Hi all,
The plant has Frame 9E gas turbine, simple cycle, 125 MW, and operates on HFO fuel. When the load reduced to about 40 MW a fluctuation happened in the signal of the flow divider and an alarm appeared (combustion trouble). The temp. of the fuel is constant at 75 c , so why this problem is appear? And how can it be solved?

 

AhmedWaleed,
HFO is usually pretty obnoxious fuel, and I believe you've mentioned it might be crude oil at your site. That means there are all manner of contaminants which will be flowing through the liquid fuel system, including rocks, sand, heavy metals, etc. Some of these can make it through filters--and sometimes, when filter differentials aren't properly monitored the filter elements can become so dirty they eventually rupture, which reduces the differential pressure to near zero and some people mistakenly believe all is okay when this happens (which isn't true).

Hiccups in flow divider feedback can be caused by air in the piping--the single most common problem--and entrained contaminants (the aforementioned sand, rocks, and heavy metals--some of which can actually plate out on other metals, though usually only at elevated temperatures but it has occurred at very low, near-ambient temperatures as well). Combustion trouble occurs when there is a larger than normal differential between exhaust thermocouples that is the result of uneven fuel flows, or uneven air flows, to one or more comustors. Usually it's the fuel nozzles that get plugged with sand or rocks or cold HFO (such as when the turbine trips while running on HFO).

Those are the normal causes of flow divider feedback problems and combustion trouble alarms. And, many times when one occurs the other also occurs, just depends on the severity and length of the problem.

If you didn't already know this, operation on HFO (crude or residual) requires more frequency maintenance and replacement and refurbishment of liquid fuel system components and fuel nozzles and hot gas path parts. In some cases, the increased maintenance costs--along with the required outages and loss of electrical production--can outweigh the lower cost of the fuel. While HFO operation may seem economical, it is rarely so--unless there is no ability to refine the crude or sufficient quantities of the crude to produce the more desirable LDO. Or, unless there is an abundance of residual HFO which makes it extremely inexpensive to obtain and burn. Most HFOs require some kind fuel treatment (including centrifuging and heating, as well as liquid additives to reduce plating on turbine nozzles and buckets). They're just not nice fuels and can be very costly, as well, because of the additional maintenance costs and treatment(s).

Lastly, operation on HFO is also not as simple as LDO. A good deal of care must be taken to ensure the temperature is maintained and treatment is maintained and filtration is maintained.

As for the flow divider, it's function is to send the same amount of fuel to each of the fuel nozzles/combustors. It does so using a geared arrangement of wheels designed to equally split the total amount of liquid fuel into the required number of individual flows to the nozzles/combustors (a Frame 9E has 14 combustors and 14 fuel nozzles). The idea is that no flow wheel can spin any faster or any slower than the others. If something blocks one flow wheel for a split second or a large volume of air enters the flow divider (which can be as equally divided as liquid fuel (LDO or HFO)) then the flows to the individual nozzles will be interrupted or uneven (in the case of air in the line). It's very important to keep air out of the liquid fuel supply piping--very important. Usually, when something blocks one flow wheel, they are all blocked, but sometimes a small particle can cause a short-lived problem to all of the nozzles before the flow divider is free to turn again and that can cause a combustion trouble alarm.

So, fuel temperature isn't really going to cause the two things you described--not under normal circumstances. And you didn't provide much in the way of circumstances (how soon after transfer from LDO did this occur; was there a problem with the HFO supply pumps/piping; has there been high filter differentials; were the filters recently changed (and possibly the air wasn't properly vented from the filter canister); etc.).

Hope this helps!

 

Dear Ahmed,
In addition and repeating to what CSA had mentioned try to focus on the following to trouble shoot:

1- check to be no air on the fuel lines by venting LPF's well.

2- you need to make HFO pressure slightly above LDO pressure

3- check air pressure in transfer valve

4- when the fluctuation happened try to remove one coil of the servo coils and see if the fluctuation stops

5- fuel temp. preferred to be above 80 degC to decrease the density(thats for crude) and for (HFO) its preffered to be >85 degC

6- sometimes a shortage in fuel flow cause this kind of problem so make sure that the strainers at suction of HFO pumps are clean.

 

Thank you very much for both advice, I shall try these.