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2005 Gas Machinery Conference Page 10 9/30/2005
Enginuity LLC
4.1 Compressor Duty
The service duty plays a key role in the applicability of the four horsepower options. Application
design parameters such as pressure ratio, and volume flow, and the expected variability in these
parameters are critical factors influencing the horsepower / compressor selection. The expected
duty is really what drives the compressor selection due to the fundamental performance
differences between the reciprocating and centrifugal compressor design.
4.1.1 Pressure Ratio
While natural gas compression is considered a low-pressure ratio application (1.2 – 1.8),
there are exceptions such as gas storage facilities. A rule of thumb used by some in the
industry is to consider both centrifugal and reciprocating compressors below a pressure
ratio of 1.4, and default to a reciprocating compressor above 1.4. There are centrifugal
compressor designs that operate in high pressure ratio applications, however, they have not
been widely accepted in the natural gas transmission industry.
4.1.2 Throughput
The centrifugal compressor has earned the moniker of a big “fan” because its specialty is
high volume, low pressure ratio throughput. The higher the flow / horsepower ratio, the
more the application tends to favor a centrifugal compressor selection.
In the high flow reciprocating compressor arena, the slow speed integral generally out
performs the high-speed compressor options. While the high speed, reciprocating
compressor is generally operated at 3-4 times the speed of the integral (750 – 1000 rpm vs.
250 – 330 rpm), the integral advantage of higher displaced volume due to the longer stroke
and larger diameters generally trumps. Due to continued advancements in the high-speed
market by Dresser, Ariel, and others, the performance difference between the high and slow
speed compressors is shrinking.
4.1.3 Variability
While development efforts have dramatically improved the operating range of centrifugal
compressors, they are still subject to efficiency islands in their performance capability. As
the operating conditions move away from the design point, the performance drops.
The reciprocating compressor maintains higher performance over a relatively wide range of
operating requirements. This is due to the basic design of reciprocating compressors as well
as the ability to reconfigure the compressor with the use of unloaders and clearance volume
pockets. The advantage of the slow speed, reciprocating compressor over the high speed
compressor hold in this area as well. The larger displaced volumes, compressor valve areas,
and available real estate to install unloaders and clearance volume pockets gives the slow
speed compressor the advantage.
Therefore, high variability in pipeline conditions such as those experienced at the market
and supply ends of the pipelines tend to favor the use of reciprocating compressor packages,
and it could be said, favor the use of the slow speed integral compressor. In many market
areas, the requirement for flexibility and higher pressure ratio compression is becoming
more critical due to the emergence of swing power generation facilities. By definition,
these facilities are turned on and off as the electrical demand swings, which dramatically
impacts the volume flow requirements in the pipe. The gas turbine power generation
facility is also demanding higher and higher supply pressures that require higher pressure
ratio compression capability – again favoring the reciprocating compressor.