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Oil Problems With Vacuum Pumps

We routinely get inquiries from the field about why oil is found in the intake section of the pump or even in the system being evacuated. Sometimes this accompanies problems with the pump starting. These types of problems are routine for pumps not employing anti-suck back valves. Even when an anti-suck back valve is present, it depends on the effectiveness of these valves, sometimes variable in performance.  The HyVac HyperVac series of direct drive pumps employs a new generation anti-suck back valve.

Generally, the problems seem to go like this:

We started the pump on a vacuum oven ("any system") we reached "X" vacuum pressure and then turned off the pump. After running the procedures in the evacuated chamber, we then went to start the pump. The pump would not start. The next day we noticed oil in the chamber. What is wrong with the pump???

All vacuum pumps are designed to evacuate a given space from atmospheric pressure to some low pressure say 29" mercury vacuum or 15 micron, whatever. HyVac vacuum pumps are all designed to run continuously. When tested at HyVac they are run for 5 days 24 hours per day prior to final testing and release to finished goods inventory. They, like most oil sealed rotary vane vacuum pumps are designed to pump air or gas, not necessarily liquids. Because the manufacturing tolerances of the pumps are so tight, they will not function properly and may actually lock up attempting to pump liquids.

When the decision is made to maintain the system being evacuated at a vacuum pressure and then stop the pump; either intentionally or because of a power outage, then certain things can potentially occur. First, the pump rotation and it's creation of vacuum stops. The inlet side or intake of the pump is then at negative pressure (-14.7 PSI) and the exhaust or outlet side of the pump is at atmospheric pressure. The atmospheric pressure on the exhaust side of the pump will try and get into where the negative pressure is present anywhere on the intake side of the pump. This can be in the whole system or even just in the intake port of the pump. The oil in the pump is what is between the positive pressure and the negative pressure. The only thing that holds back the oil is the machined tolerances of the vacuum pump. Unfortunately, oil molecules are generally smaller than most manufacturing tolerances and will find a path through the machined tolerance to get into the negative pressure space on the intake side of the pump or even beyond into the system.

What restricts, stops, or limits this action are "anti-suck back" devices such as an anti-suck back valve present in the HyVac HyperVac series of direct drive vacuum pumps. You may also use a set of two way solenoid valves or 1 three way solenoid valve on pumps not having anti-suck back valves incorporated into the design. Other solutions can also be incorporated into most systems as optional equipment such as vacuum rated manual valves employed on the intake side of the pump. Most belt drive vacuum pumps like the standard HyVac series pumps or older generation direct drive pumps do not incorporate anti-suck back devices. Without any of these devices the pump manufacturing tolerances (clearances) and the oil in the pump are all that stands between the exhaust pressure "positive" and the intake pressure "negative".

Oil, present in the pump, has a great affinity to migrate. Oil will migrate or slide by most of the tolerances in the pump and start moving towards any negative pressure space present in the pump or system. This will continue to occur until a air leak passageway is created between the intake and exhaust pressures and they are allowed to equalize.

If and once this oil migration takes place, the pump may not start. Oil migration may actually proceed up into the whole inlet side of the pump and may even invade the system being evacuated.

SOLUTIONS to all these issues.

Simple Procedures

1. Don't stop the pump. They are designed to run continuously. You will not hurt the pump running them all the time. Make sure the oil level is maintained in high pressure (low vacuum) systems or operation.

2. Manually bring the system to atmospheric pressure before turning off pump.

Procedures to maintain systems at vacuum and shut off the pump.

3. Install (2) manual 2 way valves. One to close system to maintain pressure the second to vent the pump inlet to atmospheric pressure before stopping pump.

** I think this has good flexibility.

4. Install (1) manual 3 way valve. When closing system to vacuum pressure it automatically vents the pump inlet to atmospheric pressure.

5. Install (1) 2 way "normally open" solenoid valve between pump and system. This is okay, but oil may still end up between pump and start of solenoid valve and may cause problems unless inlet sees atmospheric pressure on start up. Many of this type setup are in use today with success in this configuration. Solenoid should be rated for high vacuum. We carry 2 way solenoids for many of the pumps.

6. Install (2) 2 way solenoid valves one "normally open" between pump and system. One "normally closed" between pump and atmospheric pressure. Solenoid should be rated for high vacuum. The "normally closed" valve can be small in size.

7. Install (1) 3 way solenoid valve that vents pump to atmosphere at the same time closing system off to maintain pressure.  This can cause pressure loss on the system due to general time lag in closing and opening the common port.

Of all the solutions outlined we feel the best are either 1., 2., 3., 6., or 7.

The least expensive is option 1., and 2.

The most expensive "best idiot proof" is option 6.

The one I would consider for cost, performance and flexibility is option 3.


The PUMP IS LOCKED AND WON'T START. It was fine yesterday.

** Please read fully before doing anything.

A. If the pump is locked and will not start, vent pump inlet or system to atmospheric pressure.

B. Is the pump low on oil?

Check to make sure that there are not "huge" amounts of oil in your vacuum system. If so, the vacuum pump may now be low on oil. If the pump is low on oil make sure intake is vented to atmospheric pressure and actuate the on/off switch a couple of times to pull any oil from the intake port back into pump. Leave the pump off when actuating, just turn pump off and on a couple times till it looks like it might run ok. Check the oil level again. If still low turn off pump go to step C.

B. Is pump oil level OK?

If pump is not low on oil actuate on/off switch of the pump a number of times until pump rotates on it's own. Even though the pump is not made to pump oil, (which is now throughout the pump), actuating the on/off switch should allow the capacitor start motor to force the oil back into the pump. Go to step C.

C. Can you see the oil level meniscus in sight glass?

Check the oil level if you can not see meniscus in sight glass window. The oil level is either to low or too high. You should have a meniscus in sight glass window before starting pump. *If you have too much oil in pump, can't see meniscus and start pump anyway, you may get showered with pump oil. Don't worry it is non-toxic (at least ours is).

Get an old disposable rag handy, watch your clothes, this could get ugly. Place the rag bunched up and held loosely over exhaust port of pump. Wrap the rag around the base of exhaust dome if installed. This is to help keep things clean. With pump inlet open to atmospheric pressure turn pump on. If it does not actuate, try again. Try again. For belt drive pumps remove beltgaurd and with inlet open manual actuate pump using pulley. MAKE SURE PUMP NOT PLUGGED IN WHEN MANUALLY ACTUATING PUMPS. Under normal operation the pump should operate freely, it may make a low gurgling sound and it may create oil mist (smoke) or oil droplets from the exhaust port of pump.

Cover intake of pump to allow pump to reach vacuum pressure. Use a rubber stopper, original shipping cap, your thumb or any suitable good sealing procedure. Wait 1 minute.

The pump should now be making very little noise and may show a little oil mist or smoke from exhaust port. With the pump at vacuum pressure and running, adjust the oil level either by adding or draining oil with pump running at vacuum so that the level is just past halfway in the sight glass window.

If you still have problems after all these steps please feel free and call we will try and help you out or direct you to send the pump in for evaluation.

Good LUCK.

11/27/2005 Copyright HyVac Products, Inc. All Right Reserved
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