Sciencemadness Discussion Board

vacuum pump trouble.

havarti_gouda - 28-1-2010 at 11:15

I'm using a diaphragm pump that has a pressure gauge attached and normally (when i first got it and even recently) would go up to about 29 (out of 30). I believe it is listed in inches, I'll have to check, but I'm pretty sure.

After using it for a variety of vacuum distillations, vacuum filtrations, and possibly some other silly purposes, it seems like the pull has reduced greatly. In fact, where it used to read at around 29, it's been reading at like 5-7.

Funny thing is that this happened before. And it was acting funky, yet, still seemed to be pulling slightly higher than what it was displaying. And then one day I'm cleaning up some solutions to retrieve used solvents and distilled a combination of HCl/water/o-xylene. About ten minutes after I turned on the vacuum at around 90C, it goes from registering around 5-10 and shoots up to 25 in a second as distillate FLIES over through the head and condenser into the receiving flask. I might as well have been pouring the solution through the condenser it was coming over so fast. I am assuming that over time there has been some sort of build up of gunk which one of the solvents, maybe the xylene or maybe the acidic solution, broke down and, in essence, cleaned out.

It ran great for about two or three weeks after that and then recently since I've been distilling over formaldehyde fractions (i think this is what did it), it's been only pulling around 5-7 again. I took the vacuum apart and tried to find anywhere there could be build up, but it appears the build up, if that's what it is, is in some of the pieces that I can't disconnect.

I am looking to getting a new vacuum of slightly higher quality sooner than later, but in the meantime would like to be able to use this one efficiently.

Any help would be greatly appreciated. Thanks!

Lambda-Eyde - 28-1-2010 at 14:31

What brand is the pump? Do you have any specifications? How old is it?

I have no knowledge with vacuum pumps, but my advice would be:

* Take it apart, clean it and change the oil.

* And, for Pete's sake, use a cold trap if you're not using one already.

entropy51 - 28-1-2010 at 15:10

I never saw a diaphragm pump that used oil, but maybe I missed something.

Are the diphragms made of, or coated with, teflon? If not, it may not stand up to xylene, HCl, and other stuff.

If the diaphragms are OK, I would check for (1) leaks somewhere in the system and (2) gunk that's causing the pump valves to stick.

I would not distill a mixture of xylene and aqueous acid. I would separate the xylene layer, dry it, and distill it without the water. Did you have a stir bar in the distillation? I think 90 C is too hot to distill either HCl or xylene under vacuum. It sounds as if you heated to 90 C and then applied vacuum, which is the reverse of the normal sequence. The pressure may have remained high because of evolving HCl gas, and when you had pulled all the HCl off the pressure dropped and everything in the (superheated) flask flashed to vapor due to the lower pressure.

[Edited on 29-1-2010 by entropy51]

havarti_gouda - 29-1-2010 at 01:00

It's a Thompson diaphragm vacuum pump. Not sure of the model number. I need to check on that, sorry for not having all the necessary data before posting. Regardless, it's not a laboratory diaphragm pump. Literally I found it on ebay from a company who had another 12 of them for like $160.00 or some amount around there. It has actually worked pretty good for the most part as a beginning vacuum pump.

Entropy is right, diaphragm vacuum pumps don't use oil and that is the case with this one too.

No, they are not coated with PTFE and I'm guessing that you're right about that. Regarding the process I was using, it was definitely far from standard just like you noticed. I was just experimenting while in the process of cleaning up solvents for re-use. The acid water fraction was far from gone and i was trying to experiment with how xylene/water would come over. And yes, I had a stir bar in that mixture. I always use stir bars as I've never attempted to use boiling chips before.

I was reading my lab bible (Zubrick, Org. Chem Survival Guide) today to try and figure out if there was anything I was missing and read about how glass where can get small chips or other damage in the joints which will ensure that a tight seal is impossible. Any thoughts on this?

watson.fawkes - 29-1-2010 at 04:43

Quote: Originally posted by havarti_gouda  
I was reading my lab bible (Zubrick, Org. Chem Survival Guide) today to try and figure out if there was anything I was missing and read about how glass where can get small chips or other damage in the joints which will ensure that a tight seal is impossible. Any thoughts on this?
The most basic principle of tracking down a problem like this is divide and conquer. First figure out if you've got a pump problem or a glassware one. Disconnect the pump. Plug the inlet with a vacuum gauge. Turn on and measure. If you can pull 29 mm Hg, your pump is fine. If not, service the pump, which starts with learning how to take it apart and put it back together. You may have questions about gaskets at that point.

entropy51 - 29-1-2010 at 06:42

If not Teflon, I don't think it will handle what you are throwing at it. If it's not ruined you should try a cold trap to keep solvent vapors out of it. I would avoid acid fumes altogether. Dry your solvents using anydrous MgSO4, which is easily made from Epsom salts. Follow the bible lest you anger the chemistry gods; they are a spiteful bunch in the best of times.

As always, Watson's advice is a good plan.

havarti_gouda - 29-1-2010 at 11:49

Indeed, on to basic troubleshooting. :-)

I need to get an external vacuum gauge. There is one attached to the vacuum pump, but I question if that is even functioning properly, because even when I've turned it off, the gauge stays stuck around 5 (sometimes). It's a very finicky creature, hah.

So, I'll pick up a vacuum gauge and try and figure out if it's an issue with the vacuum. And then I guess if it's not a vacuum issue, I should just be working backwards i.e. attach a flask to my vacuum adapter, and cap the other end. Then add the condenser and cap the end of that, etc. etc.

I even wonder about the nipple on my vacuum pump. The piece is a nipple on the one end and then there's a threaded male joint on the other end which screws into a bar on the vacuum. Directly across is the air outlet from the vacuum apparatus and spaced equally up the bar is a max/min turning knob and then the pressure gauge on the end. I wonder about gunk being in there or vapors having caused some damage. I *definitely* need to get a cold trap.

I'm going to see what I can do to clean it a bit and then do some testing. Thanks for the information!

[Edited on 29-1-2010 by havarti_gouda]

BromicAcid - 29-1-2010 at 12:47

We don't use diaphram pumps but once in a while on our oil pumps we 'power flush' them by running methanol through them a few times, this actually conditions the rubber seals in ours and cleans out the junk, afterwards they run like champs.

entropy51 - 29-1-2010 at 13:55

One place I worked the oil pumps would freeze up with gunk in the oil from acid fumes from distillations. We would drain off whatever oil had not solidifed, fill the pump with dichloromethane, and let sit until we could turn the shaft by hand. Repeat as necessary, flush with new oil, then fill with new oil.

I always thought this a bit extreme, but I suppose a siezed shaft requires drastic treatment. It never failed to work, and apparently didn't kill the pump. I don't recommend it, however.

jock88 - 5-10-2014 at 22:23


Have a filthy Welch DuoSeal vacuum pump. The oil got it hard to come out the bung at the bottom.
I am going to have to clean it out. I do not see any 'cleaning oil' in my neck of the woods.
I have already run some diesel oil through the pump to clean out some of the gunk.
Any got any other recommendations?
I was thinking of putting some alcohol through it to get rid of any water solubles, just a brief run of say 20 seconds.

There is vacuum oil at the link which seems very cheap. Would it be OK?
I only got the pump a shorty time ago and do not know what actual state it is in.
The 'good' vacuum oils cost a fortune.

http://www.ebay.co.uk/itm/Smith-Allan-Airtech-V100-Premium-V...

Little_Ghost_again - 5-10-2014 at 23:42

You might be able to get a new diaphram pretty cheap. I got a very old one stage vacuum pump, its been slightly slow but great. saves me having to mess with mums two stage vane pump.
The old one I have is great for abuse, I run it on hydraulic fluid (we have loads fr the tractor) so I just empty all the oil out if I do anything dodgy with it. Saves messing with traps etc lol

chemrox - 6-10-2014 at 11:36

Go online and do a search for vacuum oil. Try ebay. Eventually you'll find some flushing oil. I think a couple of rinses through with diesel is not too bad an idea providing the diesel is extra pure. You will have to chase that out. I don't care for running alcohol through. Too much likelihood of being hard to flush out. I found a similar pump and flushed and flushed until I now pull 0.3 microns. These old Welch pumps keep on trucking...

Little_Ghost_again - 6-10-2014 at 12:20

Tractor hydraulic fluid or compressor oil is pretty close to vacuum pump oil, I was told as the pump gets older they run better with slightly thicker oil

jock88 - 6-10-2014 at 12:57


Thanks for replys. I have lots of hydraulic oil (used in hedge cutter).

The pump is the same as this one
http://www.ebay.co.uk/itm/Welch-R-1397-Duo-Seal-Vacuum-Pump-...

Its a rotary vane pump. No diaphram.

J88

Actinium - 6-10-2014 at 19:47

if your getting any kind of fumes in your pump your definitely getting it in your gauge. having a gauge that can be closed off from the rest of the system is probably a good idea or at least behind the cold trap.
Maybe add a trap that has a drying agent behind the cold trap to capture any residual moisture to prevent corrosion as moisture will mess up your gauge as well. Just a suggestion as I've had that problem with the gauge on my AA 75x sterilizer never going back to 0.

IrC - 9-10-2014 at 13:37

I think he is merely gluing his reeds down, or at least making them sticky, or creating a buildup making a good seal impossible. Assuming he did not eat a hole in the diaphragm, usually unlikely. He then does some other experiment which dissolves the 'glue' buildup on the sticking reed, or gunk buildup blocking a proper seal, and it takes off for a while. Very simple to take the head apart, remove reeds, clean everything and reassemble. Being sure to position the reeds properly. Some fumes can eat the metal reeds altering the flexibility. Most often a reed is either not vibrating (cycling) properly or not sealing against the hole from gunk buildup.