Gas turbine base load decreased 146 MW to 140 MW after diverted to combined cycle. Why and how can we solve?
Please clarify: Did your GT load change from 146 MW to 140 MW (loss of 6 MW) or did your GT load change from 286 MW to 140 MW (loss of 146 MW)?
If you lost 6 MW (4%) in GT load you probably don't have a problem - it is probably due to the increased back pressure on the exhaust through the heat recovery boiler. Also, the exhaust temperature control curve is usually changed when changing from simple cycle to combined cycle. You should be adding about 70 to 80 MW from the steam equipment (turbine or process).
If you lost 146 MW, you have a very serious problem somewhere. I would need much more information to be of any assistance on that.
load decreased from 146 MW to 140 Mw (loss of 6 MW). the exhaust temperature control curve not changed when changing from simple cycle to combined cycle.
This back pressure caused also damage of the expansion joint in GT exhaust diffuser and we expect it will damage again due to this back pressure. Do we have to do something or we should accept?
If the back pressure is increasing enough to cause problems with the expansion joints it would indicate a problem in the heat recovery system. What fuel are you running on? Do you have soot blowers and, if so, have you used them? Is there any back pressure protection on the GT? (You didn't mention equipment suppliers; GE provides pressure switches [63EA and 63ET] to alarm and trip on high back pressure.)
Is there a stack cover on the boiler that maybe isn't fully open? You may need to do maintenance on the boiler.