Example

A pipeline is flowing 3.6 standard million cubic feet per day. The gas is made up of the following components: 85% methane, 10% ethane, 4% butane, I % nitrogen. The values are given as a mole percent. The flowing temperature is 80°F and the pressure is 300 psig.

The problem is to calculate the suction conditions for a proposed booster compressor. Values to calculate are flow in cfm at the flowing conditions, the mixture molecular weight, mixture specific heat ratio, and the compressibility of the mixture.

Step 1. Convert the flow to standard cfm using 24 hours per day and 60 minutes per hour.

Qstd - 2500

Step 2, Convert scfm to flowing conditions using Equation 2.6. Standard conditions:

P, = 300+ 14.7 = 314.7 psia Tj = 80°F + 460°R = 540°R

Step 3. Substituting into Equation 2.6, using Q} for and solving for Ch.

14 7 540

Q1=i2iLX —X2500 314.7 520

Qs = 121.3 cfm (flow at the compressor inlet)

Step 4. Change the molal percentages to fractions and substitute for xm then use Equations 2.20, 2.22, 2.26, and 2.27 to construct Table 2-1.

Step 5. Solve for mixture specific heat ratio km, using Equation 2.21.

km=1.26

Table 2-1 Gas Mixture Data

Gas

xn

mcp

raw

xnmw

■ c

xnTc

Pc

xnpc

Methane

.85

8.60

7.31

16.04

13.63

344

292.4

673

572.1

Ethane

.10

12.64

1.26

30.07

3.01

550

55.0

708

70.8

Butane

.04

23.82

.95

58.12

2.33

766

30.6

551

22.0

Nitrogen

.01

6.97

.07

28.02

0.28

227

2.3

492

4.9

Mixture

1.00

9.59

19.25

380.

670.

Step 6. Using Tcm = 380°R and Pcm = 670 psia, substitute into Equa tions 2.28 and 2.29.

Trm=i.42

P 314/7

Step 7. From the general compressibility charts in the Appendix, Z = .95.

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