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Source: Presentación de compresores. Prof Manuel E. Cabarcas

RECIPROCATING COMPRESSORS

Reciprocating compressors

use pistons to "compress"

high pressure gas. These

systems are common in thehandling of natural gas and

other high pressure systems.



Operate with the adiabatic principle by which the gas is introduced

into the cylinder through the inlet valves, is retained and

compressed in cylinder and exits through the discharge valves

against the discharge pressure.

Source: Presentación de compresores. Prof Manuel E. Cabarcas





Start of cycle at point A. Piston has

reached end of stoke and moves to

the right. Suction and dischargevalves closed. Cylinder filled with

gas at discharge pressure.

As piston moves to the right, the gas

in the cylinder at the start begins to

expand and its pressure drops. Atpoint B, pressure in cylinder equals

suction pressure and suction valve

opens.

Source: CAMPBELL M John,Gas conditioning processing Vol 2, pag 214




Suction valve reminds open,

and gas flows into cylinder.

When piston starts to move

to the left, the pressure inthe cylinder rises and the

suction valve closes.

Piston moves from D to A,

gas flows out of the

cylinder into the discharge

line. At point A, cyclestarts over.

Piston moves from C to D,

pressure in cylinder rises. At

pint D, pressure reaches

discharge pressure anddischarge valve opens.

Source: CAMPBELL M John,Gas conditioning processing Vol 2, pag 214




DESIGNPISTON DISPLACEMENT

It can be calculated from:

= ()()()

2200

= ( )()()2200

Head end displacement:

Crank end displacement: PD: piston displacement, cfm

S: stroke lenght, in

rpm: compressor speed, rpm

dc:diameter of cylinder,in

dr: diameter of rod, in




DESIGNVOLUMETRIC EFFICIENCY

The flow rate is not directly equal to the piston displacement. Volumetric efficiency

is the ratio of actual volumetric flow at inlet temperature and pressure conditions

to piston displacement. It is given by:

: 96 ∗

∗

1

Ev: stage volumetric efficiency, %

R: compression ratio (Pd/Ps)C: cylinder clearance, percent of piston displacement

Zs: compressibility factor at suction, psia

K: ratio of specific heats, Cp/Cv




DESIGNCYLINDER THROUHPUT CAPACITY

Using a known piston displacement and efficiency, the gas throughput can be calculated

from:

= ∗

qa: gas througput at suction

conditions of temperatura and

pressure, ft3/min

Ev: volumetric efficiencyPD: piston displacement,

ft3/min

= 35.4 ∗ ∗

qg: gas throughput at

standard conditions, scfm

Ps: suction pressure, psia

Ts: suction temperatura, ºRZs: compressibility at

suction conditions

= 0.051 ∗ ∗

Qg: gas throughput,

MMscfd




DESIGNROD LOAD

It depends on rod diameter and material, and will be quoted by the manufacturer

Single-acting cylinder, crank end

Single-acting cylinder, head end

Double-acting cylinder

= ∗ + ∗

= ∗ ∗

= ∗ + ∗

= ∗ ∗

= ∗ + ∗

= ∗ ∗

RLc: rod load in compression, lbRLt: rod load in tensión, lb

ap: cross-sectional área of piston, in2

Pd: discharge pressure, psia


Pu: pressure in unloadead área, psia

ar: cross-sectional área of rod, in2




DESIGNGAS DISCHARGE TEMPERATURE

The discharge temperature it neglects heat from friction, irreversibility effects,

etc., and may be somewhat low, but the values obtained from this equation will

be reasonable field estimates.

= ∗

−

Pd: discharge pressure, psia


K: ratio of specific heats




DESIGNHORSEPOWER REQUIRED

= . 0857 ∗ ∗ ∗

∗

1 ∗ (

)

− 1

= ( +

2 )

BHP: brake horsepower

Qg: gas flow rate, MMSCFD

Ts: suction temperatura, ºR

Zav: Z average

Zs: suction compressibility factor

Zd: discharge compressibility factor

E: overall efficiency

K: ratio of specific heats


Pd: discharge pressure



Field downstream of the separator from 800 psig at 100°F to 1,000 psig. Anengine-driven separable compressor is available from surplus. The engine israted for 1,600 hp at 900 rpm. Horsepower is proportional to speed. Thecompressor frame has six 7-in. bore by 6.0-in. stroke double-acting cylinderswith a minimum clearance of 17.92%, a rod load limit of 25,000 Ib, and roddiameter of 1.75 in. Assume k = 1.26, Zs = 0.88, and Zd = 0.85.

Calculate:

Compute discharge temperatureVolumetric efficiencyRequired clearance

Rod loadRequired horsepower for the given conditionsCalculate the lowest suction pressure at which this unit can compress 100MMscfd.

EXAMPLE



EXAMPLE

Calculate the gas discharge temperature:

= ∗

−

= 560 ∗

1015

815

.−.

= 586 ºR

=126ºF



EXAMPLE

Calculate the volumetric efficiency

: 96 ∗

∗

1

= 1015

815 = 1245

: 96 1245 17.92 ∗ 1245

. ∗0.88

0.85 1

:.%



EXAMPLE

Calculate the requires clearance

= (2 ∗ )()()

2200

= (2 ∗ 7 1.75)(6)(900)

2200

= 233

= 6 ∗ 233

=



EXAMPLE

Gas troughput

= ∗ = 0.051 ∗

∗

= 0.906 ∗ 1398 = 0.051 1267 ∗ 815560 ∗ 0.88

= = .



EXAMPLE

Calculate required rpm to give desired throughput:

100 = 0.051 ∗ 815

560 ∗ 0.88

=

=

0.906

=

. =

=

=

= ∗ 2200

6 ∗ ( 2 ∗ )

= 218 ∗ 2200

6 ∗ ( 2 ∗ 7 1.75)

=



EXAMPLE

Calculate the clearance that would be needed to reduce the throughput from 106.9

MMscfd to 100 MMscfd:

=

=

1186

1398 = .

Now back calculate for the clearance that must be added to produce this volumetric

efficiency.

84.8: 96 1.245 ∗ 1.245

.

∗

0.88

0.85 1

= %



EXAMPLE

Calculate the size liner required to reduce piston displacement:

= 1186

0.906 = 1309

1309

6 =

2 1.75 ∗ 6 ∗ 9 0 0

2200

= .



EXAMPLE

Calculate the rod load

= .

= 7.5 ∗ 1015 815 ∗ (1.752

)∗1015

= 7.5 ∗ 1015 815 + ∗ (1.752

)∗815

= .



EXAMPLE

Calculate the required horsepower needed for the given conditions:

= . 0857 ∗ ∗ ∗

∗

1 ∗ (

)

− 1

=

0.88 + 0.85

2 = 0.865

E= ∗

E= . ∗ . = .

= . 0857 ∗ 0.865 ∗ 100 ∗ 5600.83

∗ 1.261.261

∗ (1015815

).−

. 1

=1.137 hp



EXAMPLE

Calculate the lowest suction pressure

Using the minimum clearance

=

= 0.051 ∗ ∗

∗

= 100 ∗ 560 ∗ 0.880.051 ∗ 1267

=747.9 psig



• In a gas plant before use is necessary to subject it to a compression process to

raise its energy level, the main function compressors is increase the pressure of

gas, so the gas submits the compressor even work so that increase the total

energy of the same.

•

In compression facilities, must be installed a primary separation equipment orslug catcher upstream suction manifold to avoid the possibility that any liquid

can reach the compressor.

CONCLUSIONS



•

ENGINEERING DATA BOOK. Gas Processors Suppliers Association. Volumen I.Edición — FPS. 2004.

• SURFACE PRODUCTION OPERATIONS. Volumen 2. Design of Gas-HandlingSystems and Facilities. Ken Arnold, Maurice Stewart. Segunda Edición. 1999.

• CAMPBELL M John,Gas conditioning processing Vol 2.

• HERRERA F. BLADIMIR, SALAZAR M. LAURA C “Estudio de los problemasasociados con el proceso de separación flash en las plantas típicas de

compresión de gas natural”, TESIS DE GRADO UNIVERSIDAD DE ORIENTE.

BIBLIOGRAPHY



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re cipro can tes

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