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Settle Out Concept Applicability For Reciprocating Compressors


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#1 SawsanAli311

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Posted 14 May 2020 - 02:23 PM

Dear All, 
 
I have analyzing reciprocating compressors and came across various discussion threads where specialists confirmed that settle out concept is not applicable for recirpocating compressors considering the machine's cylinder is having its own integral suction and discharge check valves and there is no retro flow through the machine as in the case of centrifugal compressors. 
 
However, on the other hand, I have been still seeing many Engineers claiming that reciprocating compressors do require settle out calculations which quite confuses me. The spill back control bypass valve which is employed for maintaining the compressor's suction pressure DOESNT open when the compressor shutdowns. Additionally, it shall be Fail safe open position and hence the suction equipment (scrubber or cooler) shall either be rated for the full design pressure of the discharge side or equipped with a PSV for its overpressure protection. 
 
My question is.. is there really settle out calculations applicable for reciprocating compressors.. do you foresee the requirement for having a bypass SDV/BDV which is to be opened when the compressor stops such that the pressure at the suction and discharge of the reciprocating compressor. Accordingly, designer himself would be imposing the concept of settle out although it may not be technically justfied. 
 
Thanks


#2 breizh

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Posted 14 May 2020 - 07:53 PM

https://www.cheresou...t-recycle-mode/

 

Hi,

You may find good pointers in the link above.

Breizh



#3 fallah

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Posted 20 May 2020 - 02:45 AM

 

 
I have analyzing reciprocating compressors and came across various discussion threads where specialists confirmed that settle out concept is not applicable for recirpocating compressors considering the machine's cylinder is having its own integral suction and discharge check valves and there is no retro flow through the machine as in the case of centrifugal compressors. 
 
However, on the other hand, I have been still seeing many Engineers claiming that reciprocating compressors do require settle out calculations which quite confuses me. The spill back control bypass valve which is employed for maintaining the compressor's suction pressure DOESNT open when the compressor shutdowns. Additionally, it shall be Fail safe open position and hence the suction equipment (scrubber or cooler) shall either be rated for the full design pressure of the discharge side or equipped with a PSV for its overpressure protection. 
 
My question is.. is there really settle out calculations applicable for reciprocating compressors.. do you foresee the requirement for having a bypass SDV/BDV which is to be opened when the compressor stops such that the pressure at the suction and discharge of the reciprocating compressor. Accordingly, designer himself would be imposing the concept of settle out although it may not be technically justfied. 
 

 

Hi,

 

The “settle out” pressure isn't applicable to a reciprocating compressor same as what is related to a centrifugal compressor.  The reason is that a reciprocating compressor is inherently a positive displacement machine and discharge gas cannot flow from the discharge back to the suction through the compressor same as what might easily happen in a centrifugal machine.  This is because both the discharge and suction valves on any reciprocating compressor cylinder are check valves hence don't allow gas to flow upstream.  



#4 Chemitofreak

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Posted 20 May 2020 - 02:54 AM

I 100% agree to what Fallah has said.

 

but once I was forced to calculate settle out pressure for a reciprocating compressor by my lead engineer. It was a proposal stage though.



#5 SawsanAli311

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Posted 22 May 2020 - 11:47 AM

Thanks all for your views..

 

I totally agree that settle out concept is not applicable in reciprocating compressors.. however, why designers tend to calculate it and set the design conditions of the suction equipment upstream the reciprocating compressor superficially. 

And do you think it is required to introduce an external quick opening valve bypassing the compressor and opens when it is shutdown.. is there the way for unloading the reciprocating machine.

 

Regards, 



#6 Sharma Varun

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Posted 27 May 2020 - 04:06 AM

In my experience a Major Licensor has specified for a Reciprocating type RGC to be designed to restart from settling out conditions. Further various equipment & piping are designed to take care of same.



#7 SawsanAli311

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Posted 12 June 2020 - 09:48 AM

Thanks Varun and All, 

 

But I would like to inquire your views on the below:

-good engineering practice calls for designing the suction side of each compressor stage to the design conditions of the discharge side specially in presence of the spill back capacity control valve which can fail open inadvertently. do you agree to the same or you can select your suction scrubber/vessel of lower design pressure with a PSV set at a lower rating. 

- For 4 stages of compression, API 521 section 4.4.2.3 clearly states that any loss of downstream compression stage will lead to shifting the compression load to the preceding stages.. does that mean that the setting of the discharge PSV at every stage shall also take into account this scenario which makes the minimum margins reported in table 4 of API 618 need to be revisited to allow for such scenarios? 

 

thanks again for your contribution 


Edited by SawsanAli311, 12 June 2020 - 09:48 AM.





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