Page 293 - Pipeline Rules of Thumb Handbook

280 Pipeline Rules of Thumb Handbook

Figure 1. Maximum polytropic head per stage—English system.

Refer to Figure 2, p. 216 to find Z 2 , discharge compress- inlet temperature but also at the estimated discharge
ibility. temperature.
The suggested approach is as follows:
Z 2 = 0.925

1. If the k-value is felt to be highly variable, one pass
Z a = ( Z + Z ) 2 should be made at estimating discharge temperature
2
1
= 094 based on the inlet k-value; the average k-value should
.
then be calculated using the estimated discharge
Determine average k-value. For simplicity, the inlet temperature.
value of k will be used for this calculation. The polytropic 2. If the k-value is felt to be fairly constant, the inlet k-
head equation is insensitive to k-value (and therefore n- value can be used in the calculations.
value) within the limits that k normally varies during 3. If the k-value is felt to be highly variable, but sufficient
compression. This is because any errors in the n/(n - 1) time to calculate the average value is not available,
multiplier in the polytropic head equation tend to balance the inlet k-value can be used (but be aware of the
corresponding errors in the (n - 1)/n exponent. Discharge potential discrepancy in the calculated discharge
temperature is very sensitive to k-value. Since the k-value temperature).
normally decreases during compression, a discharge tem-
perature calculated by using the inlet k-value will be con-
k 1 = k a = 1,126
servative and the actual temperature may be several
degrees higher—possibly as much as 25–50°F. Calculating
the average k-value can be time-consuming, especially Determine average n/(n - 1) value from the average
for mixtures containing several gases, since not only must k-value. For the same reasons discussed above, use n/(n - 1)
the mol-weighted c p of the mixture be determined at the = 6.88.