Steam Turbine Throttle Valve Failure Overspeed

Dear my collegues;

We are using single shaft machine. We are trying to create a what/if scenario for steam turbine overspeed protection. We just have HRSG LP system isolation valve, however our boiler does not have isolation valve for IP and HP outlet. Both pressure stage superheated steam is directly goes to steam turbine IP and HP sections. Steam turbine IP and HP sections have in twos stop valves and in twos throttle valves with actuated by high pressurized hydraulic oil. Our generator is driven by GT and ST is connected to generator via SSS clutch switch. Firstly, steam turbine is unloading and stop and throttle valves are closing during shutdown and SSS cluch disconnects steam turbine from generator, Gt is still going on generation at 100 MW for 15 minutes. What we are wondering about is when ST SSS cluch removes generator from ST, if these isolation valves are passing and ST goes to overspeed because of HRSG HP and IP outlet line is still pressurized. What can be done to prevent the steam turbine from going over speed in such a situation. Because, our HRSG is sending to superheated steam to turbine for HP and IP stage without any isolation valve.

Best Regards
Hello sir,

nice scenario, to think of

i think sss clutch will re engaged as the steam turbine reach 3000rpm
Therefore, the gt will try to unload.

if not, all you can do is push emergency stop button, to stop heating the boiler and try to break the pressure by opening bypass valves to condenser (if your hrsg is equiped of) or vent out the steam by vent and drain valves

On very low load, breaking the condenser vaccum will brake the steam turbine, but LP last stages blades will overheat by friction with air