Stainless steel pipe chamber

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Chris Mullins
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Stainless steel pipe chamber

Post by Chris Mullins » Tue Jun 13, 2017 1:29 am

Would it be feasible to make a vacuum chamber out of schedule 40 or 80 stainless steel pipe? Steel pipe is available in 304/304L steel, so it seems like it should be usable at first glance. This would likely be for a cyclotron-style chamber (8 or 10" in diameter, only an inch or two tall) rather than a fusor chamber.

For top and bottom lids, I'd have to form a viton gasket grooves (and the lids themselves of course). I'm also assuming I'd have to smooth up the interior wall- maybe pickling too? Raw pipe seems pretty far removed from the typical shiny stainless vacuum hardware. Would it take a lot of finishing work to get it vacuum ready?

ian_krase
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Re: Stainless steel pipe chamber

Post by ian_krase » Tue Jun 13, 2017 3:04 am

A few inches of pipe sounds expensive and hard to buy. For something like this schedule 40 is probably enough. The main issue I see is making the face plates which will need to be quite thick. You might have an easier time bending strip into a cylinder and brazing it.

You can use flat flange gaskets or weld flanges (expensive at that size!) onto your chamber.

Chris Mullins
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Re: Stainless steel pipe chamber

Post by Chris Mullins » Tue Jun 13, 2017 10:46 am

It isn't easy to find short lengths of large diameter steel pipe, but I did find a couple online vendors who will cut to length (e.g. https://www.onlinemetals.com/merchant.c ... &top_cat=0 where a 2" long 8" diameter pipe is about $30). If it's 10" diameter, the best I found for schedule 40 was 1 foot lengths. I don't have a good way of cutting that though - would have to take it to a local shop.

For lids, I was thinking of 0.25" plate.

My biggest concern is being able to finish the inner pipe surface so it's vacuum ready, especially if there's something about typical 304 pipe that precludes that.

ian_krase
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Re: Stainless steel pipe chamber

Post by ian_krase » Tue Jun 13, 2017 8:20 pm

I'm not sure if 1/4 inch is enough for a flat plate at that size. Can try the math.

Elbow grease and scotchbrite or a drill and a wire brush wheel should be plenty.

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Frank Sanns
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Re: Stainless steel pipe chamber

Post by Frank Sanns » Wed Jun 14, 2017 12:01 am

While the surface roughness contributes to surface area that can hold moisture, it is the pores and interfaces that are more of the problem. Polishing the inside will gain some advantage for pressures lower that 10E-4 or 10E-5 torr but it will do nothing to remove the pores that hold onto air and moisture. These virtual leaks are not much of a problem for a fusor unless you are really trying to push your neutron numbers.

Chris Mullins
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Re: Stainless steel pipe chamber

Post by Chris Mullins » Wed Jun 14, 2017 2:08 am

Thanks, hopefully some elbow grease will suffice for reasonable vacuum levels (10e-5 torr). I've seen some posts here, and elsewhere (for example http://www.practicalmachinist.com/vb/fa ... ost1075051 that described more elaborate treatments, but I'm getting the impression that's more necessary for UHV beyond 10e-6.

How to you take care of air and moisture held in the pores? A bakeout (up to the limits of the gaskets) under vacuum?

ian_krase
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Re: Stainless steel pipe chamber

Post by ian_krase » Wed Jun 14, 2017 3:43 am

Bakeout and plasma cleaning (also known as "running your fusor")

Basically, a glow discharge of residual gas when pumping down can clean the chamber.

One of the FAQ's covers plasma cleaning.


(if one isn't trying to flow a gas, one can often get away with much by just pumping Really Fast.)

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Dennis P Brown
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Re: Stainless steel pipe chamber

Post by Dennis P Brown » Wed Jun 14, 2017 10:52 am

From my experience with vacuum systems and my own fusor, once a system is leak checked and easily gets into the mid 10^-4 torr, burn in with plasma will cure a lot of ills. Keeping most the vacuum system under vacuum (a gate valve to those items) and opening the main chamber as little as possible will keep the system extremely clean - my large junk steel chamber - after burn in - easily gets to the mid 10^-6 torr.

I found my so-called chamber on ebay and will some simple work, turned it into a fusor; if I was to do it over again, I think I would just try and find a approx. 2 inch inside diameter four-way vacuum connector "T".

Mark Kimball
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Re: Stainless steel pipe chamber

Post by Mark Kimball » Sat Jun 24, 2017 10:00 pm

FYI, you can buy a 6" piece of 6" diam 304 stainless tube from online metals (.12" wall, welded seam) for a little under $80 (cost as of 6/24/2017, not including shipping). If you can braze something that big you could put some ends on it for pretty cheap. Going to a shop for welding on ends/flanges would end up costing way more than the cost of the raw materials. Unless you can DIY most of it with tools you already have, you probably would be better off looking for a good deal on the internet.

Mark

Chris Mullins
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Re: Stainless steel pipe chamber

Post by Chris Mullins » Thu Jun 29, 2017 3:11 am

Thanks, online metals has some good choices. Once you get into 10" diameter, there aren't as many sources; one I found is here: https://www.twmetals.com/catalog.html?c ... id=38898-1, for schedule 40. They also have schedule 80. It's priced by the foot, so I'd also need a way to make good, straight cut (or clean up a likely crooked reciprocating saw cut).

The next step was buying an entry-level TIG welder (like this? http://www.eastwood.com/tig200dc.html) and learning to weld. The idea was sourcing cheap stainless steel raw materials so my son and I could afford to make mistakes, etc. while learning to make vacuum-ready welds. We're also learning about vacuums in general at the same time, so I didn't want to start with a material that can't even be made to work. We've achieved about 90 microns with our cheapie Harbor Freight pump + Edwards SI100 diff pump (with diff pump off, no chamber attached). We should have the diff pump operational soon, and hopefully enter the next stage of vacuum!

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