How to create oil free vacuum and tackle hydrocarbon vapors in pumps
Backstreaming vapor pump fluids, vapors of oils, rotary pump lubricants, and their cracking products can significantly disturb various working processes in vacuum. Therefore, it is recommended that certain applications use pumps and devices that reliably exclude the presence of hydrocarbon vapors.
Hydrocarbons are chemical compounds made of carbon and hydrogen which are the basis of everyday fuels such as oil and gas. Eliminating such hydrocarbons from your pump will ultimately reduce the cost of operation and maintenance. Excluding hydrocarbon vapors can be tackled by using a variety of methods. Some pump types are more suited to this process, which means that the following factors are essential to consider when choosing a pump type.
Rough vacuum region (1013 to 1 mbar)
Instead of rotary pumps, large water jet, steam ejector, or water ring pumps can be used. For batch evacuation, and the production of hydrocarbon-free fore vacuum for sputter-ion pumps, adsorption pumps are suitable. If the use of oil-sealed rotary vane pumps cannot be avoided, basically two-stage rotary vane pumps should be used. The small amount of oil vapor that backstreams out of the inlet ports of these pumps can be almost completely removed by a sorption trap (see page on accessories for rotary pumps) inserted in the pumping line.
Medium vacuum region (1 to 10-3 mbar)
For the pumping of large quantities of gas in this pressure region, vapor ejector pumps are by far the most suitable. With mercury vapor ejector pumps, completely oil-free vacua can be produced. As a precaution, the insertion of a cold trap chilled with liquid nitrogen is recommended so that the harmful mercury vapor does not enter the vessel. With the medium vacuum sorption traps described under a), it is possible with two-stage rotary vane pumps to produce almost oil-free vacua down to below 10-4 mbar.
Absolutely oil-free vacua may be produced in the medium vacuum region with adsorption pumps. Since the pumping action of these pumps for the light noble gases is only small, vessels initially filled with air can only be evacuated by them to about 10-2 mbar. Pressures of 10-3 mbar or lower can then be produced with adsorption pumps only if neither neon nor helium is present in the gas mixture to be pumped. In such cases it can be useful to expel the air in the vessel by first flooding with nitrogen and then pumping it away.
High and ultrahigh vacuum region (< 10-3 mbar)
When there is significant evolution of gas in the pressure regions that must be pumped, turbomolecular pumps, or cryopumps should be used. A sputter-ion pump is especially suitable for maintaining the lowest possible pressure for long periods in a sealed system where the process does not release large quantities of gas. Magnetically suspended turbo molecular pumps also guarantee hydrocarbon-free vacua. However, while these pumps are switched off, oil vapors can enter the vessel through the pump. By suitable means (e.g., using an isolating valve or venting the vessel with argon), contamination of the vessel walls can be impeded when the pump is stationary. If the emphasis is on generating a “hydrocarbon-free vacuum” with turbo molecular pumps, then hybrid turbomolecular pumps with diaphragm pumps or classic turbomolecular pumps combined with scroll pumps should be used as oil-free backing pumps.