Page 130 - Air and gas Drilling Field Guide 3rd Edition

5.6 Prime Mover Input Power Requirements 121

For this example,
e m ¼ 0:95
and from Equation (5-39), the actual shaft power is
155:1
_ W as ¼ ¼ 175:6 hp:
ð0:95Þð0:930Þ
The above determined 175.6 hp is the actual shaft power needed by the com-
pressor tomatch thebackpressureof150 psig(atthesurfacelocationelevation
of 6000 ft above sea level). At this surface location, the input power available
from the prime mover is a derated value (derated from the rated 350 hp available
at 1800 rpm). In order for the compressor system to operate at this 6000-ft
surface location elevation, the derated input power available must be greater
than the actual shaft power needed. Figure 5-15 shows that for a 6000-ft elevation
the power of a turbocharged prime mover must be derated by 15%. The derated
input horsepower available from the prime mover _ W i is
_ W i ¼ 350 ð1 0:15Þ¼ 297:5 hp:
For this example, the prime mover derated input power is greater than the actual
shaft power needed, thus the compressor system can be operated at this 6000-ft
surface location elevation.

(b) Surface location at 1829 m above sea level (SI units)
The volumetric efficiency e v can be determined using Equation (5-39).
n s ¼ 2
2
P i ¼ 81170 N=m abs
2
P o ¼ 1034550:0 þ 81170:0 ¼ 1115720:0 N=m abs:
Equation (5-37) becomes
1

1115720:0 2
r s ¼ ¼ 3:708:
81170:0
The specific heat ratio for air is
k ¼ 1:4:
The clearance volume ratio for this compressor is
c ¼ 0:02:
With the values just given, Equation (5-38) becomes
1
8 2 39
< =
e v ¼ 0:96 1 0:02 ð3:708Þ1:4 1 5
4
: ;
e v ¼ 0:930: