If a combustion turbine experiences reduced power output and heat rate, the usual suspect is compressor fouling. But what if cleaning the compressor through on-line and off-line wash is not enough to recover lost compressor efficiency?
Some users experience no gain in gas turbine output or improvement in heat rate even after a Major outage. A likely suspect is the first row nozzles; refurbished or degraded nozzles may be contributing to the problem. In many large combustion turbine frames such as the GE 7FA, the amount of compressed air used for cooling has a significant impact on gas turbine efficiency. The first row nozzles in a 7FA can use up to 20% of the air developed by the compressor for cooling. Turbine efficiency can be greatly reduced if these cooling holes are enlarged and use too much air for cooling. First stage nozzle cooling holes may not have been checked for hole dimensions during the refurbishment process. This can also affect your ability to tune low NOx combustion systems. The amount of air being used by first row nozzles affects the fuel/air ratio, making it more difficult to tune the unit.
The only solution is to remove the first row nozzles and replace with new or refurbished first row nozzles with correct cooling hole dimensions. It is also worthwhile to check the first row nozzles when they come back from the repair shop to ensure the cooling holes are not blocked and are the correct dimension before installing them into the unit.
Your fuel bill (and possibly your EPA permit) will thank you for it.