2.5 Stop valves
The purposes of the emergency stop valves are to cut off the steam supply during periods of shutdown and to provide prompt interruption of the steam flow in an emergency trip.
In order to ensure that the valves will close in an emergency, it is a requirement that they are tested onload regularly, at least once a month. During such testing there is a load reduction, which can be minimised by a suitable interconnection downstream of the valves. The on-load testing is carried out in sequence so that only one valve is tested at a time.
Single-seated unbalanced plug type valves are normally used, with an internal pilot valve that opens first. The pilot valve is small so that it may be opened against main steam pressure. It also provides initial fine control and reduces the loads required to operate the valve. Normally, once the machine has run-up to speed, the governor valves which have been open, close in. It is then possible to open the main stop valves as the pressures upstream and downstream of the valve are equalised. A typical emergency stop valve is shown in Fig 2.35.
On some 500 MW units and on some early 660 MW machines, flap valves were used for reheat emergency valves, where the steam pressures were moderate and the specific volumes (and hence valve diameters) were large.