Reciprocating Compressors #2
A compressor may have any number of stages. Each stage normally contains a suction scrubber to separate any liquids that carry over or condense in the gas line prior to the compressor cylinder (or case for centrifugal compressors). When gas is compressed, its temperature increases. Therefore, after passing through the cylinder the gas is usually cooled before being routed to another suction scrubber for another stage of compression. A stage of compression thus consists of a scrubber, cylinder, and after-cooler. (The discharge from the final cylinder may not be routed to an after-cooler.)
The number of throws is not the same as the number of stages of compression. It is possible to have a two-stage, four-throw compressor. In this case there would be two sets of two cylinders working in parallel. Each set would have a common suction and discharge.
High-speed units are normally “separable.” That is, the compressor frame and driver are separated by a coupling or gear box. This is opposed to an “integral” unit where power cylinders are mounted on the same frame as the compressor cylinders, and the power pistons are attached to the same drive shaft as the compressor cylinders.
High-speed units are typically engine or electric motor driven, although turbine drivers have also been used. Engines or turbines can be either natural gas or diesel fueled. By far the most common driver for a
high-speed compressor is a natural gas driven engine.
Figure 10-3 shows a high-speed engine-driven compressor package. The unit typically comes complete on one skid with driver, compressor, suction scrubbers and discharge coolers for each stage of compression and all necessary piping and controls. On large units (> 1,000 hp plus) the cooler may be shipped on a separate skid.
The major characteristics of high-speed reciprocating compressors are:
• Numerous sizes from 50 hp to 3000 hp.
• 2, 4, or 6 compressor cylinders are common. Advantages
• Can be skid mounted.
• Self-contained for easy installation and easily moved.
• Low cost compared to low-speed reciprocating units.
• Easily piped for multistage compression.
• Size suitable for field gathering offshore and onshore.
• Flexible capacity limits.
• Low initial cost.
• High-speed engines are not as fuel efficient as integral engines (7,500 to 9,000 Btu/bhp-hr).
• Medium range compressor efficiency (higher than centrifugal; lower than low-speed).
• Short life compared to low-speed.
• Higher maintenance cost than low-speed or centrifugal.