Load Hunting on GE frame 7

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Thread Starter

Ahmad

Dear all,

I'm facing a problem with The fuel and load hunting in GE frame 7
When the turbine reaches Full speed no load the fuel start hunting in about 5 LPM and we close the generator breaker it goes to a massive hunting in load and fuel.

Note: the unit is out after a major maintenance

If you have any suggestion please help me with this issue

Thank you
 
Ahmad,

I don't think you're going to find much experience here with Ovation control systems on GE-design heavy gas turbines. There are only a handful worldwide, though Emerson would like to greatly increase that number.

We don't know what version of Frame 7 you have: 7B, 7E, 7EA, or one of the F-class machines (yes; Ovation has been applied to at least one GE-design heavy duty Frame 7FA gas turbine-generator).

In any case, unless major changes were made to the liquid fuel system (and although you didn't specifically say, it seems you are talking about problem while running on liquid fuel), it still has a liquid fuel control "valve" (either a variable displacement high pressure liquid fuel pump, or a Liquid Fuel Bypass Valve). Both are almost always operated with hydraulic actuators, employing an electro-hydraulic servo-valve to control hydraulic oil flow to/from the actuator to position the pump/bypass valve.

Many times during maintenance outages, servo-valves are removed and replaced (if hydraulically-operated devices are serviced or refurbished or replaced). Some times this causes dirt and debris to be released into the hydraulic system which can negatively impact other hydraulically-operated devices.

Poor practices while replacing hydraulic oil filters (or control oil, depending on the particular unit) can also release dirt and debris into the hydraulic system which can have bad effects.

And, lastly, if there are LVDTs, or RVDTs (Rotary Variable Differential Transformers) on the liquid fuel pump/bypass valve they are quite commonly "calibrated" during maintenance outages. This causes a LOT of problems if people don't fully understand what the purpose of "calibration" is, and if it's not done correctly can cause a LOT of problems on re-start after a maintenance outage.

LVDTs (and RVDTs) are no different than temperature switches or pressure switches. When temperature switches or pressure switches are "calibrated" during a maintenance outage the <b>FIRST</b> thing that is done is to record the as-found condition after applying temperature or pressure and observing the switch operation. <b>THEN--AND ONLY THEN--</b> and only <b>IF NECESSARY</b> is the setting of the temperature or pressure switch changed to match the specified setting for the device.

When people go to "calibrate" LVDTs or RVDTs on GE-design heavy duty gas turbines during maintenance outages they quite frequently never even set up a device for measuring the actual position of the pump swash plate or the bypass valve plug. They don't even have someone stationed at the device to observe the operation/position of the device. They just sit in front of the PC and click on some targets ("buttons") and pronounce the "calibration" a success! They don't know what the as-found condition was; they don't know what the as-left condition was; they don't know how the calibration "offset" and "gain" changed from the as-found calibration. They just "click and go".

Consequently, a lot of problems are caused--especially for machines with DLN combustors.

I don't have any personal experience with Ovation control systems (though I'm hoping to get some soon). I can't tell you how an LVDT or RVDT is performed--or even checked--using an Ovation control system. So, I'm afraid I can't be of much help. I have told you the above because those are most of the (self-inflicted) problems after maintenance outages.

If the unit has a liquid fuel flow divider, it has speed pick-ups which the Ovation uses to determine how much fuel is flowing to the unit and compares that feedback to an internally calculated reference. If the speed pick-ups were disturbed during the maintenance outage, it could be that they were not gapped (installed) properly and could be causing erroneous flow feedback.

Loose wires and terminations can also cause these kinds of problem, so if you haven't already done so--check the terminations all along the interconnecting cabling/wiring of the liquid fuel system (servo-valves; LVDT or RVDT; liquid fuel flow divider; etc.).

Lastly, if the unit as an electric fuel pump clutch, they can slip. About the only way to tell if one is slipping is to use a strobe light to observe the coupling between the clutch and the pump to see if it's smooth or jerky.

Also, if the pump was removed and replaced or refurbished during the maintenance outage the coupling bolts might not have been tightened properly.

Other than air in the supply piping, that's about all of the major problems which can occur to a liquid fuel system. Troubleshooting is sometimes a process of elimination--and until you eliminate several things sometimes you can't get to the root cause.

When posting to a forum like this, you need to provide as much information as possible about the equipment you are working on. There are LOTS of GE Frame 7s, and while they all work on the same physical principles they have very different auxiliaries and configurations.

If you're having problems, and have done some troubleshooting, tell us what you've done--and more importantly, what the results of your troubleshooting was/were. You did very good to tell us this problem started after a maintenance outage, but there wasn't too much other information.

One more thing you didn't tell us was how was the firing and acceleration up to FSNL? Was it smooth or faster or slower than normal, or was it jumpy and erratic?

Write back to let us know what you find and how the problem was resolved!
 
Ahmad,

One other very common cause of instability and hunting is liquid fuel supply pressure. Many times, there is a pressure regulator used to set and maintain the liquid fuel supply pressure to the Liquid Fuel Stop Valve on the turbine accessory base. This pressure should be approximately 65 psi, and stable.

Air can (and quite frequently does) get into the actuator used to control liquid fuel supply pressure, and when that happens the supply pressure can be unstable.

So, make sure that air is removed from ANY high-point in the liquid fuel supply system. A high-point is any element (regulator; filter canister; etc.) that has a pipe or tubing connection that is higher than the adjacent piping. This includes liquid fuel supply pipe(s) which may go up into overhead pipe racks and then down to the Liquid Fuel Stop Valve.

If equipped with a high-pressure liquid fuel filter, this, too, can have air trapped inside which can cause pressure--and flow--fluctuations.

Air, is, in fact, the biggest cause of instability and hunting during liquid fuel operation. Especially after a maintenance outage.

Again, please write back to let us know what you find.
 
> And i Forgot to say that the unit is working on Ovation system by Emerson

Is the Ovation system controlling the Frame 7 GT directly or is it a DCS, with a data link (Ethernet?) to a Mark V or Mark VI GE turbine control?
 
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Dear Ahmad

We had a same problem at one of our GE Frame 5 gas turbine. As soon as breaker used to close, load hunting was observed. This hunting increased with the demand in fuel.

The reason was the slight leakage of control oil from SRV (stop ratio valve). The leakage was not visible as the valves(SRV & Fuel control) were behind bolted plate. After calibrations and testings it was decided to check the valves and thus the leakage was observed.
 
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