Page 632 - Air and Gas Drilling Manual

11-70 Air and Gas Drilling Manual
The mechanical efficiency,
, is
m
090
.
m
This is a four-stage reciprocating piston compressor. Therefore,
n s 4
The first stage compressor ratio is
1
356 5 . 4
r s
.
12 685
.
r 230
s
The volumetric efficiency (only for the reciprocating piston compressor), v , is
determined from Equation 4-38. The compressor clearance volume ratio, c, is
assumed to be 0.06. Equation 4-38 becomes
1
096 1 006 230 14 . 1
.
.
.
v
.
0 913
v
˙
From Equation 4-39, the actual shaft horsepower, W , required by each compressor
as
is
250 .2
W ˙
as
(. 0 90 ) (. 0 913 )
W ˙ as 304 .5

The above determined 304.5 horsepower is the actual shaft power needed by
each of the two compressors to produce the 356.5 psia pressure output at the surface
location elevation of 4,000 ft above sea level (while drilling at a depth of 10,000 ft).
At this surface location, the input horsepower available from the Caterpillar Model
D398 prime mover is a derated value (derated from the rated 760 horsepower
available at 900 rpm). In order for the compressor units to operate at this 4,000 ft
surface location elevation, the derated input power available must be greater than the
actual shaft power needed. Figure 4-15 shows that for 4,000 ft elevation the input
power of a turbocharged prime mover must be derated by approximately 10 percent.
˙
The derated input horsepower, W , available from the prime mover is
i