03-Gas Compression Fundamentals

Gas Gas CompressionCompression

FundamentalsFundamentals

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TERMINOLOGYTERMINOLOGY(Jargon)

Gas Compressor Industry Language

Compression = The act of pressing or compacting into a smaller space.

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TerminologyTerminologyR/Rc = ratio of compression

Ps/P1 = suction pressure

Pd/P2 = discharge pressure

Ts/T1 = suction temperature

Td/T2 = discharge temperature

Ta = ambient temperature

Hp = horse power

HE = head end

CE = crank end

VE = volumetric efficiency

K = ratio of specific heats of a particular natural gas - a thermal dynamic value

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TerminologyTerminology

Ratio of Compression (RC)

RC = Discharge Pressure(Pd)psig + 14.7 = ??? psia

Suction Pressure (Ps)psig + 14.7 = ??? psia

Note: Elevation or Barometric pressure must be taken into account when figuring Rc.

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TerminologyTerminologyFinding Ratio of Compression (RC)

Example 1: Ps = 200 psig, Pd = 600 psig

Rc = 600 psig + 14.7 = 614.7 psia = 2.863

200 psig + 14.7 = 214.7 psia

Example 2: Ps = 20 psig, Pd = 600 psig

Rc = 600psig + 14.7 = 614.7psia = 17.7

20psig + 14.7 = 34.7psia

Note: If we had not converted to psia the ratio

would have been 30:1.

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TerminologyTerminologyFinding Approximate HP Requirement:

To find the HP required for a single

stage unit (example 1):

Approximating HP Formula using

Example #1

HP = 21 x Rc x S x Q

Where:HP = Horse Power

21 = Standard (Constant)

Rc = Ratio Of Compression

S = # of Stages

Q = Quantity in MMCFD

HP = 21 x 2.86 x 1 x 2 = 120.12 or 121

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TerminologyTerminologyFinding Approximate HP Requirement:

To find the HP required for a two

stage unit (example 2):

Take the square root of the ratio found earlier:

Rc = 17.71 = 4.20

HP = 21 x Rc x S x Q

HP = 21 X 4.2 X 2 X 1

HP = 176.4

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SA = Single Acting - The act of compressing on one end of

the compressor cylinder.

DA = Double Acting - The act of compressing on both ends of

the compressor cylinder.

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Terminology

Clearance Volume = The volume remaining in the compressor cylinder at the end of the discharge stroke. Normally expressed as a % of piston displacement.

Displacement = Area of piston X length of stroke X # of strokes per minute. For DA

compressors, the displacement on the CE is less than the HE due to the rod area. Normally expressed in cubic feet per minute or CFM.

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Terminology

Capacity = (or flow rate) Volume of gas compressed and delivered at specified conditions of temperature & pressure measured at the compressor inlet. Usually expressed as volume/unit of time - cubic feet per minute or cubic feet per day.

Approach = 30°F approach means the cooler will cool the gas to 130° when ambient

temperature is 100°F.

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TerminologySwept Volume

= The volume swept by the piston during the forward and/or backward stroke.

= Compressor speed is not a factor.

= Normally expressed in cubic inches and calculated as follows:

Swept Volume HE = Area of Piston X Stroke in Inches

Swept Volume CE = (Area of Piston - Area of Rod) X Stroke in Inches

Total Swept Volume = (2 X Area of Piston - Area of Rod) X Stroke In Inches

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Total Swept Volume Example (DA):

6” Diameter x 7” Stroke2.5” Piston Rod Diameter

Total Swept Volume = (2 X 28.274 - 4.909) X 7 = (56.548 - 4.909) X 7 = 51.639 X 7 = 361.473 cu. In.

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TerminologyTerminologyUnits of Measure:

PSI = pounds per square inch

PSIG = pounds per square inch gauge

PSIA = pounds per square inch absolute

= gauge + atmospheric pressure

MMCFD = million cubic feet per day

MMSCFD = million cubic feet per day @ the

standard conditions of 14.65psia & 60°F

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In-Hg = inches of mercury pressure

= can be a manometer or mercury filled gauge, used where pressures are low

& extreme accuracy is desired

= 1” Hg = .491 PSI

In-H2O = inches of water pressure

= usually a manometer filled with water

= used for measuring very low pressure

= 1/13.6 as heavy as mercury

13.6” of H2O = 1” of Hg = .491 PSI

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Temperature:

Temperature =

A measure of molecular energy…the higher the temperature the more molecular energy.

Degrees Fahrenheit (°F) =

Temperature scale where, at sea level, the freezing

point of water is 32° & the boiling point is 212°. The

distance between these two points is 180°.

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Terminology

Temperature:

Degrees Centigrade or Celsius (°C) =

A scale used worldwide which relates to the metric system. At sea level, the freezing point of pure water

is 0° & the boiling point is 100°. The distance between these two points is 100°.

Temperature range:

Centigrade = 100° = 5

Fahrenheit = 180° = 9

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TerminologyTerminologyTemperature:

Converting from ° F to ° C expressed as follows:

° C = 5/9 X (° F - 32)

Example: Convert 100° F to ° C

° C = 5/9 X (100 - 32)

= 5/9 X 68

= 37.8° C

18Separable Engine

VConfiguration

InlineConfiguration Horizontal Opposed

Configuration


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Separable Compressor Frame

ClickHere


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Compressor

Engine

Separables Coupled


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INTEGRAL ENGINE-COMPRESSORS

Left Bank Right Bank

RH

LH

1

2

Flywheel

Sheave

3

Flywheel

2L

1L

3L

4L

5L

1R

2R

3R

4R

5R

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Rod Load DefinitionsRod Load Definitions• External Rod Load

– The external rod load of a reciprocating compressor is a calculation considering the unit in a static state. The calculation for deriving the ERL is:

– ERL = PD(HA) - PS(CA) where:• PD = Discharge Pressure at the cylinder flange• PS = Suction Pressure at the cylinder flange• HA = Head End surface area of the piston• CA = Crank End surface area of the piston

• Internal Rod Load– The internal rod load is often noted as the internal gas rod load of a reciprocating

compressor and the terms are synonymous. The IRL is a dynamic rod load calculation based upon the internal gas pressures within the cylinder bore. These gas pressures take into account the dynamic pressure drop characteristics found across the valves and gas passages. The IRL is calculated through 360° rotation of the crankshaft with the highest values being used.

• Net Rod Load– The net rod load is considered a dynamic rod load rating. The NRL is the sum of

the IRL calculation and the inertia loads of the reciprocating weights. The NRL is calculated through 360 ° rotation of the crankshaft with the highest values being used.

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2.5”

ERL CompressionERL Compression

Pd=814.7 PSIA Ps=214.7 PSIA

Area of piston in square inches times dischargepressure, minus area of piston, minus area of rodtimes suction pressure.

6”

= 28.274 x 814.7 - (28.274 - 4.909) x 214.7= 23,035 - (23.365 x 214.7)= 23.035 - 5016= 18,019 #

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2.5”

ERL TensionERL Tension

Pd=814.7 PSIAPs=214.7 PSIA

Area of piston in square inches, minus area of rodtimes discharge pressure, minus area of the pistontimes suction pressure.

6”

= (28.274 - 4.909) x 814.7 - (28.274 x 214.7)= 23.365 x 814.7 - (28.274 x 214.7)= 19,035 - 6070= 12,965 #

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Piston Rods Exceeding Rod LoadPiston Rods Exceeding Rod Load

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Rod ReversalRod Reversal

Crank PinCrosshead Pin

Connecting Rod

Piston Rod

Crosshead

Oil

Oil

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• Data needed for Compressor Sizing:– Operating Conditions– Gas properties

• Approximate HP per application:– Suction Pressure (PS)– Discharge Pressure (Pd)– Quantity of gas in MMCFD

• Conditions Assumed:– Suction Temp (TS) = 80°F– Ambient Temp (Ta) = 100°F– Discharge Temp (Td) required

approach = 20°F– Altitude = < 1500’– Atmospheric Pressure = 14psi– Specific Gravity = 0.64– “N” value = 1.26– Sweet Gas

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AnyQuestions?

03-Gas Compression Fundamentals

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