We are facing the problem of hunting in VC-3, FSR, Fuel pressure and FQL1 observed in liquid fuel operation of Frame-9 GE turbines. We have done complete venting of liquid fuel line from all possible points and there is no air in the piping. Normally we run the machines on Gas but when we started the machine on liquid fuel, flame established in first try and all parameters were normal till FSNL. As soon as 14HS picked up, heavy hunting observed in all parameters of liquid fuel system and subsequently machine tripped after around 8 mins on loss of flame. We did venting at the time of hunting also from HP filter vent and forwarding skid vent but no air appeared and hunting was persisting. Need to know the possible cause for that?

 

You didn't provide the alarms (Process and Diagnostic) which were existing just prior to the hunting and which alarms (Process and Diagnostic) were annunciated during the hunting event.

From the information provided in your message, it would seem you need to review the sequencing or application code (we don't know which because we don't know which control system you have on your turbine) to see what happens when 14HS picks up. I have seen this problem occur a couple of times when there was a problem with the sequencing in a Mark V that actually shut off the Liq. Fuel Forwarding Pumps when the unit reached 14HS (that was in early sequencing provided with some Mark Vs used on Frame 9Es).

I've also seen a similar problem caused by a problem with the Liq. Fuel Forwarding Skid pressure regulator; air can collect in the top of the actuator of the pressure regulator.

Sometimes there is air trapped in liquid fuel piping in high points which have no vents and which can't be easily removed until the fuel flow-rate gets to a certain level (that level varies depending on many factors and is not the same for every unit).

I've also seen a problem like this caused by a plugged strainer at the inlet to the fuel forwarding skid. It occurred very rapidly and the differential pressure switch across the strainer was not working properly so there was no alarm to indicate a problem. This was also on a dual-fuel unit which rarely ran on liquid fuel. The strainer was plugged with a very unusual moss-like, fibrous material. We had to clean the strainers several times over several days as whatever the material was it kept getting sucked in to the strainers for several days. As the machines was started and loaded we could stand and watch the differential pressure increase very quickly.

This problem has also been attributed to an improper gain value for the LFBV servo regulator.

If the unit isn't operated on liquid fuel very often, it's common for the LFBV servo-valve to have problems because of oil-related issues.

Lastly, you should verify that the speed feedback from the liquid fuel flow divider is stable (from all the speed pick-ups).

Sometimes, if you have the patience of supervisors and management (which isn't usually very often) you can use the FSR Gag function to try to stop the LFBV from hunting to try to see if the problem is the LFBV or the liquid fuel supply pressure. The problem with doing this is that usually one has to gag the FSR to the point that speed drops below 100% before it stabilizes. Just be careful to not let the speed go below 14HS drop-out (usually 94.5% speed) or the unit will go into a fired shutdown (usually).And, it usually takes some time to get the LFBV to stop hunting.

Another possible contributor to the problem might be the hydraulic accumulator isn't properly charged or has lost its charge. The Aux. Hyd. Pump shuts down when the unit gets to 14HS (or at least it should) and problems with the accumulator can cause issues when there is other stimuli (like air in the piping or unstable liq fuel supply pressure/flow).

Problems with liquid fuel operation on dual-fuel units which aren't operated on liquid fuel very often are more common than we would like to believe. Often, it's not just one issue, but two or three that combine to cause instability and so can be very difficult to resolve. It's a process of elimination of possible causes, one at a time, unfortunately, and that can be painstaking. And time-consuming. This is one reason that GE recommends frequent and periodic fuel transfers and operation on liquid fuel for dual-fuel machines in order to ensure the liquid fuel system--and all of its components, many of which are not monitored by nor controlled by the Speedtronic panel--is and remains in working order.

 

When did the problem start ? Has the unit ever run successfully on liquid fuel ? Have you changed types of fuel ? Any recent maintenance or parts replaced ? Is this a recent unit.

I have come across this once many many years before on an initial start up and the problem was to high a regulator gain. Reducing it solved the problem. During startup the accel pid is active but at 14HS speed comes into control and included actual TNH in the scheme of things.

In our case loss of flame was within about 20 seconds. Eight minutes seems a long time.

What make of fuel valve do you have ? How many units at site ?

 

The unit is almost 2 years old and we have performed 1 combustion inspection around 6 months ago.

We have 6 units at site and all of GE Frame-9 MAKE. We are running on gas fuel from last 1 year but we changed to liquid fuel for trial.

Previously we were using Euro-3 HSD for combustion but this time we changed the fuel to Euro-1 HSD.The physical properties were almost same when we carried out the fuel analysis.

Are u talking about reducing the gain for VC-3?

 

We are using MARK-VI for control operations.

There were no diagnostic alarms at the time of hunting. Fuel line HP filter DP switch was toggling very frequently. We have carried out the venting at the time of hunting and prior to start-up from all possible vents. A clean stream of air was coming from all the vents.

It does not seem a venting problem as the machine reached 14HS from flame establishment normally?

One more peculiar thing was that the glass windows of adjacent building were vibrating but machine vibration and spread were normal.

We are operation on Gas fuel from last 1 year and took changeover to liquid fuel for trial. Previously we operated the machine on liquid fuel too.

Do we need to check the LFBV?

 

You can say or believe whatever you want about the presence (or absence) of air in the liquid fuel supply piping. It's the primary cause of hunting, and, again--sometimes it's very difficult to remove air from piping which was improperly designed or installed until the liquid fuel flow-rate gets high enough to push the air past elbows and high points. Simply venting filter canisters is not sufficient in many cases.

Another place that's very difficult to remove air from is the pressure regulator on the liq fuel forwarding skid. Usually, it's a simple spring-and-diaphragm type of valve, and air can get into the upper area of the diaphragm. There is usually a needle valve which can be used to dampen pressure pulsations once the air is bled out, but to do that it's usually required to loosen the compression fittings (which isn't the easiest or cleanest or best way to bleed air out of the actuator, but unfortunately it's the only way unless someone installs a vent valve (hint)). I have seen a couple of Frame 9E installations which didn't have pressure regulators on the fuel forwarding skids, but those were special cases with some very over-sized pumps and motors.

Yes; we are talking about the regulator gain for the LFBV, which is sometimes identified as VC-3. It's not clear if the value is too high or too low; so it's not sufficient to say it needs to be reduced. You would need to look in Toolbox at the regulator configuration for the LFBV servo-valve output and tell us what the value is; then we could comment on whether it might need to be increased or decreased.

Again, this is a very common complaint for dual-fuel machines which are rarely operated on liquid fuel--which is against the GE recommendations. This topic has been covered many times on control.com and you can use the 'Search' feature in the right hand corner of the Menu bar at the top of every control.com page.

I doubt this is a problem with the LFBV itself, or the actuator for the LFBV. It might be a servo-valve problem (again, they can be contaminated or plugged with oil varnish during many months of inactivity). It might be a problem with the servo gain.

It would also be helpful for you to know how well the unit ran on liquid fuel when it was commissioned, or when it last ran on liquid fuel. Many times there are so many problems with liquid fuel operation during commissioning of dual-fuel units that it just kind of gets dropped since the unit isn't going to be operated on liquid fuel very often anyways. And, then, it becomes an issue again at a time like this when the unit is required to run on liquid fuel or liquid fuel operation needs to be demonstrated to the bankers or the utility or grid operator....

Best of luck with your problem! Write back to let us know what you find.

 

With your current fuel distribution set up, do you have individual fuel strainers (witch-hat type, etc)prior to the injection nozzles at the combustion chamber? If so, you may want to double check and make sure the right type was installed. This could cause flow limitations on higher demand, and/or clog up if the particulate count in your fuel is high. Check your particulate size count from your fuel analysis and make sure the higher micron level particulate is not greater than your strainer/filtration capability...