GT Load limitation Due to Fuel Restiction


Thread Starter


We are having a single Generator, frame V (21 MW) Machine with mark VIe controller. Running in temp IGV Temp control, generator in droop mode, normal load varies 16-19 MW in Naphtha Fuel. Along with GT we are having Grid connection which normally having 4- 5 MW load. Both source work independently.

We are facing load limitation of machine due to fuel flow restriction.
One month back machine could not able to go beyond 20.2 MW, max open IGV 75 with Exhaust temp 512 deg C, while increasing the load FSR1 increases to 75% , LCV servo current responded according to increase in FSR and current goes negative, but magnetic pick up % and fuel flow stop increasing beyond 20.2 MW. Based on trip live data BGGTS (service agency) trouble shoots and asked to replace whole LCV assembly (actuator) with servo valve. We replaced it with new actuator and old (MI removed) servo valve after taking a shut down.

But yesterday again when we tried to take machine to base load with grid synchronization. Machine restricted its load max to 19.5 MW with IGV max opened to 83 deg, fuel flow 1.66 kg/cm2, Magnetic pick up 58 %, FSR1 raised to 72 % with fuel inlet Pr 5.1 Kg/cm2(normal Pr) to 6.2 kg/cm2(We raised it to check any fuel pr problem).

GT screenshots with trip live of yesterday data with grid synch can be access from below mention link.

Please help us to analyze the problem.

As LCV with servo assembly has been replaced, what could be the fuel limitation factor in machine?

Thanks in Advance

Many liquid fueled units have one or more filters in the liquid fuel system--both the supply (low pressure) and the high pressure (after the high pressure liquid fuel pump). Many liquid fuel systems also have a strainer on the suction of the pump(s) that provide the liquid fuel to the turbine (Accessory Compartment's high pressure fuel pump). Dirty filters will restrict the liquid fuel flow-rate, as will dirty fuel strainers on the suction side of the low-pressure pump(s). It could also be that what ever system is supplying the naphtha is flow-restricted--and it's not the control system's fault (including the high-pressure liquid fuel system).

Also, you talk about a LCV (Liquid fuel Control Valve, I presume), and many newer machines have a LFBV Liquid Fuel Bypass Valve). Older machines may have a high-pressure liquid fuel pump with a variable pump displacement control, while newer machines will have a fixed displacement high pressure liquid fuel pump that uses a liquid fuel bypass valve to control the flow through the liquid fuel flow divider and to the fuel nozzles by recirculating a portion of the liquid fuel coming out of the high-pressure liquid fuel pump back to the suction side of the pump as necessary. We don't know what system is in use at your site from the description provided.

I'm unable at the present time to open the .rar file and look at the information provided. I can say that if there is a flow restriction UPSTREAM of the high pressure liquid fuel pump and control valve (either type of control mechanism (variable displacement pump or bypass vale or whatever)) will cause the Mark* to increase (in the negative direction) the servo current to try to increase the liquid fuel flow-rate if there is a flow restriction upstream of the high pressure liquid fuel system. (And if there's a high-pressure liquid fuel filter that could also be the cause of the restriction if it's plugged (choked).)

There have been cases of incorrect LFBVs being installed causing flow restrictions (the valves were too small for the required flow).

The bottom line is: If the flow to the unit is restricted, the Mark* will increase the servo current (in the negative direction--negative servo current increases fuel flow-rate) above the normal operating condition (which should be approximately -2.67% per processor, a total of -8.0% for all three processors). If fuel flow-rate doesn't increase as the servo current decreases (grows in negative magnitude) that means there some kind of flow restriction--either in the supply or in the control valve/mechanism.

It could be that there is something wrong with the liquid fuel flow divider (not likely, but still possible), and/or with the fuel nozzles and/or the liquid fuel check valves at the fuel nozzles--though that is also unlikely because unless there are high exhaust temperature spreads it would require all 10 check valves and/or fuel nozzles to be experiencing the same problem at the same time.

I don't have a lot of experience with naphtha, so I don't know what is done for filtering or forwarding. But, I suspect (without being able to look at the information in the .rar file yet) that something is restricting the flow to the unit. There is usually some kind of transfer valve to shift between distillate and naphtha--it could be that the transfer valve isn't fully in the naphtha position or has some flow restriction upstream of it. The limit switches might be indicating the valve has fully moved to the naphtha position, but it might not be physically in the naphtha position. I would suspect at a minimum there is some kind of "straining" of the naphtha before it gets to the high-pressure liquid fuel pump and control valve/mechanism, and that may be choked (plugged). But, this is most likely NOT a controls problem, it's most likely a true restriction in the supply to the unit, not at the unit.

I will try to get some time and a file extractor to open the .rar file to look at the data. But, from the text provided it would seem the Mark* is trying to increase the flow-rate but something is limiting the flow-rate--and it's not likely the Mark*, or even the LCV (if the problem existed before you changed the LCV and its servo--so it's been a problem for both valves/servos).

Please write back to let us know what you have done to eliminate the supply system (including the distillate/naphtha transfer valve) as the cause of the problem.