2.6 Governor valves
The governor valves provide accurate control of the steam flow rate entering the turbine, thus controlling the generator load when the machine is synchronised to the grid.
The present practice for the control of the steam flow rate during turbine run-up to speed is also to use the governor valves; however, on earlier 660 MW machines and also on 500 MW units, run-up is controlled by use of the emergency stop valve pilot-valves, in conjunction with the governor valves. On 660 MW machines, the interceptor valves can also be used in a governing mode, if required, to use the stored energy in the reheater to provide a quick response to frequency variations.
The diameters of valve opening are generally calculated to give maximum steam velocities of about 120 m/s for the valves. Valve head designs are such that they are not subject to steam-induced vibration during operation.
Figure 2.36 shows a design of governor valve in which the upstream steam pressure is prevented from acting on the back of the valve by a fine annular clearance. The space behind the valve head is usually vented downstream of the valve by means of holes drilled in the valve head.
A similar design of governor valve is also used for the reverse flow type of governor valve, see Fig 2.37.
As with the stop valves, it is essential that these valves close in an emergency; they are therefore similarly onload tested.
Figure 2.38 shows the 'mushroom' type of governor valve which has a profiled head to give a more linear flow/lift relationship. A 'guided flow' type of valve head has also been developed in order to control the flow downstream of the seat so as to minimise noise and vibration, see Fig 2.39.