Vacuum Pump Expert Don McGregor Answers Your Most Pressing Questions
Question #1: Pump Oil Problems
I sent my Edwards Pump to your shop to check out. It is a 2 CFM which is backing a 20CF vacuum oven and the oil is really thick and dark. It looks like maple syrup, thick and ‘goopy’. After you look at my pump, please let me know what you think is causing this in my pump oil?
I looked at your pump and I think I have figured it out. The ‘contamination’ that was noted is really a breakdown of the oil. The residual oil in your pump was thick and dark brown, like honey or molasses. This occurs when oil has a lot of air blown through it, which causes the molecular string to get longer and longer, thickening and darkening the oil. This is referred to as ‘polymerization’. For more details on this, see our recent blog post, Tips for Maintaining Your Oil-Sealed High Vacuum Pump.
The root of the problem is that the Vacuum Pump is too small for the application, so it is forced to run at high pressure for long periods of time. A larger pump and substituting a higher-temp oil will solve your problem.
Question #2 Micron Mystery
I have a Welch Vacuum Pump I recently bought new. When connected to my 9.5 vacuum oven, it does not pull down below 2,000 microns. I have let it run for over 3 hours, but no luck. Any ideas?
In either a 9.5 or 4.5 CF chamber, the vacuum reading after 48 hours should be close to the capability of the pump. On a 1402 Welch, under 5 microns would be considered good. However, there are many variables that can come into play. Cleanliness of the chamber, the conductance of the piping connecting the chamber to the pump and vacuum integrity of the chamber can all be a hindrance to an ultimate vacuum being achieved.
The biggest hurdle is often ‘virtual’ leaks. A virtual leak is a trapped volume of gas connected to the vacuum side of a chamber
that can’t be pumped out easily. As the chamber pump down cycle crosses over from viscous to molecular flow, the gas molecules within the trapped volume can only be pumped out at a rate proportional to the conductance of the path between the trapped volume and the chamber volume. This rate is the “virtual leak”.
A virtual leak won’t be detected in a normal helium leak testing cycle either. The presence of a virtual leak will become apparent during the system pump down cycle. If the Vacuum Pump can’t keep up with the gas load from the virtual leak and ultimate pressures can’t be reached or take an excessive time to reach.
The shorter answer is that if you have pump pulling 5 microns at the inlet, it should get pretty close to that number when evacuating the chamber. If it is not close even after 48 hours, the problem is outside of the pump.
Question #3: Pump Replacement
I’m pretty confident that our Stokes Vacuum pump is toast and needs to be returned and a replacement purchased asap. We had a bunch of guys looking at it this morning and it was pulling 100 amps and was way too tough to rotate the shaft.
The two most common problems are contamination or an oil flow solenoid coil gone bad. Contamination can be checked just by
draining off some of the oil and looking at the condition. The oil we use is almost clear, so contamination would be easy to spot. If it is a contamination problem, the pump would have to be torn down, cleaned and re-assembled.
One bit of information is ‘WHEN’ did the pump seize? Was it while it was running or was it shut down and then next time you tried to start it, it would not turn?
The oil flow solenoid coil is a bit harder to check. If it goes bad you won’t see any flow in the glass dome on the side of the Vacuum Pump. One other thing to consider. If the solenoid valve sticks in the ‘open’ position, that is equally bad. What will happen is oil will flood the bottom end of the pump when it is not running. Then, the next time you try to start it, it will act like it is seized up. The motor can’t start the pump because so much oil is in the way. If this is the case, you have to repair the solenoid so no oil is flowing when shut off, then get the oil out of the bottom end by turning the pump by hand. This is a bit tough but not impossible. If the pulley can be rocked by hand, rock it back and forth until it gets easier and then can be turned 360 degrees.
Question #4: Fuse Fiasco
I have a Stokes model 412 Roughing Pump here that is ready to be picked up. We pulled it from one of our EB Welders. It had been previously repaired by another company and was only in service for about 10 months. We started having issues where the 30 amp fuses feeding the pump kept blowing. We checked the voltage, verified wiring connections were tight, rung out the contractor and overload, megged out the motor, replaced the solenoid valve coil even though it ohmed out good. Ran amp checks while running…nothing over 7.5amps even though the motor is rated for 12.5 I believe. Found no indication or explanation as to why the fuses keep blowing. Swapped out the pump with a known good unit we have here and so far so good. I suspect there is a mechanical issue with this pump that we are not seeing and were not able to catch. Hopefully, you guys can shed some light on this for us.
It sure sounds like the problem is mechanical, not electrical. Just the fact that you swapped out the pump and now all is well, tells us it is not a starter or electric-service problem. And the tests you did on the motor and solenoid kind of eliminate that as an issue.
As pumps wear they often start to become harder to turn. This is what causes the amperage to go up until the fuses start to pop. Though not scientific, you might try one other test. With your known-good pump off, take the belt guard off and try grabbing the belts and turning the pump by hand. It is not easy, but doable. Then do the same thing with the problem child. See if that one feels any more difficult to turn.
You didn’t mention WHEN the fuses pop. If it is when you first try to start the pump, there is one other thing to check. As you probably know, these pumps have an oil flow solenoid ( that is the coil you have been checking ) These are supposed to shut off oil flow when the power is cut. If this is wired wrong, I.e. power is not being cut to the coil when power is cut to the pump, oil will continue to flow even though the pump is shut off. Oil will fill the bottom end of the pump, then next time you try to start it, it will be so hard to turn, due to oil jamming the lower end, you may start to pop fuses.
Also, if there any dirt under the seat of this solenoid, you will get the same symptoms—oil continuing to flow after the pump is shut off. I hope this helps.