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Ref.1: Kumar, Gas Production Engineering, Gulf Publishing Com., 1987, Chapter 9. Ref.2: GPSA Electronic Data Book, Gas Processors Association, 1998, Chapter 13. Ref.3: Mokhatab et al, Handbook of Natural Gas Transmission and Processing, Gulf Publishing Com., 2006, Chapter 8. 1 Gas Compression Reciprocating Compressors Number of Stages: 1. Calculate the overall compression ratio (rt). If the compression ratio is under 4, consider using one stage. If it is not, select an initial number of stages (ns) based on the following equation (3% pressure drop was assumed in each coolers): r rt1/ ns / 0.97 2. Calculate the discharge gas temperature for the first stage. If the discharge temperature is too high (more than 300 oF), either increase the number of stages or reduce the suction temperature through precooling. Gas Compression Reciprocating Compressors BHP of each stage: Calculate the ideal head of each stage based on the Isentropic assumption, then calculate the GHP and BHP with using isentropic and mechanical efficiency: is 0.83 0.93, m 0.88 0.95 Gas flow rate at suction conditions: Gas flow rate at suction conditions is required for capacity calculation of each stage. Psc q g1 (Mcfm ) q g sc P1 T1 Tsc o z T ( R )q g sc (Mscfd ) 5 1 1 z1 1.9671 10 P1 (psia) Gas Compression Reciprocating Compressors Capacity (or speed) of each stage: For single acting: Piston diameter Stroke length q1 ( / 4)d p2 Ls Sv q1 (Mcfm) 5.454106 d p2 (in ) Ls (ft) S (rpm)v For double acting: q1 (Mcfm) 5.45410 (2d d )Ls (ft)S (rpm)v 6 Rod diameter 2 p 2 r Gas Compression Reciprocating Compressors Volumetric efficiency: The term “volumetric efficiency” refers to the actual pumping capacity of a cylinder compared to the piston displacement. z1 1/ k v 0.96 0.01r C r 1 z2 Where: ClearanceVolum e C StrokeVolum e Subtract about 0.05 from volumetric efficiency when a nonlubricated compressor is used. Gas Compression Centrifugal Compressors Number of Stages: The total polytropic head (Hpt) is an indication of the number of stages required for centrifugal compression: ns H pt / H pstage , lb ft H pstage f 15000 1500M g0.35 lb m Or from Fig 13-39: The discharge temperature must be less than 350 oF ft lbf 1.0 lb m Nm 3.0 k g Gas Compression Centrifugal Compressors BHP of all stages (one unit): Calculate the ideal head of all stages based on the polytropic assumption, then: 1- Calculate the GHP with using polytropic efficiency: 2- Calculate the BHP with using mechanical losses: MechanicalLosses 0.75(GHP) 0.4 BHP GHP M .L. Gas Compression Centrifugal Compressors Capacity (or speed): Gas Compression Centrifugal Compressors Performance Curves: Generally, the performance of a centrifugal compressor follows the “affinity laws.”According to the affinity laws, as the rotational speed of the centrifugal compressor is changed, the inlet flow and head vary as the speed and the square of the speed, respectively qg S , H S 2 HP S 3 q g new S new S new q g old , H new H old S old S old 2 As the speed deviates from the design speed, the error of “affinity laws” will increase. Gas Compression Centrifugal Compressors Gas Compression Centrifugal Compressors Surge Line Control: In all modern centrifugal compressors, a recycle line with a control valve that allows the flow to increase through the compressor is used for surge control. The flow through the compressor must be 10-20% greater than surge limit. SC Gas Compression Example Pressure of a gas stream (30 oC, 200 MMscfd, 90% C1, 10% C2) must increase from 600 psia to 1100 psia. a) Design a reciprocating compressor (dp= 25 cm, Ls=1.2 m, dr=3 cm, single acting, Clearance fraction = 0.15) b) Design a Centrifugal compressor Solution: T1 546 o R, r 1.833, M g 17.45, g 0.602 Ppc 674.3 psia, Tpc 355.5 o R, Tpr 1.54, Ppr 0.89 z1 0.93 q1 3.33 Mcfd, k 1.271 Gas Compression Example: Solution a) Reciprocating: The values in the parenthesis are calculated by HYSYS r 4 ns 1, assume:is 0.83 T2 637 o R 177 o F 300 ft lbf lbm v 0.85 (0.88), S (rpm) 1825(1763), H is 29070 GHP 6783(6647), assumem 0.88 BHP 7708 b) Centrifugal: p 0.715, H p 29848 ns 3, T2 237o F 350 GHP 8085(7935), M . L. 27 BHP 8112, S 11000rpm