1.3.5 Overspeed testing
Overspeed testing to prove the actual value of trip setpoint of overspeed protection systems is an essential subsidiary function of governing systems. The testing is carried out on an unsynchronised turbine. Initially, the operator has to operate a key-locked test switch which permits a higher than normal governor speed range up to +13% rather than +6%.
The normal governor raise button is used to increase speed to the trip set-points. The full method of testing is described in Section 3.5 of this chapter.
The ability to test the correct functioning of each turbine steam valve is necessary for both on-load operation and also prior to run-up. The latter is described as valve exercising, the requirement being to stroke each valve open and closed individually, to test valve freedom of movement. As far as the operator is concerned this uses the same controls as on-load testing, with the exception that selecting 'exercise' ensures that the valves can only be opened if the boiler stop valves are closed.