Dear Experts,

My plant is a single shaft combined cycle unit, and it is a GE 9FA machine. noticed that the Turbine Exhaust Pressure has increased from 1.027barg to 1.037barg recently. The question here is what is the possible causes and the effects from the incremental turbine exhaust pressure? Unit will be scheduled for Hot Gas Path Inspection next year meaning the unit is running without any major outage while this "phenomena" rising. There is also trip logic if this turbine exhaust pressure exceed its limit. Really appreciate if there is someone to explain the importants of this turbine exhaust pressure to me for my understanding.

many thanks and regards
Fai

 

The usual cause for an increase in exhaust pressure is an obstruction in the exhaust system. In your case, I would inspect the HRSG tubing in the exhaust duct - especially the superheater and reheater sections. You might also inspect the stack cover (damper) to make sure it is fully open (I know there are limit switches that will trip the turbine if it is not fully open, but sometimes the damper moves a little without moving the shaft that the limit switches are on.)

You didn't say whether you had noticed any change in steam production, which could indicate dirty tubing; you didn't say what fuel you are using, but liquid fuels tend to produce more particulates in the exhaust gases which can foul HRSG tubing.

 

I've also seen several sites where insulation from behind either exhaust transition duct plating, or from exhaust seals (not F-class units), had been pulled/blown out and was found to be obstructing the superheater tubing as well. On one site, some insulation from boiler side walls between the superheater section and the HP section was found to have obstructed the HP section, as well (that was an oddly-designed boiler for a small footprint location).

But, as otised aptly said, if there is a decrease in steam production that would also be an indicator of some blockage/obstruction of tubes.

 

Dear Sir,
Thanks for the prompt respond. I've trend the steam generation and found that the steam flow is not significantly reduced, but I've noticed there's some power loss from the Plant. Could it indicate that Gas turbine blades and it casing cleareance got widened and caused the Turbine exhaust pressure to go high and power loss from it due to aging factor?

regards
Fai

 

If this were the case, the GT output would be down. Not very likely.

 

There is Possible build up of deposits on the LP preheater tubes on the gas side. Even if your Hrsg exhaust temp is above the flue gas temp dew Point there can be localised cold areas if condensate temperature is on the low side. I have observed sulphurous deposits on a mainly natural gas fired GT installation.