Please i would to someone to give and cover or explanation Gas Turbine Exhaust Pressure.

i have gas turbine frame 5 mark VI heavy duty gas turbine. the Exhaust Pressure 96EA reading the pressure negative?

 

The reading should be positive, at least any time the unit is running. (If shut down, and there is wind blowing across the top of the stack, you might get a small negative reading.)

You don't say what, if any, equipment is in the exhaust system. If you have heat recovery equipment like an HRSG, the exhaust back pressure might be 20 inches (50 cm) of water or more. If you have silencers, maybe 6 to 8 inches (15 to 20 cm) of water.

The transmitter is measuring the gauge pressure just downstream of the exhaust plenum.

So if it is reading negative, and the transmitter is good, then you likely have the transmitter incorrectly tubed up or maybe an isolation valve is closed.

 

need to more explanation please ..
what time the pressure become negative or positive ?

 

Hai

Gas Turbine Normal operating condition, exhaust gauge pressure in the terms of +ve only. So, positive draft is provided by the Gas turbine and flue gas will sweep out by the natural draft system (Stack).

Same thing if you compare with the conventional boiler, Air will supply to the furnace by the Forced draft fan and Combustible gases carry out by the Induced draft fan through stack (draft creation is the sum of the fan and stack), this arrangement called- Balanced draft system. Normally Induced draft fan capacity (Power consumption) is 1.5 times of the forced draft(Supply air fan) capacity. The capacity of the Induced draft fan is based on the amount of flue gas flow to be handled by the fan as well as the temperature of the flue gas at the suction of the ID (Induced draft) fan.

Amount of flue gas flow is sum of mass flow of air and fuel. So, amount of flue gas flow is based on the air to fuel ratio. Air to fuel ratio with conventional boiler (coal fired) is vary from 7:1 to 12:1.

But in the case of Gas turbine Air to fuel ratio vary from 40:1 to 60:1. So, amount of flue gas flow to the stack is very high for a kg of fuel combustion in the Gas turbine. To carry out this total flue gas by using an external fan will consume very very huge power. So, Natural draft (Stack) is using to carry the flue gases by maintaining the high stack height. Natural draft is based on the height of the stack.

More stack height also not possible Due to withstanding/constructional problem, one compromise need to maintain balanced draft by providing the positive pressure at the gas turbine exhaust (At what location in the Flue gas path in the HRSG contains -ve pressure of the flue gas-need to be checked. But at any load condition -ve pressure is not possible at the gas turbine exhaust due to lower quantity of flue gas flow also).

Both conventional boiler and HRSG exhaust temperatures will be the same at stack inlet. So, main difference will be the amount of flue gas handle by the stack due to high air to fuel ratio with gas turbine due to high calorific value.

So, At gas turbine normal operation, Exhaust flue gas pressure (gauge) is always positive.

If gas turbine not in service- IGV min.opening for GE-7FA is 25Deg, due to this opening exhaust flow always will maintain 14kg/sec unless stack damper close. If the stack damper is in open condition Stack will create more -ve pressure at the Gas turbine exhaust due to lower quantity of air flow (14kg/sec at stand still and 440 kg/sec at base load) through the Gas turbine compressor.

I don't know at what condition you observed the -ve pressure at the exhaust of the gas turbine.

If the gas turbine exhaust pressure -ve at the time of normal operating condition, suspect the transmitter (why because at any load condition -ve pressure is not possible at the gas turbine exhaust due to lower quantity of flue gas flow also).

More than the design expansion in the gas turbine- simply impossible.

I hope it is useful for you,

Waiting for your reply..

 

Whenever the gas turbine is rotating (in the correct direction) the measured exhaust pressure should be positive. The air is flowing from the GT exhaust plenum up the stack to the atmosphere; therefore the pressure at the exhaust plenum must be positive. Otherwise the air would flow down the stack!

 

still i am convenience with reading please can you tell me which condition that reading become negative or positive ?

I hope CSA clarify but topic ?

 

Maint, dude,

What part of: "The exhaust back-pressure should be positive unless the gas turbine is rotating in the wrong direction" don't you understand?

Because that's all everyone has been trying to tell you: It should never be negative. It "might" be negative if there's a big updraft when the gas turbine is shut down and the HRSG and stack are hot, but it would probably be such a small negative value that the transducer wouldn't even report it.

I believe if you look at the Device Summary for the unit you will see that the exhaust back pressure transmitter (usually device 96EA-1) should be calibrated from zero to some positive value, not from a negative value to a positive value.

So, there's something wrong with the differential pressure transmitter calibration, the tubing connections, or the type of probe(s) used in the exhaust duct, or the placement of the probe(s) in the exhaust duct, or the location of the differential pressure transmitter, or the valving to the sensing lines of the differential pressure transmitter.

Usually, these transmitters measuring exhaust duck back-pressure are differential pressure transmitters, with the low side port "connected" to atmospheric pressure, and the high side port connected to the pressure probe(s) in the exhaust duct. And, an examination of the piping schematic (P&ID) for the differential pressure transmitter in question (usually the Performance Monitor Package piping schematic (P&ID)) would also show how, and where, the differential pressure transmitter should be located in relation to the probe(s) in the exhaust.

There have been other posts on control.com about exhaust duct pressure sensing. Search for "+pancake +probe" (without the quotes).

But, get real, dude. A gas turbine is going to exhaust to atmosphere at some pressure higher than atmospheric, because poop flows downhill unless it is pumped uphill. (In other words, flow is always from an area of higher pressure to an area of lower pressure.)

Use the documentation provided with the turbine to help you understand how the equipment works. The Device Summary and the piping schematic drawings (P&IDs) are about the most important pieces of documentation provided with the turbine and have a TON of information.

And, neither are difficult to understand. One is calibration settings, and the other is a graphical representation (and a pretty simple graphical representation at that) of systems and instrumentation.

 

Dear gents,

We are operating GE 7FA DLN2.0 Gas turbines, frequently we are facing the problem with negative exhaust pressure reading. we found the moisture buildup in the sensing line. now we have provided one drain v/v in sensing line & time to time we are draining it & it works fine!!!!

Regards
Parag