Welch Vacuum 8925 Direct Drive Rotary Vane Pump - Manual (Page 18)

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Section 6: MAINTENANCE
6.01 Vacuum Problems
Inability to attain sufficient vacuum in a system is usually due to leakage, contamination, or unusual
outgassing. A system must be thoroughly clean and free from leaks to operate efficiently. If the system
is found to be clean and leak-free, but vacuum problems still exist, the pump should be checked.
A simple way to test the pump is to measure its ultimate pressure capability. This can be done by
disconnecting the pump from the rest of the system and connecting a pressure gauge directly to the
pump intake. (Be sure to seal the pump intake from the atmosphere, and be sure the gas ballast valve
is closed.) The gauge can be any type that is suitable for the pressure levels expected. Run the pump
until the gauge indicates no further reduction in pressure, and compare the pressure reading to the
pump’s ultimate pressure rating.
If the pump meets its ultimate pressure specifications only when disconnected from the rest of the
system, the fault must be elsewhere in the system. If the pump’s ultimate pressure is unusually high,
the pump may be badly contaminated, low on oil, or mechanically defective. However, if the pressure
is only slightly higher than the pump’s guaranteed pressure, an oil change may be all that is needed to
bring performance up to specifications. Be sure to use only DIRECTORR® Premium or Gold Oil in
vacuum pumps; the ultimate pressure guarantee does not apply if other types of oil are used.
The most common cause of efficiency loss in a vacuum pump is contamination of the oil, which is
usually caused by foreign particles and/or condensed vapors. The condensate emulsifies with the oil,
and when the oil is recirculated, the condensate evaporates. The resulting vapor then reduces the
ultimate vacuum attainable in the system.
Some foreign particles and vapors form sludges with the oil. The presence of sludge in the oil impairs
its sealing and lubricating properties, and eventually could cause pump seizure. Therefore, periodic
oil changes are necessary to maintain efficient operation of the system. The interval at which oil
changes are required is different for each set of operating conditions; experience will help you determine
the proper interval for your system and process.