Gas Compression I

Report
Ref.1: Ikoku, Natural Gas Production Engineering, John Wiley &
Sons, 1984, Chapter 5.
Ref.2: Menon, Gas Pipeline Hydraulic, Taylor & Francis, 2005,
Chapter 4.
Ref.3: GPSA Electronic Data Book, Gas Processors Association,
1998, Chapter 13.
1
Gas Compression
Type of Compressors
Depending on application, compressors are manufactured
as positive-displacement, dynamic, or thermal type:
2
Gas Compression
Reciprocating Compressors
The reciprocating compressor consists of one or more cylinders
each with a piston or plunger that moves back and forth,
displacing a positive volume with each stroke. They are singleacting or double-acting:
Clearance
Stroke length
Ar = Rod
diameter
AP = Piston diameter
Single-acting
Double-acting
3
Gas Compression
Reciprocating Compressors
Reciprocating compressors have pressure ranges up to 30,000 psi
and range from very low HP to more than 20,000 HP per unit.
Reciprocating compressors can be single stage or multistage,
depending upon the compression ratio required. The
compression ratio per stage for positive displacement
compressors is limited to 4.0 (because of the valve life and
discharge temperature).
Gas cylinders are generally lubricated, although a non-lubricated
design is available when warranted.
Typically, high-speed compressors operate at speeds of 900 to
1200 rpm and slow-speed units at speeds of 200 to 600 rpm.
4
Gas Compression
Reciprocating Compressors
On multistage machines, intercoolers may be provided between
stages. These are heat exchangers which remove the heat of
compression from the gas and reduce its temperature to
approximately the intake compressor temperature.
5
Gas Compression
Centrifugal Compressors
Centrifugal compressors develop the pressure required by the
centrifugal force due to rotation of the compressor wheel that
translates the kinetic energy into static pressure of the gas.
Centrifugal compressors can be used for outlet pressures as high
as 10,000 psia, and inlet capacity of more than 100000 cfm.
Centrifugal compressors are usually either turbine or electric
motor driven. Typical operating speeds for centrifugal
compressors in gas transmission applications are about 14,000
rpm for 5000-hp units and 8000 rpm for 20,000-hp units.
In gas pipeline applications a compression ratio of 1.5 to 2.0 is
usually used.
6
Gas Compression
Centrifugal Compressors
A compressor body may hold one or several (up to 8 or 10)
stages. A compressor train may consist of one or multiple
compressor bodies. Pipeline compressors are typically single
body trains, with one or two stages.
7
Gas Compression
Advantages of a Reciprocating Compressor
Ideal for low volume flow and high-pressure ratios
High efficiency at high-pressure ratios (about 4)
Relatively low capital cost in small units (less than 3000 hp)
Less sensitive to changes in composition and density
Have flexibility in pressure range, and can deliver compressed
gas at a wide range of pressures
8
Gas Compression
Advantages of a Centrifugal Compressor
Ideal for high volume flow and low head
Simple construction with only one moving part
High efficiency over normal operating range
Low maintenance cost and high availability
Greater volume capacity per unit of plot area
No vibrations and pulsations generated
9
Gas Compression
Compressor Selection and Rating
A Gas engineer in the field is frequently required to determine
the desired specifications of a new compressor station or
selecting the operating point of an existing one. These
specifications are:
Type, number of stages, arrangements (parallel, series, inter and
after coolers), driver, speed, efficiency, power and/or capacity of
each stages or units, duty of coolers.
For determining the above specifications, these parameters are
required: Gas Composition(or specific gravity), inlet temperature
and pressure, total pressure ratio and total gas flow rate.
10
Gas Compression
Compressor Head and Power
There are three ways in which the thermodynamic calculations
for compression can be carried out — by assuming:
1. Isentropic process, PV k = constant, k = isentropic factor
2. Polytropic process, PV n = constant, n = polytropic factor
3. Isothermal process, PV = constant
>
W 

P2
V dP
P1
11
Gas Compression
Compressor Head (Isothermal)
Head ( H it ) 
Isothermal
PV  C  V 
C
P
 H it 
C
W
m
dP
m
P1
ln( r ),
Where r 
m
P1V1  C  z 1 nRT 1  H it 
 ft  lb f
H it 
 lb m


V
P2
P2
P1
z 1 RT 1
M
ln( r )
g
 53 . 28 z 1T1 ( o R )

ln( r )

g

12
Gas Compression
Compressor Head (Isentropic and Polytropic)
PV
 H is
k
C 
C V  
P
C 
  
 P1 
1/ k
1/ k
 H is 
C
1/ k
m

P2
P
1 / k
dP
P1
k 1


P1
z 1 RT 1
k
 1 
r
m ( k  1) / k 
 M g ( k  1) / k
 ft  lb f
H is 
 lb m
k 1

 53 . 28 z 1T1 ( o R )  k
r

 1


 g ( k  1) / k 


 ft  lb f
H p 
 lb m
n 1

 53 . 28 z 1T1 ( o R )  n
r

 1


 g ( n  1) / n 


 k 1

k
 1
r


13
Gas Compression
Compressor Horsepower
Gas Horsepower
( GHP ) 
Ideal Horsepower
Compressio n Efficiency
 it  Isothermal Efficiency


Compressio n Efficiency ( c ) :  is  Isentropic Efficiency


 p  Polytropic Efficiency
Brake Horsepower
( BHP )  GHP  Mechanical
losses 
GHP
m
14
Gas Compression
Gas Horsepower
GHP ( hp ) 
 ft  lb f
Head 
 lb m

 lb 
  Mass flowrate  m 

 min 

33000  c
Mass flowrate  q g sc  g sc  q g sc  g 0 . 0764
GHP 
8 . 57  10
5
5
z av T1 ( R ) q g sc ( Mscfd )  k 1

k
r
 1 


 is ( k  1) / k


o
5
z av T1 q g sc  n 1
 8 . 57  10 z av T1 q g sc
n
r
 1 
ln r


 p ( n  1) / n
 it


8 . 57  10
15
Gas Compression
Isentropic and Polytropic Efficiency
 k 1

1
k
r

1 

  ( n  1) / n
 is ( k  1) / k 
p

1
 n 1

n
r
 1




Fig. 13-37
k 1
p 
( k  1) / k
( n  1) / n
r
  is 
k 
cv

cp
cp  R

 yc
,
 y c   R
i
i
pi
pi
1
( k 1 ) / k
r
cp
k
p
1
For natural gas : k
150
o
F

2 . 738  log(  g )
2 . 328
16
Gas Compression
Discharge Temperature
1/ k

C 
k
k 1
 PV  C , V   
T2
P2V 2 z 1  z 1  k
r

 
P 


T
P
V
z
z
1
1 1 2
 2

 PV  z n RT
 z1 
r
 T 2  T1 

z
 2
 is 
T 2  T1 is
T 2  T1 real
k 1
k
  z 1  k 1

k
r

 1
 

T 2  T1   z 2 

T1
17
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