Many chemical plants rely on reciprocating compressors. These units, also known as piston-type compressors, can generate high head independent of the gas density. They currently are the only compressors available for producing pressures above 1,000 bar. They particularly suit services with relatively low capacity and relatively high differential pressure.
The configuration preferred today has horizontal cylinder(s) with the discharge nozzle on the bottom side (Figure 1). Vertical and inclined machines still are in use but aren't popular; they generally are considered only for non-critical applications and small sizes (usually below 350 kW). Sometimes plants select non-horizontal machines because they have a smaller footprint and so can save space or because they're cheaper.
Today, oil-free (non-lubricated) piston-type units constitute around 50% of the reciprocating compressor business for small- and medium-size applications in commonly used pressures and services. They continue to make inroads, particularly where oil carryover can cause downstream problems such as poisoning of a catalyst in a reactor. Such compressors use special materials for piston rings, rod packing and other sealing systems in contact with the compressed gas to avoid any oil carryover by the gas. The bearings and crankshaft mechanism still require a lubrication oil system; dry sealing systems isolate the lubrication oil from the compressed gas. The price of oil-free machines may slightly exceed that of oil-lubricated ones — the small cost difference is continually decreasing. However, oil-free technologies do have limits in pressure, speed, size and other parameters, and may lack successful references for particular applications.
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Dry (oil-free) seal components may not last as long as lubricated sliding components because they might not stand up as well to friction and wear at sliding and contacting surfaces. However, oil-free operation may provide better overall reliability for an entire chemical processing unit, if the compressor uses suitable oil-free components (particular sealing materials) and has proven itself in comparable services.
For large, high-speed or high-pressure machines, lubricated cylinder technology may remain the first choice. A lubricated cylinder in a low-speed (around 300–350 rpm) machine may be the best option for large (>1.5 MW) or high-pressure (say, >50 barg) services, if there's no process requirement for zero oil carryover.
Traditionally, low-speed (around 330 rpm) reciprocating compressors with lubricated cylinders promised the highest reliability. However, starting about 20 years ago, many vendors began challenging the lubricated-cylinder low-speed mantra for small- and medium-size machines. Now, oil-free (non-lubricated) technology is well established. Speed remains a critical issue for both oil-free and lubricated units. High speed ranges could result in relatively small compressors with cost, weight and space benefits. However, machine reliability, particularly for contacting and sliding surfaces, could decrease. So, speed selection demands care. The optimum piston speed usually falls in 3–4.5 m/s range. Large- and medium-size machines typically operate at around 300–350 rpm, while small compressors (say, <250 kW) may run at up to 600 rpm. Oil-free compressors generally have lower speed limits than lubricated ones.
Optimum performance depends upon proper integration of the reciprocating compressor and the processing unit, and requires extensive studies of both systems. It's crucial to review all compressor performance curves, match machine curves with process plant requirements, assess mutual interactions, and simulate all operating situations.
Load curve. Try to avoid a steep load curve. A review of the steepness of the load curves can quickly identify which load steps are quite steep in nature and, thus, where small changes in the pressure can significantly alter the load and flow (capacity). Steep load curves may indicate improper sizing of the cylinders. Compressors with steep curves also can pose control difficulties.