Dear Sir,

In our CPP, there are 6 GTs (124 MW each). A few days before we were going to start up the gas turbines, it reaches to FSNL and suddenly flame (flare) and black smoke came out from the GT exhaust chimney. So I want to know in which conditions this type of incident will occur.

 

Dear,
In your CPP we have no idea of the configuration of the plant. Are there bypass stacks (to direct the exhaust around the HRSG (Heat Recovery Steam Generator)? Is the HRSG fired? Meaning, does it have Auxiliary Duct Burners which are fed from natural gas or some kind of liquid fuel or both?

Because, besides describing a rather large problem (like insufficient purge time or leaking Aux. Burners or multiple start attempts or something similar), you haven't really given us much to help you with.

Did this just occur after some kind of maintenance outage? Was it after some kind of testing or repair?

We just don't have enough information to be able to say more than, "GEE! You had a problem!"

The best replies are given to those requests that provide the most information. This ain't one of those requests.

Hence, the reply ain't so good.

Give us more to work with, and we might be able to provide a more intelligent response to what was a basically an unintelligible question.

 

Dear Girish,

The following incidents may not be direct relevance to you, but they are of significance for other readers. The set up in your plants may be different.

1. We had common drain tank for 2 sets of Gas turbines, HRSG supplementary firing skids. One day we found that naphtha flooding in acc. compartment of GT. We were lucky to get away without fire incident. Upon checking the HRSG supplementary firing skid drain was found passing. The naphtha got filled in drain tank and back flowed into GT drains then into acc. compartment.

2. In other power plant recently there was an incident of fire in GT exhaust duct upon GT start up on gas fuel. GT earlier tripped due to condensate ingress in gas fuel.

So drains connected to a common sump/tank is the root cause for these two incidents.

As you have mentioned your observations were about at the start up of GT, if the fuel was gas condensate carryover/accumulation may be the cause. If the start up was on liquid fuel it may be due to fuel accumulation or back flow from the drains probably from the exhaust drain, if it is connected to the same sump as the turbine drains.
Moreover the flame and smoke from exhaust duct will indicate that the fuel burnt was in the path after combustion and turbine path. The exhaust temperature and GT exhaust pressure data will give some hint.
I don’t think that any fuel came from HRSG supplementary firing, as the GT was in start up mode, and GT exhaust must be going to bypass stack.

 

Dear sir,

Our CPP is in the commissioning stage. In our CPP, there are 6 Frame 9E gas turbine generators. All are commissioned but not tested at base load. Because the refinery is in the commissioning stage, we run all 6 gas turbine generators one by one at the interval of a few days. A few days before we were going for the normal start-up of GT-3. After the auto (start) command given, the M/C reaches up to the FSNL normally and then suddenly flame was coming out from the GT exhaust. Also, the vibration came two times and the M/C also overspeeded. Finally, the M/C tripped on exhaust over temperature and also the EPB pressed from Mark-6.

The HRSG was not in line. The M/C was just commissioned and normal start-up was going on (the M/C was not under maintenance before start-up).

 

Mr Girish,

The main reason is fuel (liquid) accumulation in GT exhaust. Did you physically inspect the exhaust? You must check it before the next attempt. Otherwise you have to order a new system.

You did not mention the fuel fired.

If it is gas fuel condensate, carryover is the culprit.

In case of liquid fuel,

1) Unsuccessful attempt (fail to ignite), let some fuel exhaust. You must purge the GT for a few minutes.

2) Another probable element is liquid fuel check valves. Normally at 8 bar they allow liquid in to the combustion cans. If one or few check valves cracks at low pressure, i.e. 3 to 4 bar, then liquid will enter the turbine as soon as the liquid fuel stop valve opens. If liquid fuel stop valve has passing, then liquid will be going into the turbine as soon as fuel forwarding pump starts. Liquid fuel pump starts when L4=1.

Fuel accumulation after last trip/shutdown can not be ruled out.

 

We still haven't been able to intuit what fuel was being burned at the time. And, if you have Mark VI turbine controls, you should be able to tell us every alarm (Process and Diagnostic) that was annunciated from the time the unit reached 95% speed until the trip occurred, which would help with the determination of what went wrong.

If the unit oversped and also "tripped" on exhaust overtemperature it was probably caused by a failure of the fuel stop valve to shut off the flow of fuel.

I've seen this happen at two sites running liquid fuel when the liquid fuel forwarding pressure was well in excess of the maximum stated value. I've never heard of something similar happening on gas fuel, but that doesn't mean than under certain conditions something similar couldn't occur.

But this is all speculation, and we shouldn't be speculating. If the unit is undergoing commissioning the packager/supplier should be investigating the incident and working to ensure it won't happen.

It would be very good of you to write back and let us know what is finally determined. Again, we really don't have enough information to be speculating any more than we already have, and it's all just speculation because you haven't provided very much information and we can't see all the data from the Mark VI, including the Trip Log/Trip History/Trip Capture buffer info.