Many users of screw compressors hire service and contracting organizations to perform nonroutine maintenance and repairs on the machines. Major overhauls and service require the attention of specialists, but some important monitoring and service functions can be the responsibility of operating personnel. Valuable data to be recorded and analyzed include: suction pressure, suction temperature, suction superheat, discharge pressure, discharge temperature, oil pressure at the compressor, pressure drop across the oil filter, oil temperature, and oil level. Several of these items, such as the suction pressure and temperature and the discharge pressure are conditions imposed by the system, but they should be normal in order for the compressor to function properly.
A routine maintenance task is to periodically clean the oil filter, for which the shutdown of the compressor is required. The service valves on either side of the filter are first closed, then the purge valve opened to relieve refrigerant pressure. The filter canister is then opened and the cartridge removed for flushing with clean oil. After replacing the cartridge the canister is filled with clean oil, closed, and is then ready for resumption of service.
Another maintenance procedure usually handled by the operating staff is the periodic replacement of the elements in the coalescing separator. The compressor is shut down, the discharge valve closed, and the suction check valve opened in order to vent the refrigerant to the low-pressure region of the system. The coalescing elements are accessible following removal of the manway. A new gasket is provided for the manway upon reassembling.
High quality oil is essential because of the crucial role the oil performs in operation of the compressor. When initially charging oil into the compressor, two samples should be taken and one analyzed in the laboratory as the control reference. The other sample should be kept in a clear glass container and used as the reference basis for clarity of the oil extracted every 2000 hours of operation. A systematic oil analysis program should determine the viscosity of the oil at two temperatures, 38°C (100°F) and 100°C (212°F). In addition the moisture content should be measured as well as mineral content (iron, antimony, phosphorus, copper, and magnesium). The presence of minerals may indicate a problem in the refrigeration system or the compressor itself. All the data from the periodic oil analysis are used as an indication of when to change oil.