GCV HUNTING

Dear All,

We have 6b Gas turbine machine during base load trail gas control valve hunts from 73% to 62 % same as command hunted from 68% to 58%
& load also fluctuate from 34.5MW to 29MW. However GCV fluctuation reference alarm comes with effect load reduced. Later on again checked same scenario happen.

As Hydraulic oil filter checked found ok . no differential pressure/ no Hyd. pressure hunting/ No gas flow hunt checked. nor SRV hunt.

Kindly suggest.
 
makarand,

I understand your first language is probably not English, but it's very difficult to understand the circumstances of your problem.

You seem to be saying that during a "base load trial" the gas control valve is hunting between 62% and 73%. And you are saying something about a command hunting between 65% and 58%. And, also something about a GCV fluctuation alarm.

We need more information.

First, what kind of turbine control system does the unit have? Mark IV? Mark V? Mark VI? Mark VIe?

Second, what kind of combustion system does the unit have? Conventional combustors or DLN (Dry Low NOx) combustors? (If the unit doesn't have DLN combustors, then it has conventional combustors.)

Precisely, what is the alarm message you are receiving? Precisely copy the alarm text message(s) you are receiving when the fluctuations are occurring.

What is the P2 pressure doing when the GCV is hunting? (The P2 pressure is the pressure between the SRV and the GCV--the Intervalve, or Interstage, Pressure.)

What is the SRV doing when the GCV is hunting?

What is the frequency of the grid the unit is synchronized to when this is occurring?

Are you absolutely CERTAIN that the BASE LOAD button is the button which is active and has been used to run the unit up to Base Load and keep it at Base Load? (In other words, confirm you or the operators ARE NOT using a very high Pre-Selected Load Control reference (command) to try to achieve and maintain Base Load.)

When was the last time the P2 pressure transmitters were calibrated? (The typical device number for the P2 pressure transmitters is 96FG-2 (A, B, and/or C).)

When was the last time the CPD transmitters were calibrated? (The typical device number for the P2 pressure transmitters is 96CD-1 (A, B, and/or C).)

When did this problem start? After a maintenance outage (a Combustion Inspection, or a Hot Gas Path Inspection, or a Major Inspection)? After a trip from load?

I have seen MANY times when the hydraulic filters have ruptured and do not register a differential pressure (well, they do--but it's 0 psid or 0 bard). Even when the hydraulic filters are new, there is usually some small differential pressure, a few psid or 0.2 or 0.3 bard).

I have also seen MANY times when the valves of the hydraulic filters are not in the proper positions. There are filter drain valves, and filter vents, and a filter transfer fill valve (used to fill a new, clean filter and canister with oil before transferring to it) which are not in the proper positions and that can cause improper differential pressure readings.

What is the hydraulic pressure doing when the GCV is fluctuating?

If the unit has a hydraulic accumulator, is it properly charged and are the valves on the hydraulic accumulator in the proper positions (the drain valve should be CLOSED and the isolation valve should be OPEN (sometimes the drain valve is called the "bleed valve" and the isolation valve is called the "block valve" or something similar).) Also, it's very common for the charge to be lost when trying to check the charge with the charging hose and pressure gauge.

What are the IGVs doing when the GCV is fluctuating?

IF the BASE LOAD button is active and being used to run the unit up to and maintain Base Load, AND the grid frequency is stable, AND the hydraulic accumulator is working correctly then the problem is most likely either the SRV is hunting and causing the GCV to hunt OR the CPD transmitters are not working properly or the valves for them are not in the proper positions. The valves for the P2 pressure transmitters may also not be in the proper positions and that may be causing the SRV to fluctuate which is causing the GCV to fluctuate. If the hydraulic system pressure is relatively stable (indicating the hydraulic accumulator is working properly) AND the IGVs are relatively stable then the problem is likely either the SRV or the CPD sensing.

IF Pre-Selected Load Control is being used to try to reach and maintain Base Load--STOP USING PRE-SELECTED LOAD CONTROL FOR REACHING AND MAINTAINING BASE LOAD!!! Use the BASE LOAD button--that's what it's for!!!

IF the grid frequency is not stable, well, ..., then there's nothing which can be done until the grid frequency stabilizes. NOTHING.

It would be best if you listed ALL of the alarms which are active when you are running the machine--ALL of them. And, if the turbine control system has Diagnostic Alarms, it would be best if you listed all of them, as well.

Help us to help you--give us the answers to all of the questions above, and provide a complete list of all alarms which are active when the unit is running and the GCV is fluctuating.
 
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