Before considering valve upgrades, the overall compressor system must be evaluated. For example, to cure a liquid problem, the suction vessel must be looked at. Check the vessel sizing and the damper boot sizing. Check for insulation and heattracing if condensation is a possibility. Always keep the cylinder jacket water at least 10 to 20°F above the inlet gas temperature. Most liquid slugging occurs at startup when the compressor is cold. Make sure there are no dead legs where liquids can accumulate. All of these problems should be corrected before making a decision to upgrade the valves.
Valves do not pass liquids well. Liquid slugs have been known to cause broken pistons and, in at least one case, actually separate the compressor cylinder from the crankcase. The usual evidence of liquid slugging is severely damaged plates or no plates at all; only pieces. Springs can also collapse, although this is sometimes difficult to detect, since by the time the cylinder is opened the water or hydrocarbon liquid may have drained or evaporated.
Lube oil accumulation from over-lubrication can have similar effects. This is especially prevalent with some compressors equipped with plug type unloaders on the suction or top side of the cylinder. In one case, a piston assembly and valves were severely damaged when the compressor was loaded after being operated unloaded for about 30 minutes. Oil had accumulated in the valve pocket due to the design of the internal suction passages.
There are many new valve designs available from OEM and non-OEM sources. Valve problems are often design related, and they can be solved by a change in materials, or valve type. For example, one Company compressor experienced over 50 valve failures over a three-year period. Although the gas was wet, changing from strip to peek plate valves dramatically reduced the failure rate.
Before recommending replacement of existing compressor valves, ask the vendor to conduct a valve motion study on the existing application. Be sure the valve design is such that neither the valve guard nor the assembly bolting can fall into the cylinder, even if the valves assembly bolting breaks or comes loose.
Ask the prospective supplier how his valves and unloading system deviate from API 618. Confirm that the existing valves are being built to specifications according to dimensions, material hardness, etc. Whether valves are repaired by a contractor, or in-house, established specifications and procedures are a prerequisite to reliable performance.
Non-metallic valve plate materials have been developed for relatively high temperatures. These materials are only about one-sixth the weight of steel. Also, nonmetallic materials can be contoured to reduce the drag coefficient for the flow around the plate. These designs have shown a great deal of improvement in valve life at some installations. However, existing valves should not be indiscriminately replaced with these new designs.
It is important to determine whether the manufacturer designs and manufactures the unloader system or only the valves. Be careful of a manufacturer who supplies only valves but relies on others for the unloader mechanism. The unloader system is an integral part of most compressors and must be designed in conjunction with the valves.
Also, determine the manufacturer’s capability in materials engineering, finite element analysis, and non-destructive examination. Find out whether the manufacturer can perform mechanical testing including tensile, hardness, and impact tests.