TechTip 7: Rotary pump maintenance – oil changes and use of the ballast valve

Published: 20th December 2011 | © Anthias Consulting Ltd

Figure 1

Most vacuum systems comprise of a two-stage pumping arrangement (Figure 1) – an initial low-vacuum, high-capacity stage consisting of a rotary (also known as rough, mechanical or foreline) oil pump or diaphragm pump; and a second high stage, low-capacity vacuum pump such as a diffusion or turbo-molecular pump.  The first stage or low-vacuum pump requires regular maintenance to ensure optimum pumping performance.  Two aspects that are often overlooked are the use of the ballast valve and replacement of the pump oil at the correct intervals rather than just annually or bi-annually.

In normal use, all moisture, chemical vapours and solvent fumes introduced into the analytical instrument will end up through the vacuum system, dissolved in the vacuum pump oil or in the oil-mist filter.  For GC-MS or some GC detectors using a vacuum pump like a sulphur chemiluminescence detector (SCD) this will be limited to solvents and chemicals associated with the injected samples, whereas in LC-MS and ICP-MS systems the solvent contribution is much larger as a result of the LC mobile phase (99-99.9% is diverted to waste) and in ICP-MS most of the solvent will exit via the spray chamber (98-99%). 

As the contribution of volatiles builds up in the oil, the pumping efficiency of the oil is reduced. The ballast valve allows the ingress of air from the atmosphere into the vacuum pump, purging volatiles from the oil and out through the exhaust port (where they are trapped on the adsorbent trap). Whilst this is happening, the vacuum capacity of the rotary system will be limited to approximately 10-2 Torr in place of the normal 10-3 Torr operating limit. 

For general instrument use, ballasting the low vacuum pump for around 15 minutes each week should be adequate to ensure optimum pumping efficiency, however this will be dependent on the instrument type and the samples analysed, for example a single quadrupole GC-MS only uses a small rotary pump and ballasting is not usually required.  The manufacturers’ manual should be consulted for more specific information.

Even with adequate ballasting of the pump, the pump oil will degrade, indicated by a slow increase in colouration. New, fresh oil is clear and colourless; with use this darkens, slowly turning pale straw-coloured, at which point it should be emptied from the pump and replaced with fresh oil (Figure 2). If the oil is dark yellow or brown immediate action is required as it will not be lubricating well causing excessive friction resulting in premature wear on vanes and seals and scoring of the pump shaft as well as a poor vacuum resulting in poor sensitivity. Ultimately, not changing the oil early enough could result in motor burnouts and pump seizures. 

For most analytical instruments in general use, the oil will gradually change colour over a period of approximately 6-12 months and it is usual to change it at regular intervals, annually or biannually. However, how quickly it becomes dirty depends on how many samples and how dirty those samples are, therefore a weekly check of oil level and colour is strongly advised, with replacement of the oil more frequently if required.

Changing the pump oil usually requires the following simple steps:

  1. Be aware that any compounds introduced into the analytical system since the last oil change will have been concentrated in the oil being drained, it should be considered as hazardous waste and handled as such. Use suitable PPE for this operation and replace the oil in a fumehood.
  2. Either vent the system or isolate it from the pump undergoing the oil change as recommended by the instrument manufacturer, venting is usually required.
  3. Turn off the pump. The oil will be hot, so allow to cool but replace while still warm, especially if it is a darker colour and more viscous.
  4. Locate the drain/sump nut on the rotary pump and position above a suitable collection vessel in the fumehood. Unscrew and remove the sump nut (but don’t lose in the oil) to allow the oil to drain from the pump (NB the oil will be warm/hot and contain concentrated sample, so take care).
  5. Replace the sump nut and refill with fresh pump oil to just the minimum fill level. Flush the pump with fresh oil by running it for 1-2 mins (if possible), then allow the oil to settle before removing the sump nut and emptying this rinse-oil from the pump – if the oil was very dirty this step is very much recommended! The pump has now been thoroughly emptied and rinsed out of “old” oil.
  6. Replace the sump nut and refill the pump with fresh oil to 75% of the full mark or between the minimum and maximum lines – never fill to the maximum level as once turned on and hot, the oil will expand past this line damaging the pump and reducing pumping efficiency.
  7. Change the oil-mist filter attached to the pump outlet or if a maintenance kit is available perform the maintenance (if the oil is dirty this will be too).
  8. Run the pump for 5-10 minutes and check if the oil level settles after running, if necessary, turn off and top-up the oil level.
  9. Reconnect the pump to the instrument and pump down.

Remember to dispose of the oil as Hazardous Waste.
Retain the equipment used for oil draining for this purpose; it will be impossible to clean for any other use.

To learn more about GC-MS maintenance, attend:

To learn more about LC-MS maintenance, attend:

To learn more about ICP-MS maintenance, attend:

Don’t forget that you can also subscribe to our newsletters and receive useful tips straight to your inbox!