____________________________________________________________________________________ 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.