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Author: Subject: Oil lock up of rotary vane vacuum pumps
wg48temp9
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[*] posted on 16-10-2020 at 07:03
Oil lock up of rotary vane vacuum pumps


My vacuum pump is a Pfeiffer DUO 2.5 dual stage it frequently will not start. After struggling with it. I suddenly remembered an old belt driven vacuum pump I previously had with the same problem. With that belt driven pump you could feel it was locked (more accurately very difficult to turn) by attempting to rotate the pulley by hand.

Apparently what happens is after the pump has pulled a vacuum and is then turned off, at certain points in its rotation the vacuum left in the pump sucks in the oil filling the one of pump chambers. When the pump is started up next time the incompressible oil prevents the the pump from rotating.

With the pulley driven pump it was possible to force the pulley round slowly by hand after abut one turn it would turn relatively easily and would start ok electrically.

My Pfeiffer pump is direct drive so that is not possible. I suspect its so worn the oil fills up the vacuum chamber in only an hour or two. It was a used laboratory pump and bought on ebay some time ago so no chance of sending it back. Time to strip it down and see how worn it is. Perhaps I will use it as a roughing pump and buy a new one.

On a different trouble shooting problem. I just spent two hours repairing an outside lamp. It seemed to have a short between the neutral and earth in the wiring. but when I came to connect the new wire up in the distribution box I notices the neutral had been connected to the wrong neutral bar the one before the RCD. Apparently the house owner had a new electrical heater fitted recently and I suspect the wire became loose and was reconnected to the wrong bar and the next time the outside light was switched on the RCD tripped turning off most of the house.

Of cause I had asked the home owner if anyone had been fiddling with the wires for the light and was assured that it had worked fine for 15 years and that no one had fiddled with the wires.

PS: sometimes homeowners or their friends fiddle with stuff and screw it up but then do not own up.




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Heavy Walter
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[*] posted on 16-10-2020 at 07:32


Hi
In my lab I have three Pfeiffer pumps. No one shown that problem, ever.
Do a maintenance on it.
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Fyndium
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[*] posted on 16-10-2020 at 07:55


I had a similar problem with my pump. It jammed at certain position, and sometimes little tapping or repeated restarting fixed the issue, but once I had to open the oil reservoir and turn the pump a little by hand to make it work again.

Haven't had this since though when I opened up the pump, cleaned it and reassembled with fresh oil.
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Bezaleel
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[*] posted on 16-10-2020 at 16:07


Old oil in pumps is one issue. (It also tends to form a sticky carbonaceous layer on the main hub.)

Another is to open the gas ballast before switching off the pump. Though not its primary function, this allows air to flow in slowly, so that the vacuum in the second chamber is removed, which prevents oil from being pressed in. You can hear it hiss or shriek when it sucks in air through the ballast tube. Note that with some pumps it takes a little while before the oil in the gas ballast tube is sucked out, and the air is allowed to flow in. You can tell when hissing starts.

I once had a (custom made) gearwheel replaced which was very expensive. The old one broke exactly because of this issue. (I learned by doing, as often.) Ever since, I always make the first rotation by hand before I switch on the pump. If it goes with difficulty, I know I didn't switch it off properly. It may well be that there are pumps that have a mechanism in place to prevent the issue, but mine hasn't . It pulls a decent vacuum though, to around 10 Pa.
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wg48temp9
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[*] posted on 18-10-2020 at 17:06


Quote: Originally posted by Bezaleel  
Old oil in pumps is one issue. (It also tends to form a sticky carbonaceous layer on the main hub.)

Another is to open the gas ballast before switching off the pump. Though not its primary function, this allows air to flow in slowly, so that the vacuum in the second chamber is removed, which prevents oil from being pressed in. You can hear it hiss or shriek when it sucks in air through the ballast tube. Note that with some pumps it takes a little while before the oil in the gas ballast tube is sucked out, and the air is allowed to flow in. You can tell when hissing starts.


Thanks Bezalee. I tried the opening the ballast trick before turn off three times and with upto a ten minute wait, the pump start ok. Without opening the ballast it locked up three times.

An other problem is when the pump is cold (about 10C) if it starts it runs very slowly so I suspect its gummed up or the oil is too viscous. When I strip the pump down I will replace the oil with a recommended type or equivalent.

I estimate the achieved vacuum is about 0.15 to 0.1mmHg from the appearance of an electrical discharge between two steel rods in a 500mL three necked flask. That pressure needs to be about ten times lower for the diffusion pump. Hopefully I will get there with a clean pump, fresh oil and better connections with less plastic tubing.

[Edited on 10/19/2020 by wg48temp9]




I am wg48 but not on my usual pc hence the temp handle.
Thank goodness for Fleming and the fungi.
Old codger' lives matters, wear a mask and help save them.
Be aware of demagoguery, keep your frontal lobes fully engaged.
I don't know who invented mRNA vaccines but they should get a fancy medal and I hope they made a shed load of money from it.
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[*] posted on 19-10-2020 at 12:55


Hmm, I wish I had known about this issue a few months ago.

I recently acquired a Leybold D10E rotary vane pump from ebay which the seller said had been tested and was working fine. So when I got it I turned it on briefly and it wouldn't do anything except a loud humming (motor not turning). I thought there was something broken inside and the seller had ripped me off. I ended up taking the pump fully apart but didn't find any major issue. Now I'm pretty sure it was locked for the same reason as yours wg48.

Btw, I've heard of oil suckback occuring and filling whatever chamber is being evacuated, which must be a real pain. For this reason I always vent the vacuum line before turning the pump off.

I've actually taken my pump apart a second time to change some worn parts (vanes, valve flaps and springs) so I have a few bits of advice so you don't make the same mistakes I did:

- Cover your workbench with 2 layers of cardboard to absorb the oil which will inevitably leak out. Draining the oil beforehand will not suffice, i had about 50 ml left inside even after that.
- Use gloves.
- Remove the motor from the pump's body, it makes handling easier.
- Put the parts in baggies so you don't lose them. I grouped them by general location within the pump.
- Have a lot of paper towels available. I used almost a roll each time.

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wg48temp9
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[*] posted on 19-10-2020 at 14:47


Heptylene: Thanks for the good advice. I have taken all kinds of oil filled machines apart ranging from car gearboxes, pressure washers, fridge compressors and an old belt driven vacuum pump. But its useful to be reminded. Do you only use your pump for distillations ?

The end plates on the rotor housing of that old vacuum pump were so worn you could feel the edge. I contemplated lapping them flat again but never got round to it. Hopefully this pump is in better condition.

I am feeling more confident with it as I have fixed a leak on my lashed up discharge tube (a flask with three necks for two thermometer holders for electrodes and one for the hose connection) I estimated its down to about 0.01mm Hg from the discharge appearance: almost a dark discharge with a whitish fluorescence on the glass. I also checked it was the glass glowing with magnet as its hard the judge if its a diffuse discharge or glass fluorescence. I also added a piece of glass coated with phosphor from a broken fluorescent tube. It only glowed in the almost dark discharge.
I used three MOTs in series one powered and the other's HT windings as a ballast.

dis-char-1.JPG - 37kB

dis-cha-2.JPG - 86kB

I will take some pics eventually

PS: My pump has an automatic shut of valve on the inlet to stop suck back but its still recommended not to leave it under vacuum for any significant time.

[Edited on 10/19/2020 by wg48temp9]




I am wg48 but not on my usual pc hence the temp handle.
Thank goodness for Fleming and the fungi.
Old codger' lives matters, wear a mask and help save them.
Be aware of demagoguery, keep your frontal lobes fully engaged.
I don't know who invented mRNA vaccines but they should get a fancy medal and I hope they made a shed load of money from it.
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[*] posted on 20-10-2020 at 00:20


Glad to know oil-filled machinery is not an issue :) For me it was a first.

I don't use the pump for distillations as I don't have a cold trap. Liquid nitrogen is too expensive and doesn't last long enough to justify the expense. Maybe if I had a specific need once I might go through the trouble.

I'm using it for some electron beam experiments so I need a vacuum of a few microns, ideally less. Turbomolecular pumps are expensive, diffusion pumps are slow, so I'm using the mechanical pump on its own. According to my pirani gauge, it pulls down to 6e-3 mbar (= 4.5 microns) and it can almost completely extinguish a discharge. In another experiment I managed to produce x-rays, which confirms high velocity electrons are present and that their mean free path is large enough. I don't want to contaminate the oil with solvents so I'll have to resist the temptation, no matter how great, to use it for chemistry purposes!

Looking forward to seeing pictures!
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[*] posted on 26-10-2020 at 04:37


I recently won four faulty vacuum sensor heads.
Inficon BPG40X Bayard-Alpert Pirani combination gauge. They may of some Iridium filaments in them, only a few tens of miro grams unfortunately (0.1mm wire). Probably not enough metal to explore its colorful chemistry

Hopefully I can get one (just the Pirani part for now) going by replacing what I assume is broken filament with a tungsten light bulb filament. I will use a load cell amplifier HX711 with it. The HX71 has 24bit ADC which should be more than sufficient sensitivity. For 130C increase in temperature the resistance of tungsten increases by about 60%.

Attached is a pic of one of the sensor heads looking in to the FK25 flange. I think I can see a piece of the Iridium filament (hopefully)

vac-sensor-d-2.jpg - 69kB

[Edited on 10/26/2020 by wg48temp9]




I am wg48 but not on my usual pc hence the temp handle.
Thank goodness for Fleming and the fungi.
Old codger' lives matters, wear a mask and help save them.
Be aware of demagoguery, keep your frontal lobes fully engaged.
I don't know who invented mRNA vaccines but they should get a fancy medal and I hope they made a shed load of money from it.
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wg48temp9
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[*] posted on 13-11-2020 at 01:54


Below are some pics of an 8mm x 100mm discharge tube operating at different air pressures. Its driven from a 2,000V smoothed DC supply via an 82K ohms. The smoothed DC supply and series resistance produces a stable discharge.

I estimate the top pic is at about 35mbar followed about 10mbar then about 3mbar. The last two pics show the Crooks dark space. The length of which is inversely proportional to the pressure. Thats just before the discharge goes dark. You can just about see the green fluorescence of the glass near the cathode. It was difficult to take the pics. I had to switch off most of the auto function of the camera.

LP-air-discharge2.jpg - 44kB

and below is diagram designating the different parts of the discharge

DIA-DIS.JPG - 24kB

and a pic of the discharge tube taken accidentally with the flash on.

dis-tube.JPG - 45kB

[Edited on 11/13/2020 by wg48temp9]




I am wg48 but not on my usual pc hence the temp handle.
Thank goodness for Fleming and the fungi.
Old codger' lives matters, wear a mask and help save them.
Be aware of demagoguery, keep your frontal lobes fully engaged.
I don't know who invented mRNA vaccines but they should get a fancy medal and I hope they made a shed load of money from it.
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