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Reciprocating Compressor Relieving Scenario


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#1 AMPIR

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Posted 04 August 2019 - 10:39 PM

Hi; 
For a reciprocating compressor, the recycle valve failure is one of the suction PSV sizing scenarios. As during  the reciprocating compressor start-up, the discharge Shutdown valve (SDV) should be kept open ( not same as centrifugal compressor which it should be closed), so failure of both recycle valve and SDV will be a double jeopardy.  I just wanted to understand in which case this SDV can be closed so we can define the recycle valve failure scenario for PSV sizing?
 
thanks 
 


#2 Art Montemayor

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Posted 05 August 2019 - 12:02 PM

AMPIR:

 

I highly recommend you supply a P&ID or a detailed sketch of your reciprocating compressor installation, “as is”.  It is virtually impossible to discuss a scenario without these tools - much less to hold a hazards review.  You should know this, and have one already on hand.  Please submit a copy.

 

I have frequently drawn a detailed sketch of reciprocating compressors and have submitted them within many threads on our Forums.  These are not hard to do.  For example, there is always a check valve installed downstream on the compressor’s final discharge line.  Depending on your setup, you may not want your discharge SDV open upon startup.  I’ve done literally hundreds of these startups and never did I have the final SDV open upon startup.  But your case may be different - like the fact that you don’t even mention a suction line SDV.  Are you simply compressing atmospheric air?  If so, then you don’t need a suction SDV.  All the air recycles back to the atmosphere.  But if you are compressing a pressurized process fluid, how are you preventing that same suction fluid isolated from entering the suction?  Also, where exactly is your compressor recycle stream entering the suction side of the compressor?  If it is at the suction drum (as it should be), then the suction side PSV mounted on such drum should protect the suction side of the compressor - with both the suction and recycle streams entering the drum instantaneously.

 

Await your P&ID.  Without it, this thread will convert itself into many guesses, opinions, ideas, and quandaries.  No member will be able to pin point exactly what the hazard could be or how to specifically respond to your issue.



#3 AMPIR

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Posted 07 August 2019 - 06:36 PM

Hi Art Montemayor;

 

I attached my sketch.

 

Cheers

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#4 Art Montemayor

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Posted 07 August 2019 - 07:13 PM

Your system is the first one in over 55 years that I've seen of a multi-stage, reciprocating compressor with a recycle control valve on the last stage feeding the suction of the stage immediately before it and not the suction of the first stage.

 

Is this system designed and operating by you?  If it is operating, what is the purpose of locating the recycle valve where you show it?  If the system is not operating, who designed it - and why does it have the recycle valve located where it is shown?

 

I can't say the instrumentation is wrong, but if you want capacity control, that is not the usual and practical way to do it.  But since I don't know the logic behind your instrumentation design, I can't make any judgment or give an opinion.

 

If I wanted to have capacity control on such a compressor and maintain the suction pressures at set values, I would simply recycle the final discharge gas back to the first stage suction, keeping it at a set value.   Tell us what you are trying to do with the recycle valve, so we can follow your logic.



#5 Bobby Strain

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Posted 07 August 2019 - 09:15 PM

Art,

    If you look closely you will see a recycle valve from the second stage inlet to the first stage inlet, which is a typical setup. I presume there are some controllers that accomplish the usual functions. As you noted, the sketch is not very informative.

 

Bobby



#6 AMPIR

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Posted 07 August 2019 - 10:26 PM

Dear Art Montemayor

 

The system is already designed and it is in operation. The Compressor feed is from a condensate stabilizer column. The column pressure is controlled by the first stage recycle valve and compressor speed ( split range). The compressors are tandem. 

The interstate pressure ( suction of the 2nd stage) should be controlled to maintain the final product RVP. 

 

I just need to determine the relieving scenario to calculate the PSV rate. 

 

Many thanks






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