Welch Vacuum 8925 Direct Drive Rotary Vane Pump - Manual (Page 10)

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2.06 Intake Antisuckback Protection
When power to the pump is turned off, this device closes automatically, maintaining vacuum in the
system being evacuated, and vents the inside of the pump to atmospheric pressure.
2.07 Pump Lubrication
To ensure efficient operation and proper maintenance, and to minimize noise and oil vapors, it is
important to use the correct type and quantity of oil. DIRECTORR® Gold Vacuum Oil has been
especially developed to have the proper viscosity, low vapor pressure, and chemical stability needed to
produce peak pumping efficiency. The ultimate vacuum guarantee on Welch pumps applies only when
this oil is used. Therefore, DIRECTORR® Gold Vacuum Oil is the only oil recommended for use with
these pumps. Each pump is supplied with sufficient oil for filling. Additional oil is available.
See Section 9: Accessories.
2.08 Exhaust Filter
Any oil-sealed vacuum pump tends to discharge oil mist from its exhaust port when the pump operates
under high-flow conditions, such as when the pump’s intake is at or near atmospheric pressure.
Typically, oil mist in the form of a white puff of “smoke” can be seen from the exhaust port when no
filter is used. Once the vacuum level and the corresponding air flow through the pump are reduced, very
little, if any, oil mist will be emitted.
An exhaust filter is recommended for any vacuum pump installation where the pump operates at high
intake pressures for a prolonged period of time. Oil droplets entrained in the pump’s exhaust are
removed by the exhaust filter element. Use of an exhaust filter typically reduces or baffles pump noise
as well. Exhaust filters are sometimes referred to as Oil Mist Eliminators. See Section 9 - Accessories.
2.09 Gas Ballast Valve
The gas ballast valve can increase the pump’s water vapor tolerance. (The gas ballast valve is sometimes
referred to as a vented exhaust valve.) In many vacuum pump applications the gases being pumped
from a system are a combination of permanent gases and undesirable vapors such as water vapor. Under
some conditions, the vapors condense in the second stage of the pump and contaminate the oil. The gas
ballast valve reduces oil contamination by decreasing or eliminating vapor condensation.
Vapor condensation is most likely to take place when the gas compression ratio is high, i.e. when the
pump compresses a relatively large volume of gas to a small volume. Whether or not condensation takes
place is dependent upon several factors, including the proportion of permanent gases to vapors at the
pump intake. If the gases being pumped consist entirely of vapors, condensation will definitely occur
unless the gas ballast valve is opened. The gas ballast valve adds a small amount of air at atmospheric
pressure to the gas being compressed in the second stage. This reduces the compression required to
push the gas out past the exhaust valve (less reduction in volume is required), and therefore reduces or
eliminates condensation.
When the gas ballast valve is open, the pump has to work a little harder, resulting in a slight increase in
operating temperature. The increase in temperature is small, however, and is not harmful to the pump.
Also, the pump is slightly noisier, and the pump’s ultimate pressure is somewhat reduced. Therefore, the
gas ballast valve should be kept closed when it is not needed. Note that the gas ballast is not equally
effective on all diffrent types of chemical vapors, so it may not always eliminate condensation
completely.