6natural gas compression

7/25/2019 6natural Gas Compression

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NATURAL GAS COMPRESSION

K. SARKODIE


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INTRODUCTION As long as the pressure at the gas could force the gas

through the pipeline to its destination, a compressorwas unnecessary.

As soon as the pressure dropped, however, someoutside means was needed to increase the pressure.

Compressors were also essential for gas transmissionpipelines extended great distances from the gas field-extended so far that the natural well pressure couldnot force enough gas through the pipeline to supplythe demand.

When a gas has insufficient potential energy for itsrequired movement, a compressor station must beused


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Classification of compressors


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Rotary blower

Typically, rotary blowers

have a volume up to

17,000cfm, and have a

minimum intake pressure

of 10psig and a differential

pressure of 10psig


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advantages

Large quantities of low-pressure gas can behandled at comparatively low horsepower,

it has small initial cost and low maintenance

cost, it is simple to install and easy to operate and

attend,

it requires minimum floor space for thequantity of gas removed, and

it has almost pulsation-less flow.


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disadvantages

it cant withstand high pressures,

it has noisy operation due to gear noise and

clattering impellers,

it improperly seals the clearance between the

impellers and the casing,

and it overheats if operated above safepressures


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Centrifugal compressors

Are primarily used in distribution systems where the

pressure differential between suction and discharge is

not over 15psi.

Gas is then discharged

at a high velocity into a

diffuser where the

velocity is reduced and

its kinetic energy is

converted to staticpressure.


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Centrifugal compressors

Typically, the volume is more than 100,000cfm

and discharge pressure is up to 100psig


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Centrifugal compressors have few moving parts sinceonly the impeller and shaft rotate.

Thus, lubricating oil consumption and maintenancecost are low.

They also have continuous delivery without cyclicvariations, and cooling water is normally unnecessarybecause of lower compression ratio and lower frictionloss (multistage units for process compression may

require some form of cooling).

Compression rates are lower because of the absenceof positive displacement.


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They are also used in refrigeration and closed

regeneration of adsorption plants.

The blower is built of casing in which one or

more impellers rotate in opposite directions


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Reciprocating compressors

Reciprocating compressors are mostly

commonly used in the gas industry. Thesecompressors are built for practically all

pressures and capacities. Reciprocating

compressors have more moving parts and,

therefore, lower mechanical efficiencies than

centrifugal machines.


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Process of Reciprocating compression


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The process

Clearance volume

= V3

Clearance(Cl)

= V3/ (V1V3)

Work done= Area 1234

Compression ratio=

P2/P1


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Real gas compression

For real gases , a single compression stage

PV n= a constant

n = the poly-tropic exponent


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Volumetric efficiency Represents the efficiency of a compressor

cylinder to compress gas.

It is the ratio of the volume of gas actuallydelivered to the piston displacement, correctedto suction pressure and temperature.

Ev= volumetric effiency, fraction

r = cylinder compression ratio

k=Cp/Cv of the gas at atmospheric conditions(isentropic process)

Cl= clearance, fraction


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Practical efficiency Practically, certain adjustments are made in the

theoretical formula, so that a typical equation

that might be used in computing compressorperformance is

Where zs = gas deviation factor at suction of the cylinder

zd= gas deviation factor at discharge of thecylinder


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Compressor design


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Compressor design

The designs involve

Reciprocating compressor

Centrifugal compressors and Rotary blowers

The design for the reciprocating compressor isconsidered for this lecture


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Design calculations

Theoretical Horse poweranalytical method,mollier diagram

Actual horse power/ brake horse power Clearance volumes

Volumetric efficiencies

Compressor efficiency Discharge pressures and temperatures

Compression ratios


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Design requirements

Suction pressure

Suction temperature

Gas capacity Specific gravity of gas composition


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Compressor design

Theoretical Horsepower

Horsepower (hp or HP) is the work done over

a period of time.

One hp equals 33,000 ft-lb/min, or 746 watts,

or 75kg-m/s.

It is commonly used in measuring the output

of piston engines, turbines, electric motors,

and other machinery


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THE MOLLIER DIAGRAM

This is done by tracing the increase in enthalpy from the cylinder suction

pressure and temperature to its discharge pressure along the path ofconstant entropy.

This involves some care in handling and converting the various units suchas cubic feet per minute, pounds of vapor, British thermal units, andhorsepower, but it is a simple and straightforward method.

All compressor problems for which suitable Mollier diagrams exist specificgravity natural gas is shown in Fig. below


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For practical purposes, the amount of heat transferredfrom the gas to compressor cylinder and piston duringa cycle is small compared to the work involved in thecompression. Thus, one assumption in compressioncalculations is

1. Q=0

2. Lost work due to friction can be neglected. 3. Kinetic energy effects can be neglected.

The resulting energy balance is

W=H=-n(h2-h1)

Wheren = number of moles being compressed

h1 and h2 can be obtained from Browns enthalpy-entropy charts for gases


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example


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EXAMPLE

What is the theoretical horsepower consumed

in compressing 1MMScfd, measured at 14.73psia and 60oF, from 65psia and 80oF to 215

psia? What is the discharge temperature of

the gas? Assume a gas gravity of 0.6.


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solution


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Analytical method

The theoretical hp of the compressor Required

to compress a given amount of natural gas can

be calculated by assuming the system to be

either isothermal (T = 0) or adiabatic/isentropic (H = 0).

IS THIS THE REALISITC?


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DESIGN ASSUMPTIONS

ADIABATICASSUMPTION

The calculatedtheoretical Hp

gives the maximumrequired hp

ISOTHERMALASSUMPTION

the calculatedtheoretical valuegives the minimumrequired hp.


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For discharge temperature for real gases

The specific heat ratio may also be found, using Kays rule-type

calculation as


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Compressor efficiency

The efficiency, E, is the combination of the

compression and mechanical efficiencies. It is

a function of suction pressure, compression

ratio, speed, the physical design of thecompressor, and the mechanical condition of

the compression


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Compression ratio

The ratio ofp2/p1 is called compression ratio(Rc).

Since compression generates heat, this ratio isusually kept under six.

In field practice, this ratio seldom exceeds four(Guo and Ghalambor, 2005) to ensure that thecompressor performs at high efficiency

wherepf is the final discharge pressure in psia and n is the

number of stages.


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Stage compression

The total power is a minimum when the ratio in each stage is the same.

This may be expressed in equation form as

Large compression ratios result in gas being heated to undesirably high

temperatures. Therefore, it is common practice to cool the gas between stages

and, if possible, after the final stage of compression


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Heat Removed by Inter-stage Cooler


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Analytical method


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EXAMPLES

Calculate the adiabatic horse power required

to compress 1MMcfd of a 0.6 gravity natural

gas from 100 psia and 80oF to 1600 psia.

Intercoolers cool the gas to 80oF.

What is the heat load on the intercoolers and

what is the final gas temperature?

Using the entropyenthalpy diagram.

SOLUTION


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SOLUTION


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ACTUAL HORSE POWER

ONCE THE THEORTICAL HORSE POWER IS

DETERMINED THE BRAKEHORSE POWER FOR

THE COMPRESSOR IS CALCULATED.

BHP = Theoretical HP/Efficiency (E )


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Compressor ratio and efficiency


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assignment

Read more on the design of a centrifugal

compressor

6natural gas compression

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