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A differential pressure switch set to alarm when the pressure drop reaches a predetermined point protects against the loss of oil flow. In addition to the differential pressureswitch， atwo-way， three-port valve with a pressure gauge is piped in parallel with the differential pressure switch for accurate indication of inlet and outlet oil filter pressure. When a single transflowvalve is used with a cooler-filter installation， the differential pressure switch and pressure gauge assembly should span the cooler-filter system.
Water contamination in the oil system can cause serious damage toturbomachinery， and every reasonable effort should be madeto，first， pre-vent its entranceinto thesystem， and second， provide suitable removal equipment if water cannot be effectively kept out. .xperience indicates that designers and equipment operators can be more effective in keeping water out of the system. Since the main sources of contamination are atmosphericcondensation， steamleaks， and faulty oilcoolers， preventive measures should be taken.
Condensation will occur in the atmospheric vented oil system whenever the temperature in the vapor space areas drops below the dew point. This effect can take place in the return oil piping as well as the reservoir. Consoles installed in unprotected locations are more vulnerable to climatic changes than those installed inside buildings. The outside locations will be adverselyaffected by temperature cycles between daytime and night operations.also，by showers and sudden temperature drops due to other weatherchanges， especially in the fall and winter seasons. There has been great success in ""drying up"" oil systems by making a few simple alterations. The first step is to check the reservoir unit. The vent should be located in the very top of the reservoir. It should be free of baffles that can collect and return condensateto the reservoir， and the length should be kept as short as possible to provide a minimum of surface areas on which condensate can form. If it is necessaryto run the vent up and away from the reservoir， a water trap should be provided as close to the reservoir as possible to remove any condensate formed in the vent stack. The next step is to provide and maintain an inert gas or dry-air purge on the reservoir. Only 2.5 cfh is required. The reservoir purge system will not substitute for the elimination of other water sources.
Steam and condensate leaks are the most difficult water sources to preventin turbomachinery. however， it can be done， and every effort should be madeto eliminate these sources. Obviously， the first means of prevention is to maintain the steam packing in perfect condition. .xperience has shown thateventually the steam packings willleak， and steam condensate will enter the system through the bearing seals. There has also been great success in ""drying up"" a wet oil system. The procedure is to purge the bearing laby-rinths with inert gas or dry air. One method is to drill a one-eighth-inch hole through the bearing cap and intersect the labyrinth. A one-quarter-inch diameter tube is connected to the hole in the bearing cap and to a rotometer. The labyrinth is then purged with 15 cfh (0.43 cmh) dry air or inert gas.
Another method is to install an external labyrinth with purge provisions on the bearing housings of a machine that has the necessary space to accommodate the external seal.
Removal of free water from oil systems is usually done with centrifuges or coalescer separators. Centrifuging is the most costly method in both capital outlay and operating cost. The centrifuges usually are the conventional disc-type with manual cleaning. The discs must be cleaned at least once each week with one hour required per cleaning. The coalescer separators usually require much less attention. Some separators only require element changesonce a year while others may require changes at six months or three months， and in some instances once a month. The frequency appears to be related tothe amount of water in the oil system. In many instances， coalescer element changes have been reduced by the use of a prefilter in the system. This element removes the particulates (usually rust) that would restrict the two-micron coalescer element. The time required to change both the prefilter element and coalescer separator elements is less than one hour.
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燃气涡轮工程手册 Gas Turbine Engineering Handbook 3(3)
