Medicinal Chemists, Organic Chemists, Biochemists, Biologists, Molecular Biologists and other scientists rely upon vacuum-driven devices to concentrate, dry, or filter their materials.
If a Vacuum System is not performing optimally, it can slow preparation of research-critical samples by as much as 50%-100%! This can have significant impact on time-to-market, or on research paper productivity.
Doesn’t it make sense to be certain that your Vacuum Systems are operating at peak efficiency?
Welch Vacuum Pumps (a Gardner Denver Product) provides a free service to its customers: the Vacuum System Audit Program. This service is designed to raise awareness on the importance of the subject, and teach researchers the steps in the process. These steps are also outlined below.
A majority of the researchers must be committed to improving, or management must lead the initiative if the principles learned in the Vacuum System Audit are to have any lasting benefit. Laboratories that ignore the advice generated by such Audits are at risk for more vacuum pump failures, expensive repairs, downtime, lessened productivity, and even compromised yield and purity.
Laboratories can benefit from periodic or even continuous monitoring of vacuum system performance. Welch’s inexpensive Digital Vacuum Gauge (1-760 torr) #1520B-01 and #1520K-10 are now available. Digital Millitorr Gauge 1525B-01 and 1525K- 10 are now available. Contact Welch Vacuum Pumps for details!
Vacuum System Audits typically take the following format:
|Rotovaps||1-10 torr||10-20 torr||>20 torr|
|Organic Manifold||0.040 to 0.250 torr||0.250 to 1.00 torr||> 1 torr|
|Vacuum Ovens||1-5 torr||5-10 torr||>10 torr|
|Freeze Dryers||0.010 to 0.133 mbar||N/A||> 0.133 mbar (0.1 torr)|
The final component for consistently improving Vacuum System performance is, making certain Best Practices are followed. Some of these practices can be managed by facilities or outside service vendors; others remain the responsibility of the researcher:
Properly maintained vacuum pumps will provide many years of reliable, maximized performance. This article addresses simple ways to maintain such vacuum pumps and options for what to do when pump performance is compromised due to oil contamination and degradation.
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.
Two Stage Oil-Seal vacuum pumps have an upper operating limit. Welch Vacuum specifically recommends against continuous operation above 10 Torr. That is 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 “siezes”. Such pump seizures resulting from polymerized oil require extensive repair and rebuilding of the vacuum pump.
That is why Welch Application Certification addresses the vitally important concept of matching vacuum pumping speed with the application involved. In an application with much cycling between atmospheric and full vacuum, too slow a pump will have too slow a pumpdown time, and run hotter since it spends more time pumping while above 10 torr. In freeze dryer and organic chemistry manifold applications, a balance must be achieved between a sufficiently rapid pumpdown time while not defeating the efficiency of the cold trap by pulling vapors past the cold knockout zone too quickly.
In Issue 2 of this newsletter, we will address trapping acids and acetonitrile in high vacuum systems. You will learn more about why having too fast a pumping speed can be detrimental as to pump longevity, while too slow a pumping speed can sometimes not matter, or matter greatly, depending on the application.
Welch-Ilmvac Application Certified vacuum pump selections are the outcome of many decades of collective experience, and we carefully matche 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.
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.
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.
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 the day can help condensed vapors, especially water, go from condensed liquid state in the oil to a vapor state that can be exhausted.
It is vitally important to match the grade of oil used in your vacuum pump to the application involved:
Do not use Grade 19 oil or similar 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 level in the pump to excess.
For well-trapped or benign applications, a Grade 22 oil such as Welch Premium Oil is sufficient.
Synthetic or highly refined vacuum pump oils such as Welch Directorr Gold 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 (grade 22 or higher) vacuum pump oil.
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:
This assumes you have optimized your trapping system first.
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. Instructions can be found in the Welch DuoSeal Vacuum Pump Owner’s Manual, which you can download from www.welchvacuum.com. Contact your local Welch-Ilmvac Area Sales Manager for details.
Because there are several safety concerns, please exercise great caution and follow the directions precisely, and contact Welch Vacuum if you have ANY questions before proceeding.
Welch-Ilmvac Area Sales Managers are trained experts in the area of proper vacuum pump maintenance, trapping, operation, and vacuum pump selection and performance optimization. Welch is delighted to sponsor such pump advice seminars, be they separate from or in conjunction with a Vacuum System Audit, at your location. Contact your local Welch-Ilmvac Area Sales Manager for more details.
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. Welch Vacuum is available to consult further with you on any of these issues.
Finally, please note that many applications that previously relied on oil-seal technologies now are better served with dry, oil-free vacuum pumps. The new Welch Laboratory Vacuum Catalog 2008/2009 contains a Vacuum Pump Selection chart that shows which applications can use Dry Technologies at this time. Simply contact your local Welch-Ilmvac Area Sales Manager, or download this 52-page catalog at www.welchvacuum.com
Mercury in a “perfect” manometer is pushed up by atmospheric air to 760mm. This is Atmospheric Pressure (1 ATM), or 760 mm Hg, or 760 torr. 1 torr is therefore 759/760 vacuum, i.e. 1/760 residual pressure. Oil seal vacuum pumps achieve vacuum levels a thousand times deeper: 1 x 10-3 torr, or 1 millitorr.
Rough vacuum starts with a relative approach where ambient atmosphere is considered 0 inches vacuum.
Perfect vacuum, then, would be 760mm vacuum or 29.92 inches. Usually we know this in its rounded-off form of “30 inches Hg vacuum”
Evacuating to ½ ATM, or 380 torr residual pressure, gives a reading of ½ vacuum or 15 inches vacuum.
Evacuating to 2/3 ATM or 253.3 torr residual pressure gives a reading of 20 inches vacuum or 20” Hg.
Measuring pressure is an absolute measurement because it reads the same regardless of ambient atmospheric pressure levels. Conversely, vacuum readings rely on ambient pressure. In Denver, CO, for example, ambient pressure is only 25/30ths of Sea Level’s pressure. Thus a millitorr-range pump will evacuate to a reading of 25” Hg on a rough gauge in Denver while the same pump and rough vacuum gauge will deliver a reading of 30” Hg when at sea level. Users must account for elevation when using a relative vacuum gauge.