Buckling Shell and Stiffeners

It sounds as though the vessel has not been designed originally for full vacuum and on-site modifications are being made to it in a haphazard way. Why not order a vessel of the correct rating, with the correct pressure/vac test documentation for burst indemnity insurance cover purposes, and be done with it? Why take the risk?

Thanks for your reaction, but it does not solve our problem, we have to make a stiffener who is strong enough to withstand the full vacuum force.

the shell is an expansion joint !

hopefully someone has an exel programm for us

This is why firms hire competent Mechanical Engineers.

How much is the fee for coming to site, looking at the problem, reviewing the existing drawings, discussing it with the relevant people and conceiving and detailing a suitable solution?

Remember Flixborough?

This is his is the first time that we face this problem.

We need to solve this problem on short notice.

What information is needed to make a solid calculation to find out what thickness we need for the vacuum ring ?

Engineering isn't about "urgent".

To solve this on short notice, you'll need an engineer. Otherwise, you'll need to go to engineering school, which will take about 4 years. I'm guessing that would be too long for your "short notice" requirement. A halfway competent mechanical engineer with experience in vacuum vessels could solve your problem very quickly. Any money would probably be very well-spent, unless your time is worth next to nothing to your company.

Any calculations you need are provided in the website to which Lyn provided a link. If there is something you do not understand there, then you'll need to ask specific questions.

What information is needed to make a solid calculation to find out what thickness we need for the vacuum ring ?

1. A precise, fully-dimensioned drawing showing the shape of the shell, the means by which it is anchored to other pieces (and the nature of those other pieces) and any restrictions regarding where stiffeners can be placed. Show how you would like these stiffeners to work, and the directions in which stiffness is required.

700mm is one dimension of a three-dimensional object. Given that the force on a disc of that size would be over 8700 lb at sea level, (the weight of four small cars) you can see the need for knowing all the other dimensions. Maybe this shell is donut-shaped? Maybe it is not a full disc?

2. The specifications of the fabric, (fiber material and thickness, thread count, coating material, thicknesses of each, fiber orientation as installed, fabric tensile strength and stiffness). You can probably intuitively understand that the stiffening requirements would be different for a very heavy canvas versus latex the thickness of a condom.

3. Your definition of "vacuum ring". Is this a ring supporting the outer edge of the fabric (in somewhat the same way that a drum skin is supported by its tensioning ring)? If so 4mm is not close to right size.

4. What shape should the fabric assume when fully loaded to a differential pressure of around 14.7 psi? Can the cross section be roughly parabolic? "Flat" is, of course impossible.

5. A concise, clear verbal description of what the vacuum ring and fabric are intended to do.

So, get back to us with a good drawing, specs, requirements, and description. Then we can help you do a series of solid calculations.

In the mean time watch this video, to get a feel for the forces involved on vacuum vessels, even when the vacuum is partial, let alone the "full" you are claiming.

Thanks for your reaction.

The Roacks formulas P¹=(3EI/r3) gives me the uniform radial pressure on the ring

The expansion joint is mounted with two flanges and has an installation length of 265 mm. and a diameter of 700 mm.

Expansion joint is made from rubber.

Medium and temperature are in this case not important 25 degr. Celc.

If we forget the influance of the rubber, do I just calculate the Cm2 of the rubber * 1

for the total force on the ring?

The expansion joint is mounted with two flanges and has an installation length of 265 mm. and a diameter of 700 mm.

Expansion joint is made from rubber.

Does it look something like this?

This type of rubber joint often has steel reinforcement in it. Cable can work well for tension (where internal pressure is higher than external). As you can see, these reinforcements are of different sizes, depending upon location.

If internal pressure is less than external, then bucking becomes an issue, and steel cable is not such a good choice.

How was your 4mm steel wire used? Circumferentially? If so, you are concerned with hoop stresses, and the particular case where these stresses lead to buckling (requiring relatively low force) vs rupture in tension (requiring high force). A 4mm piece of steel of about 2200 long has very high tensile resistance, but essentially zero buckling resistance.

Expansion joints this large are expensive engineered products. I'd recommend talking to the manufacturer to see if you can replace the current flange with one engineered for vacuum. (Talking to someone at Deminas, or a similar company could help.)

As you can see, the logical location for a reinforcement to resist collapse of the joint pictured above would be (in this case) in the groove inside the joint. (Intuitively, I'd guess that a piece of about 20 mm cross section might work ok in a joint like the one pictured above, scaled to your size.) But getting it inside the joint, and welding the ends would not be easy. Putting a ring on the outside is very unlikely to work, because it would have to be adhesively bonded, and the bond will fail (the bond being a very small fraction as strong as the steel).

For hoop stresses, look here. Understand, however, that in structures that are composites of dramatically different materials (one weak and stretchy, the other strong and stiff) That is is hard to calculate forces, stresses, and strains at any point in the structure. FEA is the usual way to determine where to place reinforcements in a structure like this.

My thoughts mirror yours. The General Rubber site http://www.general-rubber.com/maxi-joint/style/1015.html indicates that their Maxi-Joint rubber expansion joints are rated at full vacuum, even up to 108" diameter! Doesn't indicate any special design features to achieve that.

a ga from me. i book marked that site for future refferences.

You need to "support" 23 metric tonnes per metre.

can you be more specific in what you mean.

How do I translate this to the thickness of a steel ring

and how do I calculate this

You get an engineer to take this figure and apply it to the properties of a specific material and shape of ring (possibly derived according to what burrito they had for lunch). Seriously, why are you asking this sort of critical advice form people you don't know?

people that I do not know or people who don't know :-) Your are not really helping.

The information you supply is woefully inadequate.

How many of them are there?

Are they circular ribs?

What is the spacing between ribs?

What is the realistic pressure? Full vacuum is doubtful.

This is hopeless.

How so? For 0.7m diameter I make it 7 tonne/m.

0.7 m diameter gives about 2.2 square metres per metre of tube.

1 atmosphere is just over 10 tonnes per square metre.

So my figure of 23 tonnes was the total: "all the way round" the tube. To do the calculations of how strong the steel ribs need to be I guess you would only need to consider a smaller section of the circle.

It was just a starting point really, to get a feeling for the sort of forces involved.

OK that's another way of looking at it, your figure is pi x mine. Makes no difference as long as the right load is used in any calculation.

What you describe is not really 'Buckling Shell and Stiffeners' in the usual sense. From Roark and Young's Formulas for Stress and Strain comes

p' = (3EI)/r3

for uniform circular ring under uniform radial pressure, r is the mean radius

Note that p is lb/in around the ring, not lb/in2. The geometry of your expansion joint, especially the spacing, will determine how the pressure force gets applied to the ring.

I would test the final product at full extension. Must be some fabric.

if you're pulling a full vacuum, you can calculate the pressure pushing on the tank at your altitude. the vaccumm gauge is marked in negitive pounds per square inch.

use the reading off the vaccumm guage as part of your calulations.

it's impossible to pull a full vaccumm.

Andrevanwees,

Here's a little dose of reality for you. Please go to this link. This came from bigg, I think:

http://www.pipingguide.net/2009/04/rail-tank-car-collapse.html

This is what a "full" vacuum can do. It is impossible to help someone when they know not what they do.

You give us absolutely NO usable information, and worse yet, you don't even know what information is required.

Hire an engineer.

CAN you please show cross sections in a drawing.

Some clue is -- instead of Bar- use Tube steel-Aluminum- Plastic.

L/r will increase- will hold

If U want ultra light -- use Bamboo

...and still the forum is told nothing about the process materials wetting the materials of construction nor the temperatures involved. It is no good putting a steel brace in contact with dilute sulphuric acid at 75degC, for example! Then, as there appears to be no Mechanical Engineer present, what are the chances of a Process Engineer being present too?