Bypass valve across Anti-surge valve 2

[edm123]

What can be the purpose of individual bypass valve (100% sized) across each anti-surge control valve (100% sized) on 2 x 50% process gas (LNG) single casing centrifugal compressors driven by Steam Turbine operating in parallel configuration with load sharing feature and on discharge pressure control? (That means there are total 2 bypass valves and 2 anti-surge valves for 2 single casing compressor trains.)

The Compressor control strategy states Performance Controller (i.e. load sharing) should keep operating point of all compressors with equal surge deviation.

Can it be to assist during start/stop? If yes then how?

Can it be for load sharing? If yes then why Anti-surge valve can not carry out the same function?

I don't have any documentations in FEED indicating the design basis of this bypass valve.

Thanks in advance.

 

[LittleInch]

Got a P&ID?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[edm123]

It's 3 Compressor trains (3 x 33%) running simultaneously.
I have attached PFD.

Sorry, I didn't read the note in P&ID which says "Recycle Flow Control for Turndown operation shall be sized for 100% recycle of rated capacity of Compressor." So the answer lies in the note itself.
P&ID screenshot is attached (can't attach complete P&ID due to privacy).

Now with this information, another question comes up "Why can't ASV be used for turndown?"

Which out of the two (bypass and ASV) may be used for load sharing?

Thanks

 

[edm123]

Attached P&ID screenshot

 

[1503-44]

You do not want the surge control active when you are recycling, normally at low flow conditions, such as during startup and shutdown.

"Surge is a condition that occurs in compressors when the amount of gas they are trying to compress is insufficient for the size of the compressor and the blades lose their ability to transfer energy from the shaft to the fluid, causing a reverse flow of the gas. This condition can have catastrophic effects on the machine, so compressor manufacturers include anti-surge valves that recycle gas from the discharge to the suction when a low flow is detected. Usually compressors are designed such that these valves are only open on startup or under reduced rates."

See

https://www.emersonautomationexpert...ms/controlling-surge-centrifugal-compressors/

Turndown to flow rates and pressures that cause surge cannot be controlled by the surge valve. You must get back into the normal operating envelop.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Well it possibly could but 100% recycle could be a very different set of pressure and flow conditions than an A/S valve.

So e.g the recycle line might be set to have a high discharge pressure so that operations can start.

You need to find the process description or operating philosophy to find out why and also the valve data sheet to see what CV the valves are.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Heaviside1925]

OP,
I would suspect that this decision was compressor vendor driven, meaning each compressor should have its own ASV with its own control scheme to protect each individual compressor, whereas the bypass is part of a system control scheme for all 3 compressors. Could, you physically perform both functions with one valve? probably, but I think the push back you are going to get, is the compressor vendor is going to want to protect each individual compressor, independent of the other compressors.

 

[georgeverghese]

Your first diagram shows the individual capacity recycle valve as being operated by a dedicated FIC, which presumably has no input from the master performance controller ? This arrangement, in either case, gives the operator the ability to load the compressor as he wishes, so you could have unequal loading of each train. So feed flow variations could be handled in one train by speed control, while the other runs relatively unaffected by feed flow variations, and its speed remains unaffected.

I've seen decoupling interactions during surge flow zone operation on parallel trains handled by antisurge controllers talking to each other. In this scheme, it looks like the decoupling algorithm is housed in the master performance controller, since " The Compressor control strategy states Performance Controller (i.e. load sharing) should keep operating point of all compressors with equal surge deviation." Check with documentation from controls systems programmer, which may be CCC ?

From what you've shared so far, it does sound like the ASV on each train is a combination ASV and capacity recycle valve, while the individual FIC - FCV gives you the ability to do split loading as the operator wishes. I would use the FIC - FCV as much as possible to do unequal loading in order to get better control stability and to also suit operational requirements.

 

[edm123]

Thank you all for responding