Page 467 - Air and Gas Drilling Manual

Chapter 9: Aerated Fluid Drilling 9-71
operation. The most appropriate is the semi-trailer mounted unit with a Dresser
Clark Model CFB-4, four stage, reciprocating piston primary compressor with a
rated volumetric flow rate of 1,200 scfm (see Figure 4-25). This four-stage
compressor system is operated as a combined primary-booster system with a
continuous maximum pressure output of 1,000 psig. The prime mover for this
compressor unit is a diesel fueled, turbocharged, Caterpillar Model D398 with a peak
output of 760 horsepower at 900 rpm (at API standard conditions). This compressor
system has sufficient air volumetric flow rate to meet the maximum requirement of
1,168 acfm and the maximum injection pressure of 420 psia.
The last criteria is to check whether the prime mover of this primary compressor
unit has the power to operate at the 4,000 ft surface elevation. The highest injection
pressure occurs while drilling out of the casing shoe at a depth of 7,000 ft. This
injection pressure is approximately 420 psia. Even though the required air
volumetric flow rate at this depth is approximately 120 acfm, the compressor must
output the full 1,200 acfm (the remainder is vented). The compressor is a four-stage
reciprocating piston compressor. Therefore,
n s 4
˙
The theoretical shaft horsepower, W , required by the compressors is obtained from
s
Equation 4-35a. Equation 4-35a becomes
04
(. )
() ( . ) (.685 ) 1 200 420 .0 ()( . )
,
4
12
1 4
4
41
W ˙ 1
s
04
(.) 229 .17 12 .685
W ˙ 264 .1
s
The mechanical efficiency, ∋ , is
m
∋ m 090
.
The first stage compressor ratio is
1
420 0 . 4
r s
.
12 685
.
r s 240
The volumetric efficiency (only for the reciprocating piston compressor), ∋ , is
v
determined from Equation 4-37. The compressor clearance volume ratio, c, is
assumed to be 0.06. Equation 4-37 becomes