11.1.2 Isolating valves
These valves are usually situated either side of a pressure control valve for two reasons. First, to ensure the total isolation of the control valve if it requires maintenance whilst the turbine is running; second, to provide a back-up should the control valve fail.
Isolating valves are not required to create a large pressure drop and their design is simpler than that of the pressure control valve. A typical isolating valve is shown in Fig 2.83. The valve is a parallel-slide type, so called because it does not rely on a wedge action. Instead, the discs slide over the seat faces when the valve is being opened or closed, resulting in a minimum pressure drop.
The isolating valve downstream from the pressure control valve has a back-seating arrangement on the spindle to prevent the ingress of air to the condenser under normal running conditions.