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Control valve sticking GE G# STG

Control valve sticking GE G# STG

Control valve sticking GE G# STG

(OP)
Got one for you to ponder:


We have an issue on U1. The MHC system/control valves are setup such that CV’s 1/2/4 have the same cam profile & timing and therefore all move together. CV3 has a different cam profile and is the last to open/first to close. The unit starts in full arc mode on the SVBV’s. U1 was operating normally on 5/21 and removed from service due to a boiler tube leak (controlled shutdown). The subsequent restart was on 5/26. When reset, all 4 CV’s went 100% open. When control was transferred to the governor/CV’s the behavior was as expected, CV’s 1/2/4 closed to approximately 13 -14% open, then increased to about 22% open and controlled the load while CV3 went to 0% open. The load at that point was about 40 mw. Load was then increased to about 170 mw with little to no change in CV position for any of the 4 valves. This is typical startup behavior, the load increase is due to main steam pressure being increased from about 900 psig to 3500 psig. At 170 mw the unit tripped due to a controls issue on the boiler side. The controls issue was addressed and the unit restarted about 6 hrs later. Again, on reset all 4 CV’s went to the 100% open position. When the valve transfer occurred, CV’s 1/2/4 all closed down to about 13 – 14% open and then increased to about 20 -22% to control load. Startup followed the normal path up to about 190 mw. At that point, the governor camshaft position increased and began increasing the lift for the CV’s 1/2/4. CV’s 2 &4 followed camshaft position normally and opened as camshaft position increased. CV1 however, never moved from 22%. Load was increased to 375 mw. CV’s 2&4 were approximately 32% open but CV1 was still at 22%. Load eventually was increased to 780 mw. CV’s 2&4 were at 100%, CV3 was at 45% and CV1 was still at 22%. Over the past couple days the load has been up and down between 780 and 375. Everything looks normal with the exceptions of CV1 & CV3. CV1 has never moved from 22% and CV3 opens further at high load (which makes sense, the governor cam shaft is a couple percent higher at full load which drives CV3 open more, and that is happening because CV1 is not 100% open like is should be). 10 days ago, at 780 mw, with CV’s 1/2/4 at 100% open, CV3 was at about 33% open. So, now we are seeing it run about 12% further open to achieve the same generation (and presumably the same steam flow rate).



It seems strange to me that this would be a case of mechanical binding (stuck valve) because it stroked to 100% open on the startup just 3 days ago? I looked externally and don’t see obvious issues with linkage, etc. Our control oil pressure indication shows that it is normal. At 375 mw (our normal low load bottom end), the camshaft position still calls for CV’s 1/2/4 to be above 22% open so I can’t say whether the valve will move in the closed direction or not. Although, on startup it did move from 100% open to 13% during the valve transfer. Could we have a linkage issue on the CV1 servomotor or pilot valve that would explain this? Oil leakage on pilot valve or operating cylinder that crossed a threshold point coincidently at the same time as valve transfer on startup and now we don’t have enough hydraulic pressure for CV1 servo?



Any thoughts?



The plan as of now is keep U1 in-service and perform the mini on U2. After that, we will have to make a decision because I don’t believe we can run a successful PJM capacity verification test on U1 with CV1 only 22% open. Looking for some ideas on what the scope of work would be to troubleshoot/address this issue and return unit to service (assuming that we have to shutdown and get into the servo enclosure at a minimum).

RE: Control valve sticking GE G# STG

Dear
Turbine admisson mode opperates in two modes which are partial ARC and Full Arc.
Partial arc Cv1,2 100 open and CV3 regulate itself according to the load demand and steam flow as far as CV4 remain 5%(closed)
If load varies due to any change in frequency and load picked by machine then CV3 will be 100% open and CV4 also open 20% to 25% but its abnormal situation.Normally it does not happen.
During full arc all four CVs open equally like 25% or 35%.All participate equal steam flow to the chest.
If any one CV fails the othe ones replace its demand.
During Partial arc if any CV open all of sudden the load will picked by machine due to mall operation and if CV which is regualating the load demand stucked open or closed the load regulation will be shifted to CV4 and machine might be pick the load more than target load as CV3 stuck open so CV 3 must be forced closed and in this situation Arc to be shifted to Full and also load need to be reduced but carefully not fast mode.CVs work under the load and steam demand and also speed control with their references in TSI Limiters.
BR
Syed

RE: Control valve sticking GE G# STG

Hello Turb, any possibility the position transducer is lying? Control systems can only report what they're told, and depending on the sophistication of the control system design a discrepancy between reported aggregate valve positions and summed actual unit output may produce a non-linearity / value off-normal alarm...anything looking out of whack that way?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Control valve sticking GE G# STG

crshears,

MHC - mechanical-hydraulic controller. Had this been an EHC / DEH system I'd have likely commented already and you might be right with your guess, but MHC systems don't use feedback in the form of an LVDT or whatever you have in mind. It's all cams and levers and oil and magic in the MHC. winky smile

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