Vacuum Pump Oils
Oils, greases or lubricants for vacuum pumps must meet high requirements. The vapour pressure must be low even at higher temperatures, water content and water absorption should be minimal. They should have a stable viscosity behaviour, have excellent lubricating properties and be resistant to cracking due to increased mechanical stress. LEYBONOL oils have all been tested in the our own laboratories in extensive series of tests under application conditions for their use in the respective series. In order to be able to respond optimally to the most varied applications of our customers, various types of oil are used in our vacuum pump series.
Mineral oils are products distilled and refined from crude oil. These do not consist of precisely defined constituents but rather consist of a complex mixture. The way in which the mineral oil is pre-treated and its composition is decisive as to the applications it will be suited for. Depending on the distribution of the hydrocarbons and the dominance of certain properties, mineral oils are grouped according to paraffinbase, naphthenic and aromatic.
For the purpose of attaining especially low ultimate pressures, mineral oils must be selected on the basis of a core fraction. The thermal and chemical resistance of mineral oils has been found to be adequate in the majority of applications. They offer a high degree of compatibility with elastomers and resistance to hydrolysis. Mineral oils also include the group of hydrocracked oils. These are frequently also termed semi-synthetic oils. Hydrocracked oils are produced under a very high hydrogen pressure at high temperature and are substantially free of aromatic compounds and olefins. Hydrocracking oils exhibit a higher thermal stability compared to conventional mineral oils. In most cases the intervals between the oil changes can be extended.
Synthetic oils are produced through chemical reactions. The group of synthetic oils includes liquids differing widely as to their chemical structure and composition. Correspondingly, their physical and chemical properties differ considerably. Synthetic oils are used in those cases where special properties of the oil are required which cannot be fulfilled by mineral oils.
Polyalphaolefin oils are synthetic hydrocarbons which are paraffin like, but have a uniform structure. Thermal and chemical resistance is better compared to mineral oils. Owing to their good flowing properties when cold they can be used at low temperatures. Elastomer compatibility and resistance against hydrolysis are comparable to mineral oils.
Ester oils are organic compounds which excel especially through their high thermal resistance to cracking compared to mineral oils. Chemical resistance is generally quite good, but will depend on the type of ester oil. Elastomer compatibility and resistance against hydrolysis are not so good compared to mineral oils. They should not be used when pumping acids, halogens or alkaline media like ammonia in connection with humidity.
These are oils which are only composed of carbon (C), fluorine (F) and oxygen (O) atoms. The existing C-O and C-F bonds are highly stable. For this reason PFPE oils are practically inert against all chemical and oxidizing influences. Perfluoropolyethers will not polymerise under the influence of high energy radiation. Perfluoropolyethers are used when pumping strongly oxidative substances like oxygen, ozone or nitric oxides as well as highly reactive substances like halogens and hydrogen halides. Regarding Lewis acids (for example, boron trifluoride BF3, aluminium trichloride AlCl3) they are not completely inert. Here reactions may take place at temperatures over approximately 150 °C (302 °F). Perfluoropolyethers are thermally highly stable. PFPE is not flammable. Thermal decomposition may only take place at temperatures of over 290 °C (554 °F).
Diffusion Pump Oil
Pump fluids for oil diffusion pumps must exhibit a low vapor pressure at room temperature and must be able to resist thermal decomposition and oxidization to a large extent. Surface tension of the pump fluids must be high to reduce creep of oil films. They must be chemically inert, exhibit a high flash point and evaporation heat must be low. Moreover, the pump fluids should permit high pumping speeds over a wide range of pressures and be cost effective. One type of pump fluid alone cannot meet these comprehensive requirements. It is therefore required to select a pump fluid according to the operating pressure and the requirements of the application in each case.
Lifetime - How Often Should You Change Vacuum Pump Oil?
Lubricants cannot be used indefinitely, as they age over time and their quality will degrade. This aging is caused, among other things, by high temperatures (see Temperature section), oxidation, chemical and physical reaction with process media. This can lead, for example, to the formation of sludges, resins, crystallization and acids.
An oil change within the scheduled service intervals is important for the durability of your vacuum pump. The length of the oil change intervals varies depending on the choice of oil. Synthetic oils with special additives are more resistant than mineral oils, for example. We distinguish between:
Oil change intervals
- Standard life time: Oil change after 0.5 years
- Extended life time: Oil change after 1 years
- Long life time: Oil change after 2 years
Temperature - Important Factor for the Durability of Vacuum Pump Oils
The oil durability depends on many parameters. An important influencing factor is the temperature. Mineral oils usually have a maximum temperature limit of 80 °C. Above this temperature, it can be said in a very simplified way that a temperature increase of 10 °C causes a reduction of the oil service life by half. This results in a thickening of the oil. Synthetic oils can be operated permanently at 100 to 160 °C depending on the type. PFPE oils up to a maximum of 250 °C permanent temperature, lower maximum temperatures must be observed depending on the process medium. PFPE oils are not subject to typical oil ageing, as they are almost inert (see also the section on PFPE oil)
On the other hand, if a pump is operated too cold, water vapour or other vapours may condense. Condensed liquid can cause loss of lubrication properties or corrosion in the pump.
Mixing the following oils with another type of oil must be strictly avoided:
In addition, a change from another type of oil to PFPE may only be done by authorized service centers. The pump must be completely disassembled and carefully cleaned. Seals and filters must be changed and suitable grease must be used.
In general, mixing oil with different types of oil or oil from another manufacturer will result in a deterioration of performance. Often it is even one of the main causes of system problems. Decisive for the question whether oils are mixable with each other is the distinction between the terms mixability and compatibility. Mixability merely indicates whether two liquids dissolve easily and completely in each other. Compatibility, on the other hand, is only given if two oils can be mixed and both keep their individual properties.
- Vapour pressure
- Vapour pressure is the ambient pressure below which a liquid begins to change to the gaseous state at a constant temperature.
- Oxygen concentration
- In applications where pure oxygen or concentrations above atmospheric (> 21 vol. %) are present, suitable equipment must be used. (Oxygen reacts with hydrocarbons. There is a risk of ignition with mineral oil-based lubricants and most synthetic oils. Oxygen can cause self-ignition of oils or greases).
- Additives are oil-soluble substances that can be added to lubricants in low concentrations to improve certain properties of the lubricant. Frequently used additives serve to improve or prevent oxidation, wear, corrosion, flowability and foaming.
- Viscosity is a measure of the internal friction of a fluid. Optimum viscosities are required for the formation of hydrodynamically supporting oil films, optimum oil delivery, sealing and lubricating effect and also heat input. This must be within certain ranges depending on the intended use. The viscosity is strongly dependent on temperature. With increasing temperature, the viscosity decreases, i.e. the lubricant has a lower toughness. If the oil is too viscous at operating temperature, it will not flow through the oil lines, resulting in insufficient lubrication and damage. If an oil is too thin, it will not lubricate sufficiently. The result is too rapid abrasion and a poor final pressure.
- Gas flow
- In vacuum technology, using the measure of the gas flow as the product of pressure and volume flow has proven to be helpful. The gas flow to be pumped off during the process influences the size or the nominal pumping capacity of the pumps to be used. In practice, there are several synonymous terms: gas flow rate, gas flow, pV flow or suction capacity.
- Pumping speed
- The pumping speed of vacuum pumps is usually only constant over a certain pressure range and decreases towards low pressures. The available pumping speed therefore depends on the suction pressure of the pump. In order to maintain the target pressure necessary for the process with a total gas flow, the selected vacuum pump must provide a suction capacity at this pressure that corresponds to the gas flow.
- Process pressure
- The target pressure is initially the pressure to be maintained for the actual process and is also called process pressure.
- Inert gases
- Inert gases are gases that have a low chemical reactivity. Inert gases include all noble gases as well as nitrogen and carbon dioxide.