Gas Compression II

Gas CompressionPart II

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 CompressionReciprocating 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):

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.

97.0//1 sntrr


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:

Gas flow rate at suction conditions:

Gas flow rate at suction conditions is required for capacity calculation of each stage.

95.088.0,93.083.0 mis

(psia)

)Mscfd()R(109671.1)Mcfm(

1

o115

11

11 P

qTzz

T

T

P

Pqq sc

sc

g

sc

scgg


Capacity (or speed) of each stage:

For single acting:

For double acting:

vsp

vsp

SLdq

SLdq

)rpm()ft()in(10454.5)Mcfm(

)4/(26

1

21

Piston diameterStroke length

vsrp SLddq )rpm()ft()2(10454.5)Mcfm( 2261

Rod diameter


Volumetric efficiency:The term “volumetric efficiency” refers to the actual pumping capacity of a cylinder compared to the piston displacement.

Where:

Subtract about 0.05 from volumetric efficiency when a non-lubricated compressor is used.

101.096.0 /1

2

1 kv r

z

zCr

VolumeStroke

VolumeClearanceC

Gas CompressionCentrifugal Compressors

Number of Stages:

The total polytropic head (Hpt) is an indication of the number of stages required for centrifugal compression:

Or from Fig 13-39:

The discharge temperature must be less than 350 oF

35.0

m

f 150015000lb

ftlb

,/

gp

pps

MH

HHn

stage

staget

gm

f

k

mN0.3

lb

lbft0.1


BHP of all stages (one unit):

Calculate the ideal head of all stages based on the polytropic assumption, then:

..

)GHP(75.0 4.0

LMGHPBHP

LossesMechanical

1- Calculate the GHP with using polytropic efficiency:

2- Calculate the BHP with using mechanical losses:

Gas Compression Centrifugal Compressors

Capacity (or speed):


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

As the speed deviates from the design speed, the error of “affinity laws” will increase.

32, SHPSHSqg 2

,

old

newoldnew

old

newgg S

SHH

S

Sqq

oldnew


Stone wall Limit


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:

602.0,45.17,833.1R,546 o1 ggMrT

93.089.0,54.1R,5.355,psia3.674 1o zPTTP prprpcpc

271.1,Mcfd33.31 kq

Gas Compression Example: Solution

a) Reciprocating: The values in the parenthesis are calculated by HYSYS

300F717R63783.0:,14 oo2 Tassumenr iss

m

f

lb

lbft29070),1763(1825)rpm(),88.0(85.0 isv HS

770888.0),6647(6783 BHPassumeGHP m

b) Centrifugal:

350F237,329848,715.0 o2 TnH spp

rpm11000,811227..),7935(8085 SBHPLMGHP

Gas Compression II

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