Reciprocating Compressors – Piping and Arrangements

Reciprocating compressors are sensitive to dirt. Therefore, the suction piping to each stage (from the knockout drum to the suction pulsation damper) should be “pickled” (chemically cleaned). Temporary strainers should be installed and left in place for several days during initial operation. These strainers are usually of the truncated-cone or conical type, and made of perforated steel plate with a double overlay of stainless steel wire mesh screen (often 100 mesh over 30 mesh). The screen is attached to the outside of the strainer basket so that the flow encounters the screen fist. The strainer is typically located at the inlet to the suction pulsation damper.

Liquid is also a significant problem. Suction piping must be configured so that liquids cannot be trapped in low spots. Liquids that collect in “pockets” in the piping can “slug” the compressor causing extensive damage. The suction line immediately attached to the separation device (knockout drum, scrubber) should be slightly sloped so that liquid drains back to the vessel. Inadequacy of liquid separators is a common complaint in operating plants. They may be too small, unable to take slugs, or located too far from the compressor. The latter is a very common
failing. This aspect of plant design should be studied very carefully. Very often the design is based on preliminary dew point calculations for a given gas, but later the composition changes. Accordingly, the design should include a generous safety factor.

Where there is any possibility of liquid condensation, the suction lines should be heat-traced. A second line of defense is to include a liquid separation chamber in the pulsation damper (see API 618). Pulsation dampers can be heat-traced with a “plate coil” for steam, or with electrical heaters. Bayonet heaters can also be supplied.

The distance between the pulsation damper and the compressor cylinder should be held to a minimum.

It used to be common practice to install intercoolers on top of the compressor cylinders. Each cooler would span across two cylinders, with pulsation-damping volume built into the coolers. This arrangement made a compact installation, but provided little defense against liquids, and compromised maintenance access. It also required that the suction valves of one of the two cylinders be placed in the undesirable location on the bottom of the cylinder. Currently, on-compressor mounting of coolers is no longer used, except perhaps for bone-dry gases.

Piping design should be analyzed to assure that forces and moments exerted on the cylinder flanges are within the vendor’s tolerances. Cold springing should not be allowed. (Cold springing is forcing pipe to the machine in a cold condition in order to afford relieved stresses as the pipe heats to operating temperature.)

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