1.5.4 Reheater relief valves
Reheater relief valves are supplied on all machines to prevent over pressurisation of the reheater. These are of a spring-loaded type, set to open at a defined pressure.
In addition, on some machines, there is a need to supply relatively small release valves to release steam in the reheater system to atmosphere when the governor valves and interceptor valves close suddenly in an emergency. Although the steam may not be at a sufficiently high pressure to cause the relief valves to open, any residual steam trapped in the HP cylinder and reheater could give rise to overheating of HP blading due to churning in the relatively high density steam. The release valves, located in the connecting steam piping between the reheater and the interceptor valves, are signalled to open under the appropriate conditions to prevent this from happening.
The release valves may be hydraulically or pneumatically operated. Where an electronic governor is fitted, they are opened by energising a solenoid. Conditions which signal their opening are:
- Loss of tripping pressure.
- High acceleration detected by governor.
- Closure of all interceptor steam valve relays.
These conditions cover circumstances of machine tripping or turbine load rejection without tripping, where churning could occur either during rundown of the machine after a trip or during the period of interceptor valve closure on load rejection.