Page 592 - Air and Gas Drilling Manual

11-30 Air and Gas Drilling Manual
From Equation 4-39, the actual shaft horsepower, W , required by each compressor
is
W ˙ as 232 .5 ˙ as
(. 0 90 ) (. 0 917 )
W ˙ 281 .7
as
The above determined 281.7 horsepower is the actual shaft power needed by
each of the two compressors to produce the 288.5 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
W ˙ 760 (1 . 0 10 )
i
W ˙ i 684 .0

For this illustrative example, the prime mover of each of the two compressor
units derated input power is greater than the actual shaft horsepower needed, thus,
the selected compressor units can be operated at this 4,000 ft surface location
elevation (while drilling at a depth of 10,000 ft).
Compressor Unit Fuel Consumption
The reciprocating piston compressor is not a fixed pressure ratio machine like
the rotary compressor. As long as there is sufficient power available from the prime
mover the reciprocating piston compressor will match the back pressure resistance.
It was found above that the actual shaft horsepower required by each of the two
reciprocating piston compressor units to compress air to 288.4 psia was
approximately 281.7 (while drilling at 10,000 ft of depth). Also above it was found
that the derated horsepower available from the Caterpillar Model D398 prime mover
at the surface elevation of 4,000 ft was 684.0. Drilling at a depth of 10,000 ft, the
prime mover power ratio is

.
281 7
PR ( 100) 41 2
.
.
684 0
Using Figure 4-17 the diesel fuel consumption rate for a power level of 41.2 percent
is 0.680 lb/hp-hr of diesel fuel. The total weight rate of diesel fuel consumption per
hour is