Hello,

Dear All

I work in a Combined Cycle Plant Power. These Two GT(F9E) and a Steam Turbine (SC) of Ge. We have a problem while works by the steam turbine. This is hydraulic pressure drops. During last days, suddenly hydraulic pressure began to drop on the steam turbine. Although the second pump works, the pressure continued to drop. But this problem during full load or every time doesn't occur. Within a week two time we tripped because of pressure drop. We have seen that problem is occur while HP (high pressure)control valves open to 75%. Wonder, what could be problem at the main hydraulic unit? (There isn't oil leakage from the system.) Thanks for already yours help to me..

 

A leaking hydraulic actuator piston seal would not be visible to the naked eye, nor would a worn hydraulic actuator cylinder wall. It's common for valves which are operated for long periods of time at one position to develop worn cylinder walls in that position and for hydraulic oil to leak past the cylinder seals requiring a high flow to maintain a particular position which results in a falling pressure. (You didn't say how long this plant has been operating, and how long since the last time the valve's hydraulic actuator was inspected, refurbished, or replaced.)

When hydraulically-actuated valves are operating at a steady position (when the feedback is equal to the reference) there is <b>NO</b> flow of hydraulic oil to or from the hydraulic actuator. None is required because the valve is at the required position (per the feedback), and the only time flow is required is when the valve needs to change position.

So, if the cylinder wall is excessively worn in a particular area, or the piston seal (or o-ring as the case might be) is leaking, then flow will be required just to maintain that particular position, and if the required flow is high enough then that may cause the hydraulic pressure to fall.

Most hydraulically-actuated valve systems don't have pumps rated for constant pressure at high flow-rates, for the reason stated above. Many do have hydraulic accumulators to assist with maintaining pressure during sudden flow increases.

And, that may be another problem you are experiencing: Failed or failing hydraulic accumulators, or low hydraulic accumulator charge, or incorrect hydraulic accumulator manual valve position(s).

Have a look at the P&ID for the hydraulic system, and methodically and systematically rule out each potential problem component by proving it is or isn't that component.

If you can shut the steam turbine down and stroke the valve slowly through the position at which you perceive the problem to be occurring then you can prove--or disprove--if it's that valve's hydraulic actuator.

And, if that's not the problem, just keep testing each component as necessary until you find the faulty one.

But, just because you can't see a leak, doesn't mean there isn't one.