Help - scenarios identification 2

[sheiko]

Hello,

Attached is a scheme representing my situation.

I need to find credible scenarios for sizong PSV-1, PSV-2 and PSV-2.

Could you please share your ideas?

Thank you!

"We don't believe things because they are true, things are true because we believe them."

 

[zdas04]

Sorry, when I have to do this I always start with a bit more data than you provided. You didn't even say what the gas was or what the compressor technology was.

I do this kind of analysis for a living (as do many of the people here) and I have to say that what you provided is not an adequate starting point.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"It is always a poor idea to ask your Bridge Club for medical advice or a collection of geek engineers for legal advice"

 

[sheiko]

Hi,

Thanks for fast reply.

The scheme indicates the type of electrocompressor: reciprocating

The gas is Natural Gas.

My intent is first to identify possible credible scenario based on the configuration, not yet to quantify the contengencies.

Morever, i would like to know if you already saw such a system? i believe some PSV are useless (as PSV-1) but i can't find argument.

Please let me know which data are missing for example?

"We don't believe things because they are true, things are true because we believe them."

 

[sheiko]

Please also note that the valve are on/off (not control valve).

"We don't believe things because they are true, things are true because we believe them."

 

[Abulafia]

Hi sheiko,
i work for an API 618 recip. compressor company in italy.

I agree with you, psv on compressor suction side aren't useful becuase the problems for recipr compressor is the low pressure on suction side, (shaft broke) no one proble can be caused by increasing of pressure. (decreasing of current consuption)

In my opion you can delete PSV-01, PSV-02, PSV-03 (why psv-02/03 that are downstream of psv-01 have a set point lower then psv-01?)

Note: your scheme isn't correct, beacuse the outlet of 1st stage shall be the inlet of 2nd stage, 1st and 2nd stage aren't piped in parallel but in series!

 

[CMA010]

Compressor C-2 seems to have stages in parallel. Don't you need a PSV in the discharge of the first stage of compressor C-1? Assuming that MV-5 and MV-3 are by-pass valves and are normally closed, inadvertent closure of MV-4 would lead to overpressure.

 

[sheiko]

Thanks.

Abdulafia,
Are stages always in series? isn't it possible to have them in parallel? I ask the question because this is the representation on the actualPID i have at hand...

CMA010,
Good point for the missing PSV, but there is also a vent V-2 (or blowdown? i don't know how to translate it...).

I was also wondering why could PSVs be located at suction side of electrocompressors?



"We don't believe things because they are true, things are true because we believe them."

 

[ajs1972]

Hello Sheiko:

I just thinking in possible scenarios of overpressure in compressor´s suction:

- Overpressure could causes leakage on suction side and pulls air into system creating a flammable atmosphere.

- Reverse Flow resulting in overpressure of low pressure side and loss of containment. Like CMA010 marks, in first stage of C-1 compressor, there is no PSV, this scenario is possible if MV-4 fails.

-Fire on the suction line could derivate in overpressure scenario.

I´m not agree with Abulafia, the compressors system are in series, but the diagram is confuse, but obviously, could be a dangerous scenario if the valves MV-3 and MV-5 fail open.

 

[ajs1972]

And the last point I think the PSV-1 is because if MV-2 y MV-8 fail close the inlet pipe to compressor´s system is protected.

 

[ajs1972]

Sorry, I didn´t see the C-2, in this compressor, I agree with Abulfalia, this scheme is not correct. Anf finally, another scenario where could be necessary one PSV is if MV-7 fails, I think so.

 

[CMA010]

A vent with an RO and an on/off valve are not considered to be an alternative for a PSV.

In my experience the only reason for a PSV located in the suction would be if the upstream system has a higher design pressure than the suction side of the compressor.

Your stages have to in parallel because the outlet pressure of each stage seem to be different (Set pressure of PSV-5 = 141 barg and the set pressure of PSV-6 = 282 barg).

Furthermore there is a difference in the set pressures of PSV-4 of compressor C-1 and PSV-6 of compressor C-2, which is somewhat odd in my opinion since both compressors discharge to a common line.

 

[CMA010]

Correction: The stages have to be in series in stead of parallel of course.

 

[sheiko]

ajs1972 thanks a lot.

You mentionned a scenario when MV-7 fail...but close or open or each?

"We don't believe things because they are true, things are true because we believe them."

 

[ajs1972]

When MV-7 fails closed.

 

[dcasto]

your system needs lots of work on it. On C-2, how can you have that compressor with parrallel cylinders in operation only?

On C1, there is no relief valve upstream of the 1st stage cooler, what happens if the coller freezes or plugs off?

if the C1 and C2 have pressure limits of 180 barg, then why have a 183 barg psv (1). The only reason to have a PSV on each unit suction is if there is a way the gas can go from the discharge back to the suction, there is only 1 path, thats back through the cylinders and their check valves.

there are other issues, like maop of cylinders, coolers, piping ect..