Page 142 - Air and Gas Drilling Manual

4-28 Air and Gas Drilling Manual
The above determined 168.2 horsepower is the actual shaft power needed by the
compressor to match the back pressure of 150 psig (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 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 derated input power is greater than the actual
shaft power needed, thus, the compressor system can be operated at this 6,000 ft
surface location elevation.
4.6.3 Rotary Compressor System Unit
Rotary compressors have fixed compressor ratios for each stage. These
machines have a constant pressure output. These compressors cannot adjust output
pressures to match the back pressure in the exit flow-line. The rotary compressors
will inject compressed gas into the flow-line at the compressor’s rated pressure
output. If the back pressure in the exit flow-line is less than the rated pressure
output, the gas will expand in the flow-line (or surge tank upstream of the flow-line)
to match the back pressure [1, 7].
There is no volumetric efficiency term for the rotary compressor. There is a
mechanical efficiency term for these compressors (denoted by the term ε ). This
m
term is used to determine the input power requirement. The mechanical efficiency
term is a measure of the friction losses in the mechanical application of the prime
mover power to the compressor. These losses are due to the friction in the bearings
and linkages in the overall compressor system. The values of the mechanical
efficiency for typical rotary compressor systems can vary from about 0.84 to 0.99.

Illustrative Example 4.2 A three-stage helical lobe (screw) primary air
compressor system unit is rated to have a volumetric flow rate of 900 scfm and a
maximum pressure capability of 240 psig at API Mechanical Equipment Standards
standard atmospheric conditions. The compressor has a diesel prime mover that is
rated to have a maximum power of 300 horsepower at a compressor shaft speed of
1,000 rpm (at API Standard conditions). The 1,000 rpm is the shaft speed required
for the compressor volumetric flow rate of 900 scfm. The prime mover is naturally
aspirated. This rotary compressor system has a mechanical efficiency of 0.90.
Determine the horsepower required by the prime mover to operate the
compressor against a flow-line back pressure of 150 psig for, a) a surface location at
sea level (use API Mechanical Equipment Standards standard atmospheric conditions
for sea level) and for, b) a surface location elevation of 6,000 ft (use average mid
latitudes data in Table 4-1).