Page 145 - Air and Gas Drilling Manual

ε
.
and from Equation 4-39, the actual shaft power is
151
=
W ˙ m as = 090 .4 = 168 .3 Chapter 4: Compressors 4-31
. 090
The above determined 168.3 horsepower is the actual shaft power needed by the
compressor to produce the 204.0 psig maximum pressure output (at the surface
location elevation of 6,000 ft above sea level). At this surface location, the input
power available from the prime mover is a derated value (derated from the rated 300
horsepower available at 1,000 rpm). In order for the compressor system to operate at
this 6,000 ft surface location elevation, the derated input power available must be
greater than the actual shaft power needed. Figure 4-15 shows that for 6,000 ft
elevation the input power of a naturally aspirated prime mover must be derated by 22
˙
percent. The derated input horsepower available from the prime mover, W , is
i
(
˙
W = 300 1 − 0 . ) =22 234
i
For this example, the prime mover’s derated input power is greater than the actual
shaft horsepower needed, thus, the compressor system can be operated at this 6,000
ft surface location elevation.
4.6.4 Fuel Consumption
In order to effectively plan air or gas drilling operations, it is important to be
able to predict the fuel consumption by the prime movers of compressor system
units. The consumption of either liquid or gaseous fuels is initially calculated in
pounds of fuel used per horsepower-hour (lb/hp-hr). Fuel consumption is very
dependent upon the prime mover’s power ratio, or percent power utilization of total
power available.
Figure 4-17 gives the approximate fuel consumption for prime movers fueled
with gasoline, propane/butane, or diesel [3, 12]. Most stationary prime movers can
be field converted to operate on propane/butane, or natural gas. Figure 4-18 gives
the approximate fuel consumption for prime movers fueled by natural gas [3, 12].
Illustrative Example 4.3 Determine the diesel fuel consumption rate (in
gallons per hour) for the primary reciprocating piston compressor system and
conditions given in Illustrative Example 4.1 for: a) a surface location at sea level
and, b) a surface location elevation of 6,000 ft.
a) Surface location at sea level.
The prime mover has a maximum 300 horsepower available at 1,000 rpm at sea
level conditions. The actual shaft power needed by the three-stage reciprocating
compressor to match the back pressure of 150 psig is 190.4 horsepower. Thus, the
percent load on the prime mover, or power ratio, PR, is