Centri compressor performance prediction

[Wkcoo]

OEM Performance curve for centrifugal compressor is for one specific suction condition. How do we calculate or predict the performance from the OEM curve if given another set of suction condition? Pumps have affinity law and viscosity correction which can give fairly accurate prediction, but I am not sure for compressor. Some reading shows invariant map (reduce to dimensionless curves) for anti surge control purpose but not sure if this can predict the new performance. Any input is appreciated!

 

[georgeverghese]

It would be a tall order to explain how to do this here in this forum; I have a spreadsheet for these predictions which I had to build out of necessity many years ago - took me some months. It will be well worth your time - this effort will pay off well for future work when you dont have access to a process simulator or you need to get some quick answers.

If your discharge pressure remains the same, then polytropic head would be lower with higher suction pressure, assuming mol.wt of the gas remains the same. Work out the new polytropic head and find the new operating point on the compressor curve(s). Or find the new speed that gives you this head at the same flow.

Beware of the limitations of the GPSA manual procedure; this will be a good start in any case. The ASME procedure does not rely on Cp/Cv predictions, and is more accurate, especially for high compression ratios and high operating pressures or non ideal gases such as CO2.

Else, a process simulator would give you the new operating point if you could punch in the Q vs. H vs. speed data.

 

[Wkcoo]

Which edition or section of ASME and GPSA mention the procedure?

My intention would be to gauge and assess whether the machine efficiency has dropped. Now my machine is running at, say, 600 psig suction pressure and produce x polytropic head. However, the OEM curve is for 700 psig suction pressure and produce y polytropic head. Because the condition is different i cannot compare the performance apple to apple. I am thinking if I can translate to a new curve (P suction = 600 psig) from the OEM curve, then I should see x polytropic head is produced at that flow rate. Otherwise I have good reason to believe that the machine efficiency has dropped.

 

[goutam_freelance]

It may be possible to calculate the parameters you desire from performance test data and calculated dimensionless parameters based on the above. Refer PTC-10.

Alternatively simplified method with simplifying assumptions (perfect gas adiabatic flow etc. are available when flow data, impeller and inlet/outlet geometry etc. are available. See books like Streeter.

 

[georgeverghese]

Any recent edition of the GPSA will have the GPSA procedure for compressor performance rating or new design duty estimate.

The only resource I have to the ASME procedure is the Pro II reference manual which details this in the "Compressor" routine - the original reference would be this PTC-10.

Where your new operating point is will also be influenced by the process controls and if the machine is fixed speed or VSD.