What is Oil Mist in Oil-Sealed Rotary Vane Vacuum Pumps?
Regardless of manufacturer, oil-sealed rotary vane vacuum pumps generate oil mist from the exhaust ports while running. This "mist," which is sometimes incorrectly referred to as "smoke," is a fine aerosol of vacuum pump oil that occurs when the oil is forced through the small clearances inside the pump mechanism and as a result of the higher operating temperature. This hydrocarbon oil mist could pose a flammable threat.
How Can Oil Mist Get Into the Pump's Exhaust?
Oil mist can be visible near the pump's exhaust when an oil-sealed rotary vane pump has a high gas flow, like roughing a big chamber down from ambient pressure. Other causes include pumping a high gas flow introduced in the vacuum system or operating gas ballast.
This oil mist can cause the pump to lose a substantial percentage of its oil change over time, potentially depleting the pump's oil. It can cause oil to be pulled into an exhaust ducting system or the environment in a laboratory.
Even when a vacuum pump is at maximum pressure, a tiny quantity of oil mist is usually visible at the exhaust. Most contemporary oil-sealed rotary vane pumps include an inbuilt 'air leak' that does not compromise vacuum performance but avoids hydraulic "knocking" at very low pressures. Even with a pump that seems to be producing no gas, this air leak allows a small quantity of oil mist to flow to the exhaust.
All oil-sealed rotary vane pumps should have an exhaust mist filter installed to trap oil mist and enable it to flow back into the pump oil box. Most exhaust mist allow the mist to condense into droplets that cannot "float" in the atmosphere, like aerosols.
The oil is drained back into the mist filter body by gravity. This oil can then be returned to the pump manually or automatically using an 'oil return' attachment.
Oil mist filters should have an overpressure mechanism that allows the filter to be "bypassed" if it becomes clogged.
It prevents the pump's oil box from becoming too pressurized, which may lead to external oil leaks and poor pump performance, as well as a dangerous 'overpressure' condition.
A high-quality exhaust mist filter will additionally include an odor-absorbing material, commonly activated charcoal, to absorb the oily "smell" from the pump's exhaust.
Because exhaust mist filters do not alter the nature of exhaust gasses, if hazardous gasses are pumped, they must still be controlled after the filter has served its purpose. It is also required to evaluate the exhaust mist filter's efficiency and chemical compatibility with the pumped gasses.
The filtering components are consumables that require regular replacement. A good rule of thumb would be to change them when getting routine oil changes to ensure efficient and effective operation.
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