Gas Compression Part I

Gas CompressionPart I

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 CompressionType of Compressors

2

Depending on application, compressors are manufacturedas positive-displacement, dynamic, or thermal type:

Gas CompressionReciprocating Compressors

The reciprocating compressor consists of one or more cylinderseach with a piston or plunger that moves back and forth,displacing a positive volume with each stroke. They are single-acting or double-acting:

Single-acting Double-acting

AP = Piston diameter

Ar = Rod diameter

Stroke length

Clearance

3


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


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 CompressionCentrifugal 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 CompressionCentrifugal 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

11

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Gas Compression Compressor Head (Isothermal)

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Gas Compression Compressor Horsepower

14

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Gas Compression Isentropic and Polytropic Efficiency

16

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Gas Compression Discharge Temperature

17

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18

Gas Compression Part I

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