We operate a 330MW cobined cycle power plant comprising 2 GE Frame 9Es and an STG rated at 110 MW each. When operating in Open cycle mode the exhaust backpressure AFPEP indicates 4mm H2O. During combined cycle peration, it drops to -2mm H2O. What are the normal operating values of exhaust backpressure? Should these values vary depending on operating mode(simple vs. combined cycle)? What is the essence of exhaust backpresssure and what is it indicative of? Does it have any effect on gas turbine power output?

 

Motion Control? ... Motion Control ... ???

Exhaust back-pressure restricts flow through the unit--which reduces output. Higher back-pressures cause lower power output.

Usually, open, or simple, cycle means the turbine exhaust is NOT directed through a "boiler", sometimes calles a HRSG (Heat Recovery Steam Generator). This is usually done via a "bypass stack" and some kind of damper or door which directs the exhaust through the bypass stack instead of through the HRSG.

When turbine exhaust is directed through a "boiler", backpressure is created because the exhaust gases must pass over/through the boiler tubes and then up the exhaust stack. USUALLY there is little or no restriction in the bypass stack, depending on the design of the duct work and bypass stack.

IN GENERAL, one would think that back-pressure would be less when the unit is being operated in simple cycle mode than when it was being operated in combined cycle mode--but that's a generalization that's made without the ability to understand the exact layout and construction or your plant.

This author has seen the exhaust duct pressure probes incorrectly placed and installed and cause all kinds of problems WITH INDICATION--and if the unit uses CPR instead of CPD to bias exhaust temperature control the indicated exhaust pressure can adversely affect turbine operation at Base Load.

It is recommended that the placement and installation of the exhaust duct pressure probes be very closely reviewed--after the exhaust ducts are examined to make sure there are no loose pieces of metal in the bypass stack which could be causing a higher-than-normal exhaust back-pressure. This author has heard of induced draft fans used in HRSGs to decrease exhaust back pressure, but never on units as large as Frame 9Es--but anything's possible with time and money, eh?

EVERY installation of a gas turbine has a slightly different exhaust duct pack-pressure.

markvguy

 

I've also seen delta pressure transdcuers plumbed backwards giving reverse readings.