The vacuum pump oil does more inside a vacuum oil purifier than many people assume. At the same time it is a sealing agent, a lubricant, a heat transfer medium and the carrier for the oil vapour in the gas phase. When the condition of the pump oil deteriorates the machine does not raise an alarm at once; instead the vacuum level slides down little by little, until one batch suddenly fails to meet the figures.
What the pump oil actually does
First, sealing. The gaps between the rotating and the sliding parts are filled by the oil film, and the higher the pressure in the vacuum chamber, the more demanding the sealing becomes. Second, lubrication and cooling: the oil film carries the friction heat away and at the same time suppresses the carbonisation caused by local overheating. Third, carrying: the gas in the pump chamber is compressed out together with the oil mist, and the oil takes that part of the flow away, cools it and returns it to the working surface. Fourth, mass transfer: the pump oil is itself the medium for the exchange between the gas phase and the pump chamber.
For exactly that reason the pump oil is subject to two kinds of contamination at the same time: the carbonisation caused by mechanical wear particles and the oil mist, and the water and the light fractions carried back from the oil circuit. Both of them destroy its function.
The trade-off in viscosity
The viscosity has to be chosen with both ends in mind. If the viscosity is too low, the oil film does not have enough carrying capacity, the leakage at the sealing gaps increases and the ultimate vacuum of the pump cannot be taken deep; at the same time a large amount of the oil mist is drawn away, and the oil consumption and the topping-up volume both rise.
If the viscosity is too high, the start-up at low temperature is difficult, the pump needs a longer preheating period before the oil film is established and the power consumption rises; the flow resistance is high, the pump body runs warmer, and over a long running time the oil is more likely to coke and carbonise at the hot points. The practice on site is to select the viscosity grade specified in the vacuum pump manual and to review it against the start-up and running behaviour at the actual ambient temperature, rather than copying the oil used on another model.
Three criteria for changing the oil
The colour is the visible signal: new pump oil is pale and clear, and a darkening of the colour after a period of running is a normal development; once it goes black or brown and does not recover quickly after fresh oil is added, carbonisation products have already entered the system.
The smell is the second signal: normal pump oil has a faint oily smell, and a burnt smell or a sharp sour smell point to local overheating and to acidic degradation products respectively; both of them accelerate the ageing of the internal parts of the machine.
The emulsified state is the most direct criterion: if a white milky substance is found in the oil when a sample is taken and there is a water layer underneath, it means that water vapour and oil vapour have already mixed into the pump oil, the vapour pressure of the pump oil has been raised, and the ultimate vacuum has deteriorated accordingly.
The chain reaction after the oil has deteriorated
Once the pump oil emulsifies, the saturated vapour pressure rises clearly, which amounts to an extra stream of water vapour in the vacuum chamber and directly raises the limit of the working vacuum; the efficiency of the dehydration of the oil in the vacuum falls accordingly and the share of oil carried in the condensate rises.
Carbonisation is more troublesome: the carbon particles are carried round with the pump oil into the valves and the pipework, which damages the sealing surfaces and reduces the free flow cross-section; once the atomisation of the pump oil increases, more oil and water vapour enters the vacuum chamber, the load on the fore pump rises, and a cycle of the more it is used, the worse it gets is set up. The end of this chain usually shows as an ultimate vacuum that will not go deeper and a gas content that does not reach ≤0.01%.
| Criterion | Normal state | Alarm state | Action |
|---|---|---|---|
| Colour | Pale yellow, clear | Black or brown and irreversible | Change the oil and flush the oil circuit |
| Smell | Faint oily smell | Burnt smell or sour smell | Change the oil and check for local hot spots |
| Emulsification | Uniform oil | Milky substance or a separated water layer | Change the oil at once and trace the source of the water ingress |
- Clean the pump body, the oil circuit and the strainer at the same time as the oil is changed, otherwise the residual carbon particles will spoil the new oil quickly.
- Set the oil change interval from the running hours and the degree of contamination of the oil, not from the calendar alone.
- If water is found in the pump oil, trace the source first: a leak in the pipework, water ingress on the vacuum side, or a high water content in the oil itself.
- The roots blower JRP-500 runs dry and does not consume pump oil, but the seals still have to be checked regularly.
The pump oil is the consumable that is easiest to overlook; its condition is not written into the operating parameters, it is written straight onto the vacuum gauge.