We have governor system consisting of Pressure tank, servomotor, distribution valve etc.

Our unit always sticks when a raise from 230MW to 290MW is given. After initial start the behaviour absolutely normal, but after 3-4 hrs of operation at 230MW when we try to increase load it sticks. We have seen at governor pressure system that control system raises the distribution valve fully open. As we have thoroughly checked the control system and found its behaviour completely normal. We have changed the distribution valve with new one, but no effect at all.

what do you thinks where the problem lies?
Thanks

 

sounds like the supply pressure at the valve. check the over pressure/recycle valves on governor oil pumps.

 

I have a couple of things to add you should check.

When you notice the distributing valve is full-up and the servos are stalled, do you also notice the pumping system is cycling a lot more often, or maybe even running continuously? If yes, there is excessive oil usage and since you said the distributing valve was replaced and it still stalls, the next place to investigate is the condition of the servomotor piston ring seals and the condition of the servomotor bore. If the ring seals and/or the bore are worn, oil from your pressure system and distributing valve is blowing between the ring seals and the bore. The result is decreased applied force and when this gets bad enough the servo can stall.

Check your servomotors for sampling valves. The next time the unit is shut-down, pull oil samples from each servomotor plus your pumping system oil tank(s) for analysis. If the iron content is high, you'll need to get those servos pulled apart and inspected. With a unit this size I know this is no trivial task, but servomotor stalling at any position is a big deal. If it stalls when you try to go up in power it can also stall if you are trying to unload it. You must have full and complete confidence in your hydraulic system including the servos and all linkages to the wicket gates that it can shut down that unit from any power level.

 

I will second this very well written reply, in that it seems you operate for some period of time at one load, and then move to a higher load. So, the wear could be "concentrated" at the lower load position. This happens quite frequently to machines operated at some load for long periods of time and results in increased servo current trying to pass enough oil to make the desired change of position when required.

And, also, it's just as critical, as PowerGenScott said, to have control while unloading as well as when loading.

Well said, PowerGenScott.

 

Thanks very much for all your valuable feedback especially PowerGenScott.