Steam turbine control issue

[npower0073]

On site , a CCPP with a steam turbine Franco Tossi Mechanica and Siemens ORSI IT control system we experience lateley the following event: WHile ST runs her start up sequence the speed is controlled by HP Stop (throttle) valve). After the spped of 2800 rpm the conmtrol of spped goes to HP Control valve (governor)> But for some reaosn at the stage of 2990 rpm the speed of ST decreases to 2900 rpm becasue the control goes back to the HP stop valve. This happens one or two times and then the unit sunchronizes OK. Can you share any experience on this issue?

 

[byrdj]

I have no knowledge of Siemens controls, however that sounds like a problem I've seen on another type unit. The "FA/PA transfer" circuit used position indications to verify the CV went to the transfer position. this was either via limit switches which were a real problem to set and get repeatable. the latter electronic position switches (VCs) were much better.

the problem was that the circuit could not determine if the transfer was from "SV high transfer" to CVs or the "CVs low transferr" back to SVs.

Thus I would look for position logic as the problem