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I'm interested in whether a flat rubber washer or equivalent is good enough to replace an o-ring in a low pressure application (edit: I meant low pressure differential. My bad.). The reason is that flat washers might be easier to design for if you have a dynamic seal application where one tube has to "air lock" against another tube.

The application is to mount an air sampling tube in such a way as I can automatically sample air and then, later, automatically heat the tube to 200C and pull the vapours.

The tube is just a thin walled (0.25mm) steel tube 6mm or so in diameter and 40mm or so long). Here's how I initially imagined the system:

enter image description here

So some sort of automatic airlock connector tube could move out, seal against the o-ring and then stop at the green washer. The green washers might be something like mica or teflon, something able to take the 200C while insulating the holder - I want to heat just the tube, not everything connected to it! Here's a cross section:

enter image description here

I have some issues with this - firstly I'm not convinced that a metal to metal seal (tube against holder) will air seal properly, even when pushed by the automatic connector. Perhaps another o-ring is necessary?

Secondly, mica washers aren't flexible (I think) and getting a washer into a groove like I've shown might not be possible. If I replace the washer with some form of flat seal which captures itself against a lip, perhaps this type of form:

enter image description here

As I would have to make the connector by machining or other process, perhaps this would make the design easier. But is such a v type seal effective at air sealing? The difference is that in the O-Ring, positioning is all that is necessary but with a flat washer type, you need force too (related link: What are the common failure points of waterproof cases?)

EDIT: Clarifications:

  • I believe I tried to present too much information at once and missed the important points!
  • The pressure differential should not be very high. I'm considering a flow, from outer atmosphere, of a few litres per minute through a sorbent. If the pressure diff was more than 1/2 atm I'd be surprised.
  • The green washers are really to provide heat isolation, not air sealing. If the sorbent tube is 200C I didn't want the entire system to try and be 200C too!
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  • $\begingroup$ Why can't you just use a threaded valve with a teflon seal that is designed for high temp? Then you can just open and close the valve at will to take a sample. $\endgroup$
    – William Hird
    Commented Jul 8, 2017 at 19:53
  • $\begingroup$ @WilliamHird: Yes, that would the easiest but unfortunately I have 6 of these tubes on a rotating disc. I'm supposed to move a tube to an air sampling position, then afterwards move it to an analysis position. All very roboticsy :-) Now that you say it though, I could have a set of 6 way valves (!) that would do this without moving the tubes around the place. Except I need to load and unload the tubes later. It's like reinventing the CD changer. $\endgroup$
    – carveone
    Commented Jul 9, 2017 at 11:37
  • $\begingroup$ This application looks similar to the attachment used on the Cornelius kegs used for soda fountain dispensers. They use a bayonet locking feature IIRC, but theyre also operating at higher pressures. $\endgroup$
    – DLS3141
    Commented Jul 10, 2017 at 21:33

3 Answers 3

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This really depends on what sort of vacuum / ultimate pressure you're looking for. For something in the millitorr range, this probably won't work. Maybe if you have a big pump and a small internal volume, you could get down to 50-100 torr. Your metal-metal seal here should be fine, assuming your O-ring groove is sized correctly (and for the right pressure direction). You may want to make your O-ring groove on the deep end of the tolerance range for proper sealing so it does not get pinched when your outer tube moves into place. The face seal between the outer tube and your washer will be a problem, I think.

Something like PTFE isn't usually flexible to get a correct seal in most vacuum seal applications. Mica definitely won't be. I would opt here to put a second O-ring where your washer is; I'm not sure the washer, even if it was rubber, would necessarily have the right flex and shape to seal the gap. There are high temp vacuum O-rings like Kalrez (and others that are not as pricey) that should handle 200C without a problem.

The easiest thing might be to simply embed the O-ring in the plate where you would have your washer located. I tend to opt for the conservative method of a proper O-ring because with a minimum amount of design, they do a great job of a very predictable and consistent seal. It will be much harder to guarantee that with a washer, and it's always annoying and costly to send something back to the machine shop.

Alternatively you can move the O-ring out further on the plate so it gets sealed against when your outer tube moves into place (probably the best choice, now that I think about it...), or embed the O-ring in a groove on the tip of the outer metal tube. Unloading the tubes with an O-ring in the plate seems like it wouldn't be too bad.

Terrible drawing:

enter image description here

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    $\begingroup$ Good answer although I would put the groove in the tube head instead of the plate. $\endgroup$
    – Eric S
    Commented Jul 10, 2017 at 2:15
  • $\begingroup$ Thanks Eric. Mostly I put it on the bottom to avoid the possibility of it falling out due to gravity if the tube gets unscrewed (which it sounds like it does). Might be easier to modify the tube part rather than whatever this plate is, though. $\endgroup$
    – phyllis diller
    Commented Jul 10, 2017 at 3:38
  • $\begingroup$ I think I tried to present too much information as an entire design and missed some important points (that I didn't know were important at the time) so I've added some edits. The pressure isn't very high and the plate doesn't need to be air sealed - only insulated against the tube when it gets hot. That's what the green washers are for. $\endgroup$
    – carveone
    Commented Jul 10, 2017 at 10:13
  • $\begingroup$ Oh, I see what I did - I typed "low pressure" application when I meant "low pressure differential". I'm sorry - that's a completely different kettle of fish. $\endgroup$
    – carveone
    Commented Jul 10, 2017 at 10:15
  • $\begingroup$ I think you are right that O-Rings are more predictable than flat washers for sealing applications, even when the pressure diff isn't high. It was pinching or pulling the o-ring that I wasn't sure about. If I have a ring in a groove like I illustrated, perhaps a lot of force would be necessary to get the tube on. Which is why I prefer what you are saying - seal the face to face interface with an o-ring. But if it's not held in place, surely when the head retracts, the o-ring could fall out or stick to the head!? I'm looking at every air seal I can find thinking "how does that stay in there"!! $\endgroup$
    – carveone
    Commented Jul 10, 2017 at 10:22
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A properly designed O-ring seal will be your best option, though for 200C operatiing temperature, you'll need O-rings made from a material than is designed to function at that temp, like Silicone or Fluorocarbon (FKM) rubbers. O-rings are popular because they work well in so many applications and they're cheaper overall than anything that functions as well.

With any O-ring seal, static or dynamic, the key parameter is "O-ring Squeeze" which is defined as the percentage reduction in cross sectional area when compressed from the free state. Since the o-ring acts like a spring, this squeeze is directly related to the sealing force. The greater the squeeze, the greater the sealing force, and the greater resistance to leakage...at least in general.

As with most things, more is not always better, too much squeeze can result in a poor joint since it can cause excess stress in the mating parts, excess friction (in the case of a dynamic seal), and O ring damage since there's a greater risk of the o-ring getting pinched between the two mating surfaces.

In designing O-ring seals in stainless steel with good surface finish, I would try to stick to < 15% squeeze for dynamic seals and 15%-30% for static seals using a standard NBR O-ring from Parker, but it really depends on the type of O-Ring, the materials involved and all of the other specifics of the application. In this case, you'll want to look at the properties of whatever material you choose and as with any quasi-dynamic joint like the one you describe, smooth mating surfaces and lubrication are essential. (I recommend Krytox grease, pricy, but a little goes a loooooong way. You just need enough grease to make the O-ring look wet.)

There's a lot that goes into the design of a good O-ring joint. The best resource I've found is the Parker O-ring Handbook

Source: I designed and tested engineered mechanical seals for 5yr

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  • $\begingroup$ Nice link - lots of reading! I would have attempted to squeeze more on the basis that more is better - evidently this is not the case. I can see that FKM is the US name for FPM which Dupont calls "Viton" and I can source that from ebay in various forms. Krytox has some nice properties, thanks for the name. Also some nice warnings associated with any fumes! $\endgroup$
    – carveone
    Commented Jul 10, 2017 at 15:17
  • $\begingroup$ The Parker hand book is really the Bible for o ring sealing design $\endgroup$
    – DLS3141
    Commented Jul 10, 2017 at 15:20
  • $\begingroup$ I was under the impression that solid o-rings don't compress, they deform. Can you clarify that? $\endgroup$
    – geekly
    Commented Mar 30, 2018 at 18:29
  • $\begingroup$ @geekly why would you think that? Typical O-rings are made of rubber and designed to compress $\endgroup$
    – DLS3141
    Commented Apr 8, 2018 at 0:26
  • $\begingroup$ Because the cross-section area of a solid o-ring doesn't change when squeezed. It changes shape, but not volume. So, I take issue with the statement that the cross-sectional area of the o-ring decreases. $\endgroup$
    – geekly
    Commented Apr 11, 2018 at 23:56
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My push on connector with flat washer rubber is good for 2 bar diff press. Actually, o-ring seal inside socket connection maybe easier for assembly & disassembly. Difficult fabricate, yes.

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  • $\begingroup$ "Difficult to fabricate" is what I'm trying to avoid but perhaps I'm being too theoretical and I should go and make some experiments! The trouble is figuring out how a seal can be made and then unmade without all the bits falling out. When I look at examples - eg: the oil filler cap on my car is a plastic cap with a rubber seal onto a metal engine top - you can see how precisely fitting everything is with very specific materials (eg: it's not just any plastic to withstand engine temperatures). $\endgroup$
    – carveone
    Commented Jul 10, 2017 at 11:19
  • $\begingroup$ The seal only needs to be compressed between mating flanges. Rubber cement /gasket glue can be used to stick seal to one flange. $\endgroup$
    – RainerJ
    Commented Jul 10, 2017 at 11:24
  • $\begingroup$ Thanks for the help. That's another difference: flat seals need to be compressed, o-ring seals don't (after the initial push). I found some high temperature gasket sealant (blue silicone) which will do 200C so I should start reading instructions for working a lathe now! :-) $\endgroup$
    – carveone
    Commented Jul 10, 2017 at 11:55

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