View
13
Download
0
Category
Tags:
Preview:
DESCRIPTION
gas compression
Citation preview
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
Gas CompressionReciprocating 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:
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
Gas CompressionReciprocating Compressors
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
Gas CompressionReciprocating Compressors
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
Gas CompressionCentrifugal Compressors
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):
Gas CompressionCentrifugal 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
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
Gas Compression Centrifugal Compressors
Stone wall Limit
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:
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
Recommended