Is your vacuum pump losing pumping capacity? Can't the pump reach its ultimate pressure level? Are you seeing milky pump oil? These are the signs of pump oil contamination and you would probably need to do gas ballasting.
All gas can produce vapors at certain temperatures and pressures dependent upon their physical properties.The gas stream evacuated from a vacuum chamber will always contain vapor of water and solvent, as well as other potential contaminants. The moisture vapor will condense due to the low pressure inside vacuum pump. The resulting condensate will accumulate and contaminate the pump sealing oil because the liquid form of water and solvent cannot be effectively expelled from the pump. Contaminated oil will lose lubrication, cause over-heating and affect pump's performance.
Gas ballast valve is designed to admit some air to a pump's compression cycle. The increased pressure due to the tiny little amount of added air is helpful to reduce condensation and expel the condensable vapor with the air to atmosphere in its gas phase via pump exhaust port. The pump's overall performance won't be affected by controlling the opening of gas ballast valve. Therefore, proper use of gas ballast valve can add efficiency and reduce risks of contamination.
Should gas ballast be kept open all time?
Gas ballast valve helps to remove condensable vapor from a vacuum pump, but it will also prevent the vacuum system from reaching the highest vacuum level. In practice, the gas ballast valve should be opened at a pumping cycle starts. When an equilibrium is reached, i.e., the pressure is not going down anymore, and a further lower pressure is needed, the gas ballast valve can then be closed.
What to do if pump oil has been somehow contaminated by the condensed vapor?
In such a case, a purging process by using gas ballast is necessary to clean up the pump. The purging is carried out by running the pump for a few hours with pump inlet port closed and gas ballast valve slightly opened. Note that the pump might be smoking or emitting a visible mist during the process, posing a health issue or fire hazard. Using capture filters on pump's exhaust and oil mist return kit on gas ballast intake can eliminate these risks; proper venting is also necessary to maintain a health working environment.