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Pressure Ratio in Multi-Stage Reciprocating

Compressor

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nirlt (Mechanical) 7 Aug 12 6:04

As a general observation in multistage reciprocating compressor (API 618), pressure ratio of

initial stage (low pressure stages) is higher compare to final stage (high pressure stage). e.g. for2-stage compressor, pressure ratio for 1st stage is 1.9 and pressure ratio of 2nd stage is 1.5.Why?

BigInch (Petroleum) 7 Aug 12 7:00

Of course. Doubling the pressure of a low pressure stage, say 50 psig x 2 = 100 psig, vs doublingthe pressure of a higher pessure stage, say 200 psig x 2 = 400 psig would require maybe 400/100,or 4 times the strength in materials to be provided. As pressure ratio goes up, so does temperatureas well and the gas passing through a first stage is already hot, generally making each sucessivestage a bit more inefficient, so a given amount of power does that much relatively less work foreach higher stage to the point where decreasing the ratio makes sense.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

zdas04 (Mechanical) 7 Aug 12 7:17

I do it because of BigInch's second point. Most recips have fin-fan coolers that are rated at anapproach to ambient temperature. So if it is 100F outside and the gas is at 80F in the pipelineand your cooler is designed for a 20F approach to ambient then if you do 4 ratios in both stageswith k=1.28 you get:

T(in) = 80FT(first disch) = 271F

T(interstage out) = 120FT(second disch) = 325F

That last number is higher than most people will allow. Shifting the load just a bit to the firststage fixes that.

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BigInch (Petroleum) 7 Aug 12 8:17

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There is an optimum work division between stages as well.http://www.tech.plym.ac.uk/sme/THER205-web/recips....This doc also mentions hitting the temperature wall.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

2 2Gas compression engineering - Pressure Ratio in Multi-Stage Reciprocating Compre...

2013-08-21http://webcache.googleusercontent.com/search?q=cache:KFijRGOu4acJ:www.eng-ti...