1.5 Governor valve relays - part 3
The null is adjusted by altering the spring bias applied to the armature. In the event of a loss of the electrical signal driving the servo-valve, it is essential that the valve relay and hence the power piston is driven to a closed position, so a 'null bias' is set to give a small offset in this direction.
The control characteristic of the servo-valve in this condition is shown in Fig 2.19 (a). Also plotted (in Fig 2.20) is the corresponding valve internal leakage which reaches a peak at the normal controlling point of the valve. The servo-valve is likely to be the most sensitive item in the hydraulic system owing to the small clearances, especially in the nozzles, and the alignment of the control edges of the spool with the ports in the valve body. The hydraulic fluid must be kept in a pure condition with a low contamination level, otherwise the control edges will become eroded and the internal leakage will increase substantially. If this leakage persists, it will result in an excessive fluid consumption and a reduction in the fluid pressure, eventually resulting in the need to shut down the turbine and replace all the worn components.
The needs for the control of the fluid characteristics are described in the section on the fluid pumping system. However, at this point, it should be noted that CEGB standard practice is to provide a test rig in the laboratory for checking the characteristics of servo-valves removed from the turbine.
Before going on to describe the details of the valve relays following the servo-valves, it is necessary to set out the requirements of the final drive to the steam valve. In the event of a failure of the hydraulic system, the steam valve must be capable of closing in less than 200 ms against any force acting to open the valve. The flow of steam through the valve usually generates an opening force over some part of the travel. Figure 2.21 shows a typical case where the steam force acts to open the valve over the last 70% of the valve lift. A substantial spring (or even a nest of springs) is provided to give the closing force with a large margin over the steam valve opening force to overcome frictional effects. The figure shows a typical spring force characteristic and the net force closing the valve. Frictional forces are kept to a minimum by designing the valve so that side loads on the valve spindles are minimised. To open the valve, it is necessary for the valve relay to supply a force sufficient to overcome the spring force and any steam closing force, such as that occurring over the first 30% of movement shown in Fig 2.21.