Fuel Gas Skid for Gas Turbine

[norzul]

Hi Guys,

I'm currently involved with the cogeneration project. Now under construction phase. We're going to install 4 GTG and 1 STG. Our GTG got the supply of NG for the national grid.

Each GTG has its own fuel gas skid consists of filters, dual SSOV (safety shut-off valve) and pressure control valves.

My question, for the control valve is it common to have self regulated valve or instrument air (IA) regulated valve. Someone say if using IA, the risk of tripping the GTG is higher with the failure of IA supply.

Any feedback is appreciated

Thanks

norzul

 

[monaco8774]

We have lot sof turbines all of which use air regulated valves, you will probably find the turbine needs air anyway for other on skid instruments, purging on cooldown stop and all sorts of other reasons. self regulated valves in my opinion are just as likely to drift and fail as your instrument air supply.

If you are in a remote area and dont have an air supply, if your fuel gas quality is good you can even use instrument gas actuated valves. But I would check you dont need air anyway before using this as a reaon to go to self actiated PCV's.

 

[toothless]

Our gas skid valves are controlled with hydraulic oil. This hydraulic oil comes from a gear driven pump and backed up with an AC-powered auxiliary pump. We do NOT use IA to control the gas turbines.

 

[ScottyUK]

toothless,

Are you describing the throttle valve arrangement, or the local pressure regulator? Our turbines - MW-701DA's - use a pair of electro-hydraulic servo valves in the throttle arrangement. One valve serves as the throttle, the other maintains a constant DP across the throttle to linearise the characteristic and additionally acts as the OST valve.

There used to be a local regulator driven by instrument air and controlled by a downstream PT via the DCS, but it was sluggish and troublesome. Its main function was to allow startup at a reduced gas pressure, but it caused more problems than it solved. It was scrapped and replaced by an altogether more reliable spool piece.


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I don't suffer from insanity. I enjoy it...

 

[norzul]

Dear All,

The link below will direct yu to my technical blog explaining the SSOV application. Appreciate your feedback.

http://technical2005.blogspot.com/

Thanks

norzul

 

[BSN80]

We also use the instrument air for the regulation of gas fuel valve.So far our CT has never tripped as a result of instrument air failure and i also think probability of instrument air failure is no higher than failure of any other critical circuit in plant.Usually most facilities like us have standby compressors and still if there is any failure then its better that the gas turbine gets tripped as most of the instruments in the rest of the plant will stop responding as result of this failure .

The only concern which we have with using instrument air is Slow response of valves which sometimes cause the machine to overspeed whenever the generator breaker opens on any fault......

 

[byrdj]

"...only concern which we have with using instrument air is Slow response of valves which sometimes cause the machine to overspeed whenever the generator breaker opens on any fault......"

I was recently ask to review a ST protection that used Pnuematics to trip SV. the actuator had fast dump valves, but someone had decided to increase the local regulator from the required 30 psig need to open to 90 psig. thus trip time increased geatly. After reducing air to design, trip time went from many seconds to .2s