PG 6551B GE MACHINE

K

Thread Starter

karthickprabu

Dear all,

I am working in PG 6551B GT 35 MW open cycle (10 combustion chambers, spark plug in 1st and 10th chamber, four flame scanners -2, 3, 7, 8). Last 20 days we are struggling to start GT.

On 20/11/2014 plant was blackout in night shift. After power resumed from grid we start our plant at normal procedure. But ignition failure happened more than 2 startup. We are using liquid fuel (LDO for startup and HFO for running) as our fuel. We people did following maintenance.

1. 2 sparks plugs checked by forcing logic L2TVX1 found normal. More than that ignition transformer input voltage checked normal &that is 115 v.

2. Four flame scanners checked. That is also found normal.

3. All 10 fuel nozzles removed and chocked nozzle by HFO cleaned and fitted again.

4. Air intake primary and secondary filters also replaced by new.

5. Water wash also done

After this we start the plant again. But again failure to ignite alarm came. Some white color smoke found in chimney after ignition failure. After this we did the following maintenance.

1. Calibrate the liquid fuel bypass valve VC3 and IGV (IGV opening and closing visually inspected). Both found normal during calibration.

2. Starting booster compressor discharge pressure checked, that is 0.1 bar. Discharge NRV also normal.

3. For clarification we checked also VA22 (isolation valve of starting booster compressor) valve open and close. Found normal.

GT started again, but we got same result-failure to ignite. After this we started following the maintenance.

1. 2 spark plugs replaced by new. After boroscope 1st and 10th chamber linear changed.

2. Startup time FSR value raised from 17.5% to 18% and started again but same result we got.

3. After this atomizing air flow checked and found normal. This time startup time FSR value changed from 18% to 16.5%,but we got failure to ignite as our result.

Maximum things we checked to start our plant. But ignition failure happening. I need your help for this problem.

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You say that you have some white smoke so it looks like you have fuel. Do you get any change in exhaust temp. when you try to fire? It would be nice to try a new set of fuel nozzles that have been calibrated, do you have any? You have checked a lot but I would still be a bit suspicious of the Atomizing Air system, are you completely sure that all is OK?

Good Luck and report back please.
 
K

karthickprabu

Dear glenmorangie,

There is no change in Exhaust temperature and wheel space temperature after the ignition command. Fuel nozzle and atomizing air circuit also inspected by GE ALSTOM experts, all are normal only.
 
karthikprabu,

White vapours coming from the exhaust stack would indicate that fuel is flowing to the combusors but there is no ignition. White smoke (thicker and heavier than white vapours) would indicate incomplete combustion in one or more combusors--but should be accompanied by a rise in several exhaust temperature T/C values and a small rise in the average exhaust temperature, also.

Have you checked the Tell-take Leakoff drains to be sure none of the liquid fuel purge check valves are leaking?

Have you used the manual selector valve and gauge at the liquid fuel flow divider to make sure the liquid fuel pressure to all ten fuel nozzles is the same during firing? If they are not then one or more of the liquid fuel check valves may not be working properly causing poor liquid fuel atomizing. Combined with poor atomizing air pressure and flow would make light-off difficult.

Atomizing air pressure and flow during firing (starting) are very important and very difficult to measure. The pressures are usually too low for the gauges supplied to accurately measure during firing.

How did you calibrate the liquid fuel bypass valve--specifically tell us what was done and what the results were. I'm not aware that most Frame 6Bs have LVDTs on the liquid fuel valves which can be calibrated so please describe the liquid fuel system and how it was calibrated.

What is the liquid fuel supply pressure during firing? Is it stable or is it erratic?

Finally, if the machine was running on Heavy Fuel Oil when it tripped and the HFO has to be heated for normal operation then it's very possible the HFO has thickened and is blocking the liquid fuel flow divider and the liquid fuel check valves and the liquid fuel nozzles, not all of them but a sufficient number to prevent thinner Light Distillate Oil to flow and ignite during firing. This is a problem which is known to exist with machines that burn heated HFO and trip while burning HFO. They must be re-started very quickly <b>OR</b> the liquid fuel system must be purged with LDO very quickly to prevent solidification of HFO on the lines/flow divider/check valves/nozzles causing firing problems later.

Please write back to let us know how you fare in resolving this issue!
 
K

karthickprabu

Dear CSA,

There is no leakage in check valve (VCK1 to 10) in fuel purge line. We checked the cracking pressure of check valve, that is 6 bar. We also checked the fuel line between the check valve and flow divider outlet by air, that is found OK.

For our more clarification we calculate the fuel flow from each fuel line during firing time (remove the fuel line from nozzle and connect separate hose in each line and collect the fuel). We got 700 to 750 ml fuel in each fuel lines.

We have manual selector valve and pressure gauge in flow divider for checking pressure. During firing time we were get 6 to 7 bar pressure as output in different line (pressure is constant).

yesterday we changed the HP fuel filters by new and Force the 20WP1 (close that valve for improving air pressure) in water injection purge line. After this we give the firing cmd in cranking mode. But we got failure to ignite result.
 
To ignite flame in the combustors of a GE-design heavy duty gas turbine the following three things are necessary:

1) Fuel
2) Air
a) Combustion Air (from the axial compressor discharge)
b) Atomizing Air (from the Booster AA Compressor)
3) Spark

That's it.

Of course they have to be in the proper proportions, but that's all that's required.

The liquid fuel check valve(s) should be set to open at approximately 100 psig (which is more than 7 barg) during firing. This is to force the fuel pressure to be high enough to atomize the fuel as it's passing through the liquid fuel nozzle passages (called "pressure atomization"). The liquid fuel should not dribble into the combustor, it should be sprayed in small droplets into the area downstream of the liquid fuel nozzle tip.

Atomizing air flows from the opening around the liquid fuel nozzle tip and helps to further atomize the liquid fuel by breaking it into even smaller droplets--which help both combustion as well as ignition.

Combustion air flows into the axial compressor inlet (through the inlet guide vanes) and through the axial compressor into the combustors where it enters the combustion liner around the fuel nozzles.

Spark is provided by the two spark plugs (only one is required; the second spark plug is for redundancy). Once flame is established in one or both of the combustors with spark plugs the pressure in that (or those two) combustors is higher that the adjacent combustors without flame which forces hot air through the cross-fire tubes into the adjacent combustors to establish flame in that combustor, and so on until all the combustors are lit at which time the pressure in all the combustors is relatively equal and there is no more flow through the cross-fire tubes.

There should be false start drain valves on the lower combustion cans and the lower part of the combustion wrapper. When liquid fuel flows into the combustors but is NOT ignited, it will condense inside the combustors (if they are cold), and should flow through the false start drains. This is a good way to tell if liquid fuel is getting into the combustors when there is a problem with firing. There is also a false start drain on the exhaust plenum because some fuel will also pass through the turbine section into the exhaust and will collect in the bottom of the exhaust duct, and is supposed to flow out the false start drain connected to the exhaust duct low point drain. This is another good place to check to ensure liquid fuel is flowing into the turbine if it's not igniting. (In other words, if the liquid fuel does not ignite, it doesn't just magically disappear--it has to go somewhere, and it is supposed to go out of the false start drains so it doesn't collect in the combustors, combustion wrapper and exhaust and go <b>BOOM!!!</b> when flame is finally established. So, checking the false start drains is a way to determine if liquid fuel is flowing into the combustors or not. And, there should be a relatively steady flow of liquid fuel to indicate sufficient liquid fuel has been flowing into the combustors during firing; a small dribble for a short period of time is probably not an indication of sufficient liquid fuel flow flow during firing.)

Unless the IGVs are really damaged and closed much less than 34 DGA (DeGrees Angle), it's not likely that there is insufficient combustion air. If the IGVs are open much more than about 36 DGA then the problem could be excess combustion air. So, the calibration and actual position of the IGVs during firing should be verified.

That leaves spark and atomizing air. The spark plugs are relatively easy to check. They can usually be removed without damaging the gaskets, grounded and energized to test for sparking. (They must be solidly grounded, either by laying them on an unpainted surface or by wrapping a 1.5mm or 2mm bare wire around the unpainted body of the spark plug and then attaching the other end of the wire to a solid, unpainted ground. Plugs do deteriorate over time, and this can lead to low spark. There are rebuild kits available which are simple to install. The plugs are two-position--meaning they have a large spring which forces the plug tip into the combustor when the turbine is NOT running, and when the turbine is fired and accelerating CPD will push the plug tip out of the combustor to prevent burning the tip(s). If there is too much liquid fuel flowing into the combustor during firing and/or it's not atomized very well the spark plug tips can become so wet with liquid fuel they won't spark--which is another possible, but unlikely cause.

Atomizing air, .... is very important for most liquid fuel-fired GE-design heavy duty gas turbines. It's difficult to measure with the gauges supplied with the turbines. It's a very low pressure, but at a relatively high flow (from the roots-type blower used as the Booster AA Compressor). But, it's very important. I've seen loose couplings between the motor and compressor cause problems. The blowers are positive displacement (usually) so they can't be plugged or they will essentially rupture, but they do get a lot of moisture condensation inside of them which can cause the internal clearances to wear down quite a bit which can reduce the air flow considerably. Again, this is hard to measure, but it is critical. There are usually air-operated valves and even an occasional gravity-operated valve which should be checked and verified to be operating properly.

During (actually shortly after) firing, it should be possible to see "thin" white vapors coming out of the exhaust stack when there is a failure to ignite. Some of the vaporized fuel doesn't condense and does exit the stack in the form of a light ("thin") white vapor or "haze." This usually happens after firing is complete and when there is little or no exhaust temperature rise during firing. It is an indication that there is liquid fuel flowing, but not an indication that there is enough liquid fuel flowing, and is also an indication that there may not be enough spark (weak or non-existent).

Thick which smoke during or after firing on liquid fuel is unburned liquid fuel which is being vaporized--but not combusted--by the hot combustion gases from combustors which have ignited. It's usually an indication of liquid fuel nozzle or liquid fuel check valve or purge air check valve problems (if the unit is dual fuel (gas/distillate)) or the lack of AA during firing or weak spark or problems with cross-fire tubes (the two latter are unlikely except immediately after a maintenance outage).

So, that's about it: fuel, air, spark. In the right proportions, of course. My bet is that HFO has plugged most of the liquid fuel nozzles and/or the liquid fuel check valves. Also, if the liquid fuel pressures are not above the liquid fuel check valve cracking pressure (usually about 100 psig) then little or no liquid fuel is going to flow into the combustors. Which would be evident by little or no flow coming out of the false start drain valves. You've said the unit was running on HFO when it tripped, and it seems that it took several hours before a re-start was attempted. If the HFO must be heated when running, then it's a high likelihood that it has solidified in the liquid fuel system (lines; check valves; liquid fuel nozzles; flow divider; etc.) and is causing blockages which is limiting or preventing sufficient flow to the combustors to establish flame.

Please write back to let us know how you resolve the problem. Troublshooting is a logical process of elimination. Which sometimes take some time to eliminate what is not the problem until you find what is the problem--and some times, it's more than one issue causing the problem, too!
 
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