Parrallel operation of Centrifugal compressor

[picaso32]

Hi All ... We are going to revamp the PSA for additional capacity. Presently the PSA tail gas is sent to surge drum and centrifugal compressor (two stage – 8 kNm3/hr ) discharges the tail gas to the fuel gas header. After revamp the tail gas system will have two surge drums and two centrifugal compressors ( two stage ) will be running parallely and discharges the tail gas to common header. The new compressor has the capacity of 21 kNm3/hr. When the two centrifugal compressor of different capacity ( 8 & 21 kNm3/hr ) running paralley do I need a load sharing controller between these two compressors .Does the two compressor need to talk to each other while running ?

 

[pmover]

2 centrifugal compressors of different capacity in parallell - load sharing? load sharing is usually done on units of similar construction.

since these different capacity compressors will operate in parallel mode, will the head across each compressor be the same?

the performance curves are likely different.

constant speed units or variable speed units?

drivers are electric/engine driven?

it likely can be done, but it will likely be difficult for the operation of the system as a whole. pick an operating point on a compressor curve. pick same operating point on the other compressor. note the differences in speed, efficiency, and power requirements.

recommend getting expertise from the good folks at CCC.

good luck!
-pmover

 

[picaso32]

Thanks for the inputs

Yes the head across each compressor will be same. Both the compressors have common suction and common discharge.

Both are constant speed , electric driven.

With this parallel configuration, of different capacities do we really need load sharing controller , or can we manage without laod sharing controller.

 

[pmover]

think of the operation in this manner, without load sharing, is it possible for one compressor to gradually increase in capacity while the other compressor decreases in capacity. thus possibly operate the compressor while recycling gas. somehow, someway, avoiding this possible operational scenario needs to be addressed/resolved.

also, what will the design incorporate to control capacity to each compressor? or how will capacity to each compressor be controlled?

again, the good folks at CCC or a competitor can assist you.

good luck!
-pmover

 

[tgmcg]

So long as gas MW is relatively constant, flow does not vary more than +/- 15%, you're happy with continuous throttling/recycling energy losses, and you're absolutely sure of your design conditions, then constant speed centrifs are just peachy.

However, back in the real world, centrifugal process gas compressors with constant speed motor drives, more often than not, are a recipe for trouble. However, many people install them because they are cheaper to buy than variable speed machines. The buyer often does not appreciate the potential control and operability issues. Putting them in parallel configuration adds to the number of things that can go wrong.

At this moment I happen to be having tons of fun sorting out a litany of process & control issues with a 2 process-stage constant speed centrif in high MW gas recovery unit application which was installed in 1982. We will likely convert to VFD drive.

The only place I would ever consider a constant speed centrif is air or industrial gas compression, but would avoid integrally-geared compressors due to their unbalance sensitivity.