Since we did Combustion Inspection, we have been having this problem.We change all the liners and the cross fire tubes. The TC readings is as follows:
TTXD_1-394; TTXD_2-417; TTXD_3-417; TTXD_4-413; TTXD_5-415; TTXD_6-428; TTXD_7-425; TTXD_8-419; TTXD_9-415; TTXD_10-417; TTXD_11-416; TTXD_12-402; TTDX_13-400; TTDX_14-382; TTDX_15-372; TTDX_16-395; TTDX_17-394; TNH-101.16%; CTD-364deg C; TTXSPL-159.0; TTXSP1-56.1; TTXSP2-45.0; TTXP3-34.2; TTXM-408; WATT_EX-57.1

This is what we got after changing 7no thermocouples, and checking all the thermocouple lines. The Unit still alarm combustion trouble as it is approaching 60MW. Please what could be wrong?

 

Did you check the Maintenance reports? I would check combustion parts gaps as found/as left after inspection, nozzles/burners flow test (very important!), seals gaps, perhaps you are over the gaps tolerances in your combustion parts, etc..

TTDX_14-382 and TTDX_15-372 are your coldest temp and seems to be the origin of the alarm... Did you check the nozzle/burner relative to this temp? If water or steam injection, I would also check the "donnunts" or whatever you call the mixing device... hope it helps..

 

How could you get the alarm if TTSP1 < TTXSPL?

 

>How could you get the alarm if TTSP1 TTXSPL?<

Thanks for the feedback. As the unit gets to 60Mw the allowable spread moves closer to spread 1.

 

Contrary to popular opinion, exhaust temperature spreads (especially those which occur after maintenance outages!) are not controls-related and can't be fixed with a control system modification.

The unit didn't get put back together properly, or there is a large mismatch in fuel nozzle flow-rates and the nozzles with the highest and lowest flow-rates have been placed next to each other.

Whee they GE liners or a third-party manufacturer?

Whee the liners new or refurbished?

Whee the fuel nozzles new or refurbished?

Do you have the flow-test data for all of the nozzles? Do you know which combustor each liner was installed in? Have you plotted position versus flow-rate tests to see if the highest and lowest tested nozzles are nearest each other in the machine?

Have you looked at the cross-fire tubes when the unit is running to see if any of them are hot (sometimes glowing red)? If the cross-fire tubes are properly installed and there is combustion gases flowing through them (and there should not be any flow through the cross-fire tubes during normal operation after flame is established when starting) then they will be very hot; very, very hot--again, sometimes glowing red.

The exhaust temperature spread will appear to "move" when the problem is mechanical in nature. That's because as the unit is loaded and the IGVs open and fuel flow increases the combustion gas flow through the turbine and into the exhaust will change. The cold spot will appear to move "with rotation" (i.e., in an anti-clockwise direction looking downstream) if it's mechanical in nature.

However if the spread is very high at Full Speed-No Load and gets worse with load, you may not see the cold spot move very much as you're only at about 50% load and the IGVs may not have opened very much.

If the cold spot doesn't move with load, then it could very well be a T/C or control system problem, though not very likely. But, if the cold spot moves with changes in load then the problem is most likely something wrong with the components used in the inspection or with the way they were reassembled.

As much as people don't want to believe it, most spreads which occur after a maintenance outage are the result of poor quality practices or poor quality parts, or both. Unless something was done to the control system or it coincidentally experiences a failure of the exhaust T/C inputs, high exhaust temperature spreads after a maintenance outage are not control system-related.

 

Hi,
Have you changed fuel nozzles? Chocked fuel nozzle sometime may create this problem.

G.Rajesh