Design Pressure Vessel: Vacuum, O-Ring Guidelines

[engineer23]

I am trying to design a pressure vessel for testing purposes that is 12 inches long and must have an inner diameter of 1.5 inches.
1) What is the minimum thickness the walls can be in order to attain "vacuum" pressure (10E-6 torr)? I want to make the walls as thin as possible and still be able to pull a vacuum. Is there an equation for this somewhere?
2) Sealing of the pressure vessel -- I would like to use O-rings to seal the end caps. I need guidelines for O-ring design, since I am entirely inexperienced in this design area. Anyone have a good website for a crash course on O-rings or can give me some general advice on design? I can certainly provide more information if you need it.

Thanks!

 

[FredGarvin]

1)Consult ASME pressure vessel code design guidelines. It's not so cut and dry with vacuum and it gets complicated when dealing with stresses around the ends.

2) Parker has an excellent o-ring design guide. You can find it here (page 4-20 is what you will eventually look for):
http://www.parker.com/o-ring/Literature/00-5700.pdf

 

[Astronuc]

Well in the normal atmosphere, the pressure vessel containing a vacuum will have 1 atm (14.7 psia, 0.1013 MPa) of pressure pushing in on the sides. Yes there are equations based on the geometry. The simplest geometry would be a sphere, followed by a circular cylinder with hemispherical ends, and then perhaps a torus. Squared geometries will have stress concentrations at the edges and corners, and are more prone to buckling.

 

[Q_Goest]

Fred's right about using ASME Code. Even though this isn't a pressure vessel per code definition (ie: it's a tube or pipe) the piping codes will all send you to BPV code.

See para. UG-28 for unreinforced shells and tubes. UG-28 has a nice step by step procedure, but you'll also need Subpart 3 of section II, Part D to look up values based on material. If you'd like I can walk you through that but you'll need to define your material. At a minimum you need type of material, yield and ultimate strengths. If you have ASTM material specifications for it, that would help.

Regarding O-rings, the Parker reference Fred points to is the industry standard. If you'd like help with it, you'll need to be more specific.

Note that 10-6 Torr is a very high vacuum and such things as cleanliness will be very important. Do you have any experience in trying to create that high a vacuum?

 

[engineer23]

Thanks! The Parker guide was immensely helpful and I was able to select an O-Ring we have in house for a face seal gland.

As far as the material of the pressure vessel, I am open to suggestions. I was browsing through the machine shop yesterday and came across a stock piece of Inconel that I that would fit the bill.

 

[Q_Goest]

Since you don't have a material and are open to suggestion, I'll suggest 304. Inconel isn't very descriptive, there are lots of different grades, so that doesn't help. At any rate, Inconel has roughly the same modulus, and is probably much stronger so 304 will give you a very conservative value for wall thickness, which you will see, is very thin.

If the external pressure is only atmospheric, the wall thickness can be on the order of 0.015" or even less. Here's a quick synopsis of the code calcs:

For L/Do = 12/1.5 = 8
Do / t = 1.5/.015 = 100
From Fig G, Section II, Part D, Factor A = 0.00015
From Fig HA-1 (for 304), and assuming no more than 100 F temperature, Factor B = 2000

Equation provided in UG-28 then gives Pa = 27 psi so even with a wall as thin as 0.015", there is more than sufficient strength given a 1.5" OD tube.

 

[mpbst27]

Hi does anyone here have the ASME BPV code part 2 on hand? I am constructing a vacuum vessel from fused silica glass and I need to determine the constants A and B for the design. The temperature will be from 20-80C. I am using the UG28 section on vacuum vessels. If anyone could scan the chart with fused silica, that would be uber-helpful! If not, I'd settle for just knowing the 2 constants if some enterprising engineer can post them. Thanks - Dr. Mike.