FAQ - Sealing a conflat flange and flanges in multiples

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Richard Hull
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FAQ - Sealing a conflat flange and flanges in multiples

Post by Richard Hull » Mon Dec 02, 2019 7:29 am

A big issue faces a newbie to vacuum work. This issue is how to seal two conflat flanges, especially large ones, often joining two halves of a large SS chamber or a turbo pump, etc.

This process can be simple or complex. Simple if you are tightening one flange only at a time in a single flange serial chain. Complex and tricky if you have several flanges to seal in one chamber.

Conflat flanges are the vacuum system designer's preferred joints in ultra high vacuum systems. They are expensive, but when used correctly, are the best links in any vacuum system of the many required component parts during assembly. They have knife edges that almost touch each other if pressed together. However, an annealed copper crush gasket is interposed between the flanges during assembly and bolts are tightened to force the knife edges to dig into the copper to form a flowing metal seal over the knife edges. In the best case, the flanges never have to have there faces actually touch. A gap typically remains between the flange faces as the copper seals the system well before the bolts are tightened to cause the faces to actually touch.

The knife edges on the conflats do all the real sealing. Ideally, you never tighten beyond a certain amount and never tighten so much that you have the two CF faces touch unless you have a damaged knife edge and even then, that might not seal. Proper approved bolts are created for such high end systems and are often silver plated at high end costs. They are never really needed in amateur fusor systems. I tend to procure, at vastly reduced prices, all bolts, nuts and washers of suitable or proper length at hardware stores such as Lowes or Home Quarters. In a tight, low clearance fit on a flange where the perfect stock bolt length is not to be found, use "all-thread rod" cut to length as needed and use two nuts to seal at that tight clearance hole.

I use a washer at the bolt head to broaden the surface contact. At the nut end of the bolt I use a washer again with a lock washer and then the nut against that. Tightening must be done carefully and in an opposing order working around the flange accordingly. This across and around tightening helps evenly force the copper gasket into the knife edges. At some point when the gap between faces starts to close, but remains open, I start my mechanical pump and look at my TC gauge. I know what my pump can do and if in just a few seconds I don't see it doing it, I continue the opposing tightening regime until I see a drop in pressure. I stop and see if the pressure continues down in a manner which I like. If, after a minute or two, it is still not to my liking, I continue to tighten properly.

Here, it gets tricky....Is this failure to hit my concept of "well sealed" just water sucked in when the seal was imperfect? I try very tiny tightening until I hopefully see a drop again and hopefully at or near what I consider sealed. I let the pump run a bit and play a propane flared torch flame over the shell or flange lightly. If the pressure goes up, it is water. I just stop all tightening at this point and gas ballast the chamber to remove the water a bit more.

Ultimately, I wind up with what I consider a sealed system at the fore pump to my satisfaction. Note: I still have enough gap to allow for more tightening if needed once I get the secondary pump working and suspect a micro real leak back at the main flange gasket. This is searched for with an atomizer using acetone. If located, I tighten only those bolts near the leak and very slowly.

Multi-flange sealing on a chamber or other inline component

All sealing of conflats needs to be ideally done one by one. On a cross style chamber of a large spherical chamber there are other ports. Here is a real quandary!

My suggestion is to tighten all flanges involved to a decent level beyond what you might like in a single flange test as above and pray it is sealed. Start you fore pump and watch the gauge. It surely must start to drop rapidly. (this assumes you are very intimate with your pump's normal low end pressure in a good sealed system - see below). If it is not sealed to your liking, you just tighten one flange at a time until you see a change and continue to do this until no more drop is forthcoming. Mark this flange with a grease pencil! if still not low enough, move to another flange. If no change is seen then move to another and so on until you see the pressure drop again. if still not to your satisfaction, go back to the grease penciled flange and tighten just a bit. If a drop is noticed, give it a minute and if the pressure is within some limit you can accept, stop here. if not go back to the last leaky flange and tighten it a bit. Continue until happy. Gas ballast the system while heating the cross or chamber. If the pressure rises, it is water as noted above.

Getting intimate with your fore pump

Always block off your fore pump at the head with a TC gauge after placing new oil in it. Don't be a dummy and start off for the race with a broken leg and cracked skull!!! Is your gauge really good? Are you absolutely sure the gauge is very tightly hooked (VACUUM TIGHT) to the head of the pump?? Only if you are sure of this, turn on the pump. A truly good pump will pound down into the lower micron range almost instantly. If in 20 seconds you are not below 30 microns, you probably have a bad pump or a fouled gauge or a lousy connection to the pump and gauge. A good pump with clean oil, good gauge and good connection should, within a minute or two, reach its base pressure well below 20 microns and hopefully around 10 microns or less at the head. If not, you will have to pump you chamber in a real fusion scenario for many minutes to get it to bottom out. This is never a good situation. The solution is always to acquire and maintain a truly good fore pump.

If all goes well here, you may consider yourself as being intimate with your pump and your vacuum gauge. As the system's vacuum volume increases it is normal to allow for longer pumping times to base pressure and you may never see that base pressure ever again if your pump has a lower pumping capacity related to the system size. So, as you seal on down the line, expect to give longer pumping intervals as you tighten for the gauge to drop to a newer lower level, perhaps still short of your goal. Your pump is trying its best as you throw new stuff into your system. Keep all parts of your system very short in length...A foot long hose connection between your fore pump to your system is just terrible unless you have a 18 CFM pump!!

Know your pump before attempting to seal any vacuum components into the system and do such sealing in a linear fashion flange by flange, one at a time spreading out from the fore pump.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: FAQ - Sealing a conflat flange and flanges in multiples

Post by Jim Kovalchick » Mon Dec 02, 2019 10:03 am

Several manufacturers of CF fittings ultimately recommend face-to-face as the correct seal except where you approach the yield point of your bolts. Checking torque values will help with that.

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Re: FAQ - Sealing a conflat flange and flanges in multiples

Post by Bob Reite » Tue Dec 03, 2019 1:14 am

If you are using a convection gauge as I am for coarse vacuum, or any other type that is sensitive to changes in gas composition, you can use a can of "dust off" to spray each joint and watch for a change in the gauge reading. Tighten until there is no longer a change, then check other joints if the pressure is not where you expect it to be.
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Re: FAQ - Sealing a conflat flange and flanges in multiples

Post by Richard Hull » Tue Dec 03, 2019 5:12 am

For reasons explained earlier fusor IV had its faces smashed together in the end, but not in the beginning, as noted. I have, over the time frame from 1984 to date, had to do small tweeks of the bolts until the gasket was so crushed that the faces met. This is the norm. Apparently, even fully crushed the copper gasket may creep flow over time and regardless, the faces not touching or tightly torqued together do not effect any form of useful seal. Only the gasket of copper does the sealing.

Yes, crushing the gasket until the faces meet will undoubtedly seal on first assembly, leaving no wiggle room for future adjustments should a leak develop beyond full disassembly and a re-gasket at the coupling. there is rather constant expansion and contraction in the system due to the fusor's nasty habit of dissipating virtually 100% of the input energy to heating the reactor vessel. This thermal push and pull ultimately winds up at the weakest link, the copper gasket. The coefficient of thermal expansion of copper versus 304 SS ultimately does such seals in over time with repeated thermal cycling.

This is why I suggest tightening to a good seal only via gauge and leaving a bit of wiggle room for further tightening in future if or when it is needed. Few vacuum systems get the thermal cycling that the fusor gets. With small cross systems being the rage now, this cycling is much worse with the reduced thermal mass and cooling surface area of the old spheres.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: FAQ - Sealing a conflat flange and flanges in multiples

Post by Dan Knapp » Tue Dec 03, 2019 12:42 pm

I agree with Richard on tightening conflat flanges. I’ve been using conflats for nearly fifty years, and early on my frugality led me to squeeze as much life as possible out of the copper gaskets. I typically get 5-6 uses out of a gasket (i.e. opening and resealing). I put a little sticker on each flange and make a tic mark on it to keep track of uses. I check the clearance between the flange faces with a piece of paper, and when the clearance is gone, the gasket is on its last use. I’ve found LDS to be the cheapest source of gaskets.
Years ago, a colleague reported at a users meeting that he recycled copper gaskets by re-annealing them. Apparently, copper is annealed by heating red hot and quenching (unlike steel which hardens by this process). He quenched in alcohol to reduce oxidation resulting from quenching in water. He said this ignited the alcohol. It sounded dangerous to me, so I never tried it. It would be nice to have a way to recycle the large gaskets as the ten inch ones in particular are rather pricey.

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Re: FAQ - Sealing a conflat flange and flanges in multiples

Post by Dennis P Brown » Tue Dec 03, 2019 8:08 pm

At red hot in air, what little oxygen the water might add (if any) is so many orders of magitude below what heating copper to that temp in air would already create is laughable. So, water isn't the issue here, I'd hazard to guess ( :) ).

For high vac work (not a fusor), those copper seals are one and done. But certainly, for a fusor, retightening is ok. Still, why I prefer KF connectors and std o-ring seals - instal ,then as needed dis-assemble/reassemble as often as one wants with no issues.

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Re: FAQ - Sealing a conflat flange and flanges in multiples

Post by Richard Hull » Tue Dec 03, 2019 8:40 pm

KF connectors elastomer seals are ideal where regular on and off things are part of a system, (gauges, valves, window ports, experimental input ports, etc.). This is especially true in the vacuum range down to about 10e-4 torr. Basically, KF connections are "fore line normal".

If one is shooting for 10e-6 or 10e-7 torr on a regular working basis,.... Conflats are to be preferred from the throat of the secondary pump onward in the system.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: FAQ - Sealing a conflat flange and flanges in multiples

Post by Dan Knapp » Tue Dec 03, 2019 10:07 pm

I typically operate my system at the low end of the 10-7 Torr range and still get 5-6 uses from each gasket by careful tightening.

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Re: FAQ - Sealing a conflat flange and flanges in multiples

Post by Joshua Guertler » Thu Dec 05, 2019 2:27 pm

How will I know if the bolts are tight enough but no so tight that anything will get crushed?

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Re: FAQ - Sealing a conflat flange and flanges in multiples

Post by Dan Knapp » Thu Dec 05, 2019 9:52 pm

You want the knife edges to dig into the copper to make a seal, but you don't have to do it fully the first time. The manufacturers would have you tighten flanges on a new gasket until the flange faces touch, but you can make a good seal with less compression. The key is to tighten sequentially in a 12:00, 6:00, 1:00, 7:00, 2:00, 8:00, etc. pattern with a small compression on each cycle. On ten inch flanges I use a !2:00, 6:00, 3:00, 9:00, 1:00, 7:00, 4:00, 10:00, etc. pattern. You could use a torque wrench, but with experience you develop a feel for it. The key is to tighten until you get a good seal. This only works well with undamaged knife edges. A nicked knife edge can be made to seal on a first try, but subsequent uses are likely to have the nick in a different location on the gasket.

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