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Vacuum Pump Maintenance

Vacuum pumps are used for a variety of applications in laboratories, and can be classified based on the application and its vacuum requirements, for example:

  • Laboratory/filtration vacuum pumps: low vacuum pumps used for filtering, drying, degassing, and evaporation. Most laboratory/filtration pumps have vacuum and pressure capabilities
  • Rough vacuum pumps: used in applications requiring a level of vacuum less than 10–3 Torr, such as degassing, freeze drying, and cryogenics
  • High vacuum pumps: those vacuums higher than 10–3 Torr, used for mass spectrometry, electron microscopy, evaporation coating, and space simulation

As with any other piece of equipment, proper routine maintenance to ensure these pieces of machinery are working correctly is essential for them to function and perform well in whatever capacity they are being used. 
For the purpose of this piece we will discuss rotary, foreline, diffusion, and turbomolecular pumps; all of which can be used in a variety of applications but are also used in conjunction to provide the high levels of vacuum required in LC- and GC-MS systems.

Rotary Pumps

Rotary pumps move fluid using the principles of rotation.  The vacuum created by the rotation of the pump captures and draws in the liquid.  Rotary vane pumps can be used in a variety of applications such as for freeze drying, controlled atmosphere glove boxes, or centrifugal concentrators.  Smaller sizes have a maximum flow of 3 mL/min with pressures in the range 80-120 mbar.

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Figure 1: Rotary pump.

Foreline (Rough) Pumps

Foreline pumps (also known as rough pumps) are oil-sealed rotary vane pumps in which a piston on an eccentric drive shaft rotates in a compression chamber sealed by spring-loaded vanes, moving gas from the inlet side to the exhaust port.  Pumping capacity is limited to approximately 0.1 Pa (10-3 Torr) due to the vapor pressure of the sealing oil, and at a speed of 1,400 rpm the pump will typically exhibit capacities of between 50-150 mL/min.

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Figure 2: Foreline (rough) pump.

Any pump, such as a rotary or foreline pump which contains oil requires maintenance.

Pump Oil

For wet pumps, such as primary rotary vane pumps and diffusion pumps, oil contamination is one of the most common maintenance concerns.  Oil must be carefully maintained and changed at proper intervals. Over time, pump oil can become oxidized or contaminated by vapors from the process in which it is being used, resulting in reduced lubricity.
Pump oil should be maintained at the correct level at all times.  Check the oil level weekly and top up as necessary.  The condition of the oil should also be monitored for changes in color or the presence of particulates (Figure 3).  The most common indicators that oil needs to be changed are, a dark appearance with or without the evidence of particulates, inability of the pump to reach its ultimate vacuum with the inlet valve closed, a milky appearance, which is an indicator of water/moisture being present in the system, or excessive noise.  Before changing the oil, run the pump for a few minutes, and then turn it off; the oil will flow and drain more thoroughly when warm.  If the drained oil is very dirty consider changing it more frequently.

If there is an excessive loss of oil consider the following:

  • A damaged vacuum pump
  • Excessive solvent going to the pump and displacing the oil
  • A gas ballast left open for extended periods of time

Figure 3: Change in pump oil color with use.

Change the oil when it starts to darken or routinely for every 3,000 hours of use (this is approximately every 4 months).  Change the oil as follows:

  • If the pump is attached to an instrument, for example an LC- or GC-MS, remember that any compounds introduced into the instrument 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 if possible replace the oil in a fume hood
  • Gas ballast the pump for 30 minutes (it is much easier to drain the oil while it is still warm)
  • Locate the drain/sump nut on the rotary pump and position above a suitable collection vessel. Unscrew and remove the sump nut to allow the oil to drain from the pump (the oil will be warm/hot and will contain potentially hazardous material, therefore, take care)
  • Replace the sump nut and refill with fresh pump oil to the minimum fill level. Flush the pump with fresh oil by running it for 1-2 mins, then allow the oil to settle before removing the sump nut and emptying this rinse oil from the pump. The pump has now been thoroughly emptied and rinsed of old oil.  If the rinse oil is particularly dirty repeat this process
  • Replace the sump nut and refill the pump with fresh oil to 75% of the full mark or between the minimum and maximum lines
  • Change the oil-mist filter attached to the pump outlet
  • 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
  • Gas ballast for 30 minutes

Gas Ballasting

Gas ballasting serves two important purposes (Figure 4):

  1. When rotary pumps are used to pump away solvent vapors the solvent can become dissolved in the oil causing an increase in backing line pressure.  Gas ballasting is a means of purging the oil to remove dissolved contaminants
  2. Oil mist expelled from the rotary pump should be trapped in an oil-mist filter.  This oil is then returned to the pump during gas ballasting

Gas ballasting should be performed on a weekly basis for approximately 30 minutes.  Always consult your manufacturer’s instructions for specific instructions on gas ballasting.  It is normal for the pump to make a noise when the gas ballast valve is open.  Do not leave the gas ballast valve open during normal vacuum pump operation.  Leaving a gas ballast open will cause a vacuum pump to run hotter than normal and causes the vacuum pump oil to break down or “burn”.  Furthermore, an open gas ballast valve will cause additional oil mist to be evacuated out of the outlet, this loss of oil is problematic for anything the exhaust comes in contact with but also, in the extreme, empties the vacuum pump of oil.

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Figure 4: Foreline pump gas ballasting.

Exhaust Filters

Foreline pumps can contaminate the working environment by expelling oil or chemicals that have contaminated the pump oil.  A two stage vacuum pump filtering system is highly recommended to keep high air quality levels in the instrument laboratory.  The first stage filter consists of an oil-mist filter to trap the heavy oils exiting the pump.  The trapped oil is then returned to the rotary pump during gas ballasting.  The second stage is a charcoal trap; this trap is especially designed to retain volatile organics.  Both of these elements will need changed as part of a routine maintenance schedule.

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Figure 5: Foreline (rough) pump exhaust filters.

Turbomolecular Pumps

Turbomolecular pumps consist of a system of rotating foils or blades that are angled to compress exiting molecules and progressively draw them down through the stack and out via the exhaust.  Whilst spinning at around 60,000 rpm these pumps have varying capacities, with 200-500 L/s being typical.  Turbomolecular pumps are usually sealed units and no user repair or maintenance is possible.  If the pump fails it is usually replaced with a new one.  However, always consult the manufacturer’s manual for any specific maintenance requirements of the pump.

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Figure 6: Turbomolecular pump.

Diffusion Pumps

Diffusion pumps operate by boiling a fluid, often a hydrocarbon oil (e.g. polyethylene oil), and forcing the dense vapor stream through the central jets angled downward to give a conical curtain of vapor.  Gas molecules from the chamber which randomly enter the curtain are pushed toward the boiler by momentum transfer from the more massive fluid molecules.  Water cooled diffusion pumps have coils through which cooling water circulates during operation.  The boiler will normally operate between 280-300 °C and the vapor exiting the pump will often have supersonic velocities (> 750 m/s) and temperatures of approximately 100 °C.  Pumping capacity is in the same order as turbomolecular pumps (200-500 L/s).  Oil diffusion pumps require very little maintenance.  The oil level should not drop unless a vacuum accident occurs.  The most likely failure of an oil diffusion pump is the heating plate.  This can usually be replaced by the user (the pump manual should be consulted for specific information).  High vacuum pumps are often water cooled and will switch off if the water supply fails.

In diffusion pumps, the use of hydrocarbon based oils can leave a burnt residue on the pump interior and potentially clog the vapor jets. Silicone based oils are less prone to this oxidation and are required for diffusion pumps.

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Figure 7: Diffusion pump.
 

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Dr. Dawn Watson
 

This article was written by Dr. Dawn Watson.

Dawn received her PhD in synthetic inorganic chemistry from the University of Strathclyde, Glasgow. The focus of her PhD thesis was the synthesis and application of soft scorpionate ligands. As well as synthetic skills, this work relied on the use of a wide variety of analytical techniques, such as, NMR, mass spectrometry (MS), Raman spectroscopy, infrared spectroscopy (IR), UV-visible spectroscopy, electrochemistry, and thermogravimetric analysis.

Following her PhD she spent two years as a postdoctoral research fellow at Princeton University studying the reaction kinetics of small molecule oxidation by catalysts based on Cytochrome P450. In order to monitor these reactions stopped-flow kinetics, NMR, HPLC, GC-MS, and LC-MS techniques were utilized.

Prior to joining the Crawford Scientific and CHROMacademy technical team she worked for Gilson providing sales and support for the entire product range including, HPLC (both analytical and preparative), solid phase extraction, automated liquid handling, mass spec, pipettes, and laboratory consumables.

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