications to improve the transient stability of power system networks. However, this facility is not provided on turbines supplied to the CEGB and the opening of the steam valve is rate-limited to give a time for full stroke of about 5 s. To achieve fast opening and closing, the servo stages which feed the power piston must be designed with the appropriate size of porting to provide the required flow into and out of the power piston. As well as the porting, there must be pipework of adequate size to supply the fluid flow from the high pressure source to the power piston to meet opening demands, and from the power piston to the drain line to meet closing demands. To assist in this, particularly where fast opening is required, hydraulic accumulators are often provided adjacent to each steam valve relay.
A secondary means of closing the valve relays of a main turbine unit from the unit protection system must also be provided. This will be described in detail in a later section, but from the valve relay point of view, each relay is supplied with power oil at high pressure for normal functions. It is also separately supplied with tripping fluid at high pressure, which is routed through the protection system. In the event of protection system operation, this supply is cut off to all valve relays and independently ensures their closure. Even so, the valve relays all employ sliding pistons of relatively small clearance, and a postulated failure mode exists in that it is remotely possible for a failure process in the hydraulic fluid supply unit to generate widespread fluid contamination, thus causing sticking of a number of the steam valve relay spools. To provide a back-up for this eventuality, emergency stop valve relays (and sometimes governor valve relays as well) are provided with a dump valve having generous clearances and operated by the tripping system so that safety is assured.