Horsepower to Compress Gases

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This procedure uses the Polytropic Process to determine the horsepower required to compress gases.

Example #1 - Compression of Natural Gas for the Transmission Pipeline

1. Natural Gas Properties

a. Temperature: 60 F
b. Barometric Pressure : 14.6960 psi
c. Nat. Gas Gage Pressure: 1250.00 psig (at compressor discharge)
d. Nat. Gas Absolute Pressure: 1264.6960 psia
e. Specific Gravity: 0.60
f. Molecular Weight: 17.3787 lbm/mole
g. Compressibility Factor Z: 1.00
h. Ratio of the Specific Heats (k = Cp/Cv): 1.29
i. HHV (Btu/lb) : 22,708.49
j. HHV (Btu/std ft3): 1040 Btu/ft³
k. Density: 3.940842 lb/ft³

2. Natural Gas Pipeline Flow

a. Mass Flow:

1,878,030.4628 lbs/hr
31,300.5077 lbs/min

b. Volume Flow at 1250 psig & 60 F:

476,555.6451 actual ft³/hour
7,942.5941 actual ft³/min

c. HHV Energy Flow: 42,647,230,264.40 Btu/hour

3. Pipeline Data

a. Pipe OD: 36.00 inches
b. Pipe ID: 34.7580 inches
c. Transverse Internal Area: 6.589265 ft²
d. Pipe Length:

528,000 ft
100.00 miles

e. Velocity of Nat. Gas at the Compressor Discharge 1250 psig: 1205.3839 ft/min
f. Pipe Pressure Drop: 292.8855 psi
g. Pipe Pressure at the end of the Pipe Run:

957.1145 psig
971.8105 psia

h. Temperature at the end of the Pipe Run: 60 F
i. Velocity of Nat. Gas at the End of the Pipe Run: 1568.6640 ft/min

4. Compressor Data

a. Polytropic Efficiency (decimal): n = 0.85
b. Variable Speed Drive Efficiency (decimal): 0.95

COMPGAS Computer Program

1. Polytropic Efficiency (decimal): n =0.85
2. Gas Molecular Weight: 17.3787 lb mass/mole
3. Inlet Temperature: 519.67 R
4. n/n-1 = 3.7810
5. n-1/n = 0.264480
6. P disch/Pinlet: 1.301381
7. Compressibility Factor Z = 1.0
8. Inlet Mass Flow: 31,300.5077 lb/min

9. Calculated Polytropic Head Hp (ft –lb_f/lb_mass)

Hp(ft-lb_f/lb_mass) = 12,607.67

10. Calculated Gas Horsepower (HP)

Gas Horsepower = 14,068.68 HP

11. Compressor Temperature Discharge

Temperature Discharge = 97.4966 F

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