1.3.6 On-load testing
On-load testing can only be carried out on groups of valves in turn. Thus, on a four-inlet turbine, one group of valves comprises the left-hand front HP governing, HP stop, IP governing and IP stop valves. The on-load test system includes facilities for checking the operation of all the valve relay inputs.
On HP and IP governing valves the ability to close in response to governor signals is checked and also the means of tripping the valve in response to the action of the protection system. On HP and IP stop valves, the normal tripping and resetting action from the protection system is checked and also the facilities provided as back-up for emergency closure. Testing of a group of valves is carried out sequentially, starting with the closure of the HP governing valve,
then the HP stop, IP governing and IP stop valves until the whole group is closed. Valves are opened in the reverse sequence and there is also the capability for reversing the sequence at any point when the group test is only partially complete.
On a typical wet steam (PWR) turbine, there will be four HP inlet and six LP inlets so a group cannot be defined in the above manner.
A vital part of the on-load test equipment is the provision of some means for the operator to monitor the performance of the valves. This may be done by automatic initiation of a chart recorder, giving a time-base against which the opening and closing times of the valves may be judged for conformity to the specification. Another method is to use an XY plotter with the actual valve position being plotted against the demanded position. This can be arranged to give a well-defined characteristic for a correct test, so that an incorrect test is readily identified. A third method, likely to be employed in the future, is to make use of the VDUs provided in the control room for data processing. A similar type of record to that described for the XY plotter may be obtained, using a temporarily enhanced scanning rate for the valve position signal of the valve under test.