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Compressor Polytropic Head


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#1 colt16

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Posted 28 March 2017 - 03:01 AM

Dear all, 

 

Have been checking the compressor performance in recycle gas loop for one of our units and found that compressor polytropic head is operating much higher than design. ~40% higher. 

 

I would like to check if this is the result of increase dp in the recycle gas circuit?

 

The equipment DP all seem normal so I am wondering if the design hydraulics is an issue. 



#2 Saml

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Posted 28 March 2017 - 06:58 AM

What kind of compressor? Centrifugal, Reciprocating?

What do you mean by "The equipment DP all seem normal". It means "as it has been during the last year" or "as design PFD".

What equipment? The compressor or the rest of the loop equipment



#3 Bobby Strain

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Posted 28 March 2017 - 09:55 AM

You should compare your current operation with the compressor performance curve. Assuming it is a centrifugal machine. As to differential pressure, you should have all the data; we have none.

 

Bobby



#4 colt16

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Posted 29 March 2017 - 05:30 AM

Equipment dp meaning all the other equipment in the loop seems normal.
The compressor is centrifugal type.

We have the curves but gas molecular weight has changed.

#5 Bobby Strain

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Posted 29 March 2017 - 09:03 AM

Gas molecular weight won't change the polytropic head curve.

 

Bobby



#6 Saml

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Posted 31 March 2017 - 11:29 PM

You are not providing much details, so I will be generic here too:

- If molecular weight has changed, you will have difference in the flow meters if they are oriffice plates, flow nozzles or annubar. You should be  correcting for molecular weight.

- I don't understand the fact that Delta P is "normal" but "head is higher". If the pressure drop is normal, but your calculated head is higher this means that the MW is lower. That is, for the same pressure drop you have more height. In this case, you have an increased pressure drop. Pressure drop chenges with the  MW for the same actual rate.

- If the compression ratio is low, what may be the case in recycle compressors,  temperature readings can have a big impact on head calculation. Check also the efficiency calculation.

- There may be an error in the calculation. If you are using Excel, it is not difficult at all to introduce an error in a poytropic head formula.



#7 Bobby Strain

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Posted 01 April 2017 - 10:59 AM

Try this online calculator. I was a bit hasty with my earlier comments regarding the effect of gas molecular weight. It is the polytropic efficiency that is unchanged by the gas properties. So, on this basis, you can calculate the compressor parameters for your operation. The compressor will always exhibit the characteristic head curve for your machine as a function of inlet flow. At any given flow, the head is obtained from the manufacturers curve. So your measured values can be compared to the curve(s). If the measured and curve values differ, then your machine is likely worn or damaged in some manner. Best to refer to an authoritative source for the proper equations.

 

Bobby

 

http://www.elliott-t...ssor-calculator


Edited by Bobby Strain, 01 April 2017 - 01:06 PM.





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