We just finished our Gas Turbine M.I. and HRSG shutdown. HRSG s/d was routine annual shutdown to meet with statutory requirements. Generally during previous HRSG shutdown, we use to clean our finned tubes of HRSG by air compressor. This time as per some recommendation we opted for dry co2 cleaning. After startup we have observed that our steam generation has reduced. We have Frame-6 GT of 30 MW capacity with exhaust at 560 Deg.C.

Before S/D we were getting @ 55 TPH steam at 24 MW, now we get 51~52 TPH steam at same load. Our steam/MW ratio was 2.25~2.3 now it is 2.1 also our final stack temperatures with Condensate preheater in service was @168~170 Deg.C. Now it is 190 Deg. C and it is a burning issue for operation dept. Please share views/comments/remedies.

 

I would not expect the cleaning to cause a problem unless ofcourse they blasted a load of fins off the tubes?

How badly clogged were the fins that they needed cleaning?
Are the mass flows and temperaturs you discussed based on pre-outage values, or on previous post overhaul values?

It sounds like the hot flue gas is able to bypass the finned sections of the boiler - Was any work done on the baffles that seal the finned tubes at the side of the gas path to casing, or the top / bottlom headers to floor of the gas path?

Could there be internal man doors left open in the gas path maybe allowing gases to flow in the dead spaces instead of normal gas path?

I'd be interested to know what you find.

Regards

 

Dear sir,

I have interested to help you to resolve this problem, But i need to check the parameters before MI & after MI.

If possible email all possible graphs of HRSG & GE.

 

Hi DRP,

1)Is there any change in GT parameters like Exhmass flow, Exh temp after MI? Do you have any other GT with same config and completed MI for comparison?

2) Did you get your FT recalibrated again to rule out FT error?

3) Do you have other FT to compare this FT reading? Is the difference between this two FT's same before and after MI? Normally we have two FT's at a point, do you have any other FT's.

4) What about the Preheater inlet and outlet temperatures, other parameters on water side, do you see any change? If your preheater works fine and still the Flue gas temp. is high, then the fins might be damaged during the cleaning.

 

DRP... if you mean dry ice pellets or shavings, then what was minimum pressure of blast gun?

Phil Corso

 

> I would not expect the cleaning to cause a problem unless of course they blasted a load of fins off the tubes?

> How badly clogged were the fins that they needed cleaning?
> Are the mass flows and temperatures you discussed based on pre-outage values, or on previous post overhaul values?

---- snip ---

> I'd be interested to know what you find.

Thanks for you response.
We inspected HRSG internally after dry co2 cleaning. There is no damage to fins.
Also we do not have any baffles in HRSG except main screen before Superheater which is also not damaged.

All the manholes are confirmed to be closed.

One thing we have noted that our GT exhaust pressure has reduced. (By @ 50 mm of WC) compared to previous readings.

 

>> I have interested to help you to resolve this problem, But i need to check the parameters before MI & after MI.

> If possible email all possible graphs of HRSG & GE.

Dear Dir,

What parameters do you need and please tell me your mail ID
Thanks

 

> if you mean dry ice pellets or shavings, then what was minimum pressure of blast gun?

Sir it was @ 7 kg/cm2

 

Sir,

Please Forward me HRSG's all parameters like Gas I/L temp, Gas flow, Stack temp, water temp, steam temp, all banks temp diff, if available gas flow, etc

 

> What parameters do you need and please tell me your mail ID

[email protected]

 

DRP... In my experience, pressure was held above 12.5 kg/sq cm. Also, effectiveness very dependent on nozzle selection.

Phil

 

Would be interesting to confirm that the heat absorption in the low temperature economizer is less than before the outage.

Could the economizer recirc. system be operating differently than before?