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Here is an interesting question for those who are familiar with vacuum science techniques. In the attached picture we have a damaged surface of a KF flange, which comes from the turbo pump outlet and connects to a roughing pump. We believe this is a source of major leak (our roughing pump or helium leak-checker can pump out a small 4-way cross efficiently, but the same 4-way cross attached to the turbo pump (inlet sealed) cannot be pumped down efficiently).

How would one fix a scratched flange surface like this? Tried putting some silicone vacuum grease but didn't fix the problem.

UPDATE (01/31/2019) : We tried the PTFE o-ring as suggested below, but it did not prevent the leak. My suspicion is that because the damage is on the inner diameter of the flange, the o-ring cannot simply fill in the gap resulting in the inner diameter (only the centering ring, not the o-ring, touches there I think).

UPDATE 2 (03/08/2019) : The turbopump was repaired by Duniway Stockroom. What they tell me is that the scratches were not to blame for the leak! What matters for the foreline flange is the presence of deep scratches that may connect to the outside, and in our case, no scratches were of this kind. The deep scratches that we had were closer toward the inner diameter. The actual culprit for the leak was that there was a crack in the joint between the foreline flange and the main body of the pump.

  • $\begingroup$ Do you know what's caused this? Maybe you also need to check your lab's procedures to make sure you're catching/preventing as much damage like this as you can? (I don't want to accuse without knowing anything, I'm just curious.) $\endgroup$
    – thosphor
    Jan 25, 2019 at 14:55
  • $\begingroup$ @thosphor, I have no idea, but probably because people are too lazy to put plastic caps on the flanges (actually I don't see them, we mostly cover with aluminum foils). $\endgroup$
    – wcc
    Jan 25, 2019 at 15:19
  • $\begingroup$ In addition to the answers below, if you're in a lab, factory, or facility where this kind of thing can happen (too many inexperienced or clumsy technologists, students, whatever - frequently removing your backing pipes) then you should always protect the welded flange with a short KF-KF extension or bellows that can be your sacrificial flange. If that gets damaged you can just have a competent person replace it rather than having to hack it back into shape or send the pump back for an expensive repair. $\endgroup$
    – J...
    Jan 26, 2019 at 12:32

3 Answers 3


I have fixed flanges like this by sanding. If your flange is aluminum, this won't be too hard; if it's stainless, it might be fairly tedious and painful!

Start with a coarse-grit sandpaper, around P120-P200, and sand out the visible dents and scratches. Then work your way through a couple of intermediate grits up to at least P600. Sand in a circular motion around the flange to avoid making scratches that cross the o-ring. Make sure to moisten the sandpaper (I usually use isopropanol) to keep the dust down and to plug the exhaust tube and clean its outlet regularly while you work, as you don't want any of the grit to migrate into your turbo.

As a side note, I've found Apiezon L to be a very good low-outgassing grease for ultra-high vacuum, but it's always better to fix leaks if you can rather than greasing and hoping for the best.

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    $\begingroup$ The flange is indeed stainless steel, as it is part of a turbo pump. How do you make sure your surface is flat as you sand away the rough surface? $\endgroup$
    – wcc
    Jan 25, 2019 at 3:32
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    $\begingroup$ I would use a flat piece of metal stock (like a small block of aluminum) to hold the sandpaper flush with the flange surface. I wouldn't worry too much about it being perfectly flat, as your rubber o-ring can flex to accommodate large-scale surface unevenness. $\endgroup$
    – fiddlehead
    Jan 25, 2019 at 3:38
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    $\begingroup$ A plate of thick glass is also flat and smooth. $\endgroup$
    – mmesser314
    Jan 25, 2019 at 5:32
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    $\begingroup$ To better avoid getting dust inside you should insert cotton wool or other non-shedding pad into the hole. $\endgroup$ Jan 25, 2019 at 10:19
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    $\begingroup$ I'd dismantle to the point you can lay your sandpaper on a sheet of glass, with the flange face down. Then you can easily get rid of the debris as well. (I'd maybe finish off on our polisher, but that's because we've got one) $\endgroup$
    – Chris H
    Jan 25, 2019 at 13:40

I can think of three possible fixes:

1) The easiest "work-around" involves using a teflon gasket. Teflon is known to cold flow and conform to the shape that it is clamped between. With sufficient compression, such a gasket should fill in the gaps that obviously exist in your present flange face. Assuming that this works, you will probably want to keep quite a few teflon gaskets on hand, as each time you compress the gasket, the teflon would get squeezed to thinner and thinner dimensions.

2) Have a good machinist resurface the current flange face.

3) Cut off the existing flange and weld a new one on. This is probably not an option, or you would have done it already.

  • $\begingroup$ Dear David, your proposal 1) sounds interesting. I tried vendors such as Kurt J Lesker or Duniway stockroom but couldn't find teflon gaskets for the type of flange I am using. Could you recommend where I may find such compatible teflon gaskets? $\endgroup$
    – wcc
    Jan 25, 2019 at 3:29
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    $\begingroup$ @IamAStudent The KF system uses a captive O-ring on a carrier -- you can get Teflon (PTFE) O-rings comparatively easily (e.g. from globaloring.com). As well as being incredibly chemically inert, PTFE has the nice property (for me at least and possibly for you) that it doesn't crack at lHe temperatures and can survive repeated thermal cycles. It's also highly plastic, as David says. $\endgroup$
    – Landak
    Jan 25, 2019 at 8:48
  • $\begingroup$ @Landak, thank you for the vendor suggestion. I actually ordered some o-rings from them and will give it a try next week. Regarding plasticity, can they deform to the point that it will fill ~0.5 mm notch? $\endgroup$
    – wcc
    Jan 25, 2019 at 16:57
  • $\begingroup$ @IamAStudent Unless you got some completely weird KF size, KLJ definitely has the O-rings free and mounted on centering rings. They're listed as "fluorocarbon". $\endgroup$
    – user71659
    Jan 25, 2019 at 22:14
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    $\begingroup$ @user71659, the "fluorocarbon" is actually Viton with durometer 75. I believe this is the standard o-ring material people use. The PTFE o-ring is much softer with durometer 55. $\endgroup$
    – wcc
    Jan 26, 2019 at 0:47

Use a soft metal, thin, non flexible but moldable gasket like lead. Simple.

  • $\begingroup$ Just... no. This is a terrible idea for too many reasons to list. $\endgroup$
    – J...
    Jan 26, 2019 at 12:29
  • $\begingroup$ @J... Could you give at least one? Indium is often used for such purposes. $\endgroup$ Jan 31, 2019 at 22:30
  • $\begingroup$ @JánLalinský Lead is toxic. Unless there's a compelling reason to use it over any other normal solution to this problem that alone is worth avoiding it for. Decon for pumps with lead exposure in the vacuum system gets expensive when you need them serviced. Lead suffers from creep - it's not a good material for this application. Vacuum systems aren't a mystery - there's no need for wacky DIY solutions when there are perfectly serviceable normal materials, gaskets, and fittings for this purpose. $\endgroup$
    – J...
    Jan 31, 2019 at 22:59
  • $\begingroup$ Oh, with lead I agree. I was thinking of other soft metals, which are not toxic. $\endgroup$ Jan 31, 2019 at 23:05
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    $\begingroup$ @JánLalinský Well, even indium isn't ideal. It's useful at cryogenic temperatures because nothing else really works at all, but even there I find it rare for an indium gasket to survive more than 18-24 months in service (in cryopumps, for example). Copper is a material of choice for CF fittings and it lasts basically forever if you don't need to open it again. The only reasonable gasket to use on a KF flange is an o-ring. If the flange is damaged, the right thing to do is just to fix it - not to start jamming random materials between it hoping for a seal. $\endgroup$
    – J...
    Jan 31, 2019 at 23:16

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