Properly maintained vacuum pumps, such as rotary vane pumps, etc, will provide many years of reliable, maximized performance. Below are some simple ways to maintain such rotary vane vacuum pumps as well as options for what to do when pump performance is compromised due to oil contamination and degradation.
Principles of Operation
Oil-Seal, Rotary Vane vacuum pumps pull millitorr-level vacuum (‘high vacuum”) by sweeping intake air and vapors from the intake port around to the exhaust port. Note in the diagram above how the rotor is offset in the chamber, or “stator”. The rotor is set with only 1/1000” clearance from the top of the stator. Vacuum pump oil seals this tiny gap and prevents regurgitation of the airflow. For this reason, this technology is referred to as “oil seal, rotary vane” vacuum pumps. Vacuum pump oil also lubricates the vanes, which are spring-loaded so they always push to the inside wall of the stator, allowing for very efficient sweeping action.
In a “two-stage” pump, the exhaust from the first stage chamber is fed into the intake of the second stage and lowers the vacuum level achieved down to, or below, 1 millitorr (1 X 10-3 mm Hg) residual pressure. When a vacuum pump is first evacuating, the oil vapor pressure is high enough that a visible amount of oil aerosol or “mist” exits from the exhaust port. As the pump pulls vacuum below 1 torr, this oil mist dissipates, as does the gurgling noise associated with pumping down a chamber.
The Effects of Higher Pressures on Vacuum Pump Oil
Two-Stage Oil-Seal vacuum pumps have an upper operating limit. Continuous operation above 10 Torr is not recommended. That's because, at this higher pressure, the pump sweeps enough air that during the later portion of the vane rotation, that the vapor molecules are compressed into a smaller and smaller volume. This compression heats up the vapor stream; the more vapor molecules there are, the more heat is added to the system. This excess heat has a significant impact on standard vacuum pump oil.
The excess heat causes carbon bonds to dissociate, then reassociate in longer and longer carbon chains. This oil polymerization leads to vacuum pump oil that is brown and viscous. Excess viscosity adds friction and therefore adds even more heat load in system cascading into even more oil polymerization. The more viscous oil also fails to seal as well, again compromising vacuum levels and adding more vapor to the system, and as a result, more heat. Ultimately, the polymerized oil becomes so viscous that, if left unchanged, it becomes a hardened mass inside the pumping module once the pump is turned off and the oil cools. At this point, the rotors cannot turn when the pump is turned on, and the pump “seizes." Such pump seizures resulting from polymerized oil require extensive repair and rebuilding of the vacuum pump.
In an application with much cycling between atmospheric and full vacuum, too slow a pump will have too slow a pump downtime, and run hotter since it spends more time pumping while above 10 torrs. In freeze dryer and organic chemistry manifold applications, a balance must be achieved between a sufficiently rapid pump downtime while not defeating the efficiency of the cold trap by pulling vapors past the cold knockout zone too quickly.
A-VAC has decades of experience, and we carefully match the application and sample specifics to the vacuum pump(s) with the proper vacuum levels, technology, and pumping speed. Included in these considerations are application, chamber size, sample composition, sample amount, sample temperature, budgetary/space considerations, and desire for useful enhanced features.
The Impact of Corrosive Vapors, Acetonitrile, and Dichloromethane on Vacuum Pump Oil
When not trapped, corrosive vapors, and also ACN and MeCl2 can quickly cause a vacuum pump to fail. Well before any metal or
seals are attacked, these compounds attack the oil even faster than heat does. As it does when overheated, the oil polymerizes when exposed to these chemicals. Such polymerized oil does not lubricate as well, adding friction and therefore heat to the equation. Also, polymerized oil no longer seals the small gap in the rotary vane system, leading to regurgitation and greater compressed vapors. All this adds to the heat load in the system. Now we have both ingested, damaging vapors, and heat contributing quickly to oil degradation The end result of unchanged oil is accelerated polymerization, compromised vacuum efficiency, and ultimately, hardened oil and pump seizure. You will want to avoid this or face expensive pump rebuilds costing 40% to 60% the price of a newly purchased pump.
Primary Modes of Vacuum Pump Failure
Vacuum Pump Oil Degradation is the primary cause of most oil pump failures. This can occur, as detailed above, from prolonged overheating, from acid ingestion, or from ingestion of Acetonitrile and Methylene Chloride. In all of these instances, the heat or corrosive or organic agent(s) cause(s) the vacuum pump oil hydrocarbons to polymerize into long chains. This increases oil viscosity, which increases heat, which only causes more polymerization.
- Again, if left unchanged, such oil can polymerize to such an extent that it hardens upon cooling down. If this occurs, the rotor cannot turn, and the pump seizes or fails to rotate.
- The motor, in trying to turn such a frozen rotor, will heat up, then overheat, then shut off as its overtemperature protection circuit deploys.
- Rotors frozen by extensively polymerized oil are very costly to repair.
- Mechanical failure is in rare instances the cause of vacuum pump failure. Vanes can become wedged and prevent rotor rotation.
- Leakage of gaskets (rare) or shaft seals (3-5 years of use) can lead to vacuum pump failures. In these instances, loss of oil leads to an unlubricated vacuum pump which overheats and again, quickly polymerizes the oil to the point of hardening upon cooling.
- Other modes of failure include motor malfunction and belt failure. Both are rare occurrences.
Trap Damaging Vapors
Properly trapping vapors that otherwise are ingested and harm vacuum pump oil is a vitally important part of properly maintaining vacuum pumps at optimum performance. Mechanical and manually maintained cold traps, molecular sieve traps, and other traps are used to keep these damaging vapors out of the pump oil. Opening the Gas Ballast for 30-60 minutes at the end of the day can help condensed vapors, especially water, go from condensed liquid state in the oil to a vapor state that can be exhausted.
Match Oil Grade to Application
It is vitally important to match the grade of oil used in your vacuum pump to the application involved:
- Do not use lesser quality grades of oil. The molecules have a high ratio of unsaturated carbon sites where polymerization can rapidly develop. They have a high ratio of light hydrocarbon fractions which can evaporate quickly and lower oil levels in the pump to excess.
- For well-trapped or benign applications, Techni-clear oil is sufficient.
- Synthetic or highly refined vacuum pump oils which are either fully synthetic, but usually are simply more carefully refined hydrocarbon oils where all carbon-hydrogen and carbon-carbon bonds are saturated. Such refined oils resist heat and acid breakdown far better than standard oils. This can be cost-effective if it lowers oil change frequency enough. Organic chemists, proteomics researchers drying peptide samples in lyophilizers or concentrators, and other similar users should consider using such highly refined grade (Techni-clear or higher) vacuum pump oil.
Change the Vacuum Pump Oil—Once a Month!
Sufficiently changing vacuum pump oil is vital to prolonged vacuum pump life. Pumps that are properly maintained and receive frequent oil changes can last 10 years or more with minimal service if they are direct drive pumps. Properly maintained belt drive vacuum pumps have been known to last over 40 years.
If uncertain how often you should change your vacuum pump oil, start with a once-a-month frequency. You can then adjust to more or less frequent changes based on the following factors:
• Has the ultimate vacuum pressure gone up since the oil was first changed? If so, change more frequently.
• Is the oil dirty or brown or viscous or loaded with condensed water? If so, change the oil more frequently.
• Does the oil have any odors picked up from the vapors it has pumped? If so, consider changing the oil more frequently.
This assumes you have optimized your trapping system first.
Basic Techniques for Changing the Vacuum Pump Oil
Change While Still Warm:
- Turn the pump off and unplug it.
- Disconnect the intake hose.
- Disconnect any exhaust hoses if appropriate
- Unscrew the oil fill port.
- Unscrew the oil drain plug or turn the oil drain valve lever. If plugged, apply pressure on the plug so that when you are close to fully unscrewing, this pressure keeps oil from leaking. Have an oil drain receptacle of sufficient volume ready to catch the draining oil.
- Drain the oil. When the oil flow is nearly zero, carefully tip the vacuum pump up to drain the remaining oil. Caution: some pumps are so heavy that this is not practical.
- Properly dispose of the waste oil according to your company’s safety and hazmat regulations.
- Replace the oil drain plug, taking care not to cross-thread, or close the drain valve.
- Take an oil fill funnel and place it in the oil fill port. Pour oil into the funnel.
- Watch the oil level and stop filling when the oil level in the sight glass is above the “empty” line and near but below the “full” line.
- Replace the oil fill plug, taking care not to cross- thread.
- Reconnect intake and exhaust hoses, and plug the vacuum pump in.
- Clean up any spills, and restart the pump when ready.
Advanced “Forced Oil Flush” Technique Purges Internal Oil Sludge
There is a more advanced technique known as a “power flush” or a “forced flush”. This forced flush is very effective at flushing away the internal residue of polymerized oil. Because there are several safety concerns, please exercise great caution please contact your A-VAC representative before proceeding.
Properly selected Oil Seal Vacuum Pumps that receive sufficient oil changes and are adequately trapped and properly sized for flow rate can give many years of trouble-free performance. Conversely, neglect in these areas can lead to pump failure in a short amount of time. A-VAC is available to consult further with you on any of these issues.