Material thickness pressure vessels

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<Cough> The principles can be worked out from some O-level physics <splutter>. Extending from there, there are a number of different codes for the detail of pressure vessel design. Best bet is to get a mentor to take you through one for the first time, because whatever is being designed may have client-specific features <cough, cough>. There are some software packages out there too. Remember to get insurance company review (at least) of the calculations, 'cos they will be taking the risk on it <splutter, sneeze> and won't want it going bang, not with that pressure and volume; it could do a lot of damage.

Don't need 'em for cardboard boxes <wheeze>.

I would make a rather thik wall!

You would need to specify the type of material from which the vessel is constructed, as the yield stress affects the minimum thickness. The basic equation is

t = (PD)/(2xSy) where

P = Design pressure[Pa]

D = internal diameter[m]

Sy = yield stress[Pa]

t = thickness[m]

However, the vessel would need to be designed, fabricated & inspcected in accordance with a recognised code of manufacture eg. ASME VIII Div 1, which states that

t = [PR]/[S*E - 0.6*P]

with the symbols as above and

S= allowable stress value as per ASME

E = joint efficiency

R = internal radius

many thanks for the reply but -

How do i work out what the -Sy = yield stress[Pa]

The Hammer,

You are right, but for equation t = (PD)/(2*Sy), I'd like you to take a factor of safety, where you can't substitute with yield strength Sy directly into equation, but such a factor of safety like 1: 3 or 1:4 must be taken, depends on the code required to be applied.

For ASME code, an allowable tensile strength, S is to be used and can be extracted from ready made tables at ASME section II, Part D.

Glen,

If you tell me the type of material used in design of vessel (according to ASTM), I can tell you how much its allowable tensile strength, S.

Hi;

You are right regarding the FOS. The first equation was a sort of "general approach" whereas the second equation from ASME does specify that Sy is the ALLOWABLE stress value.

Thanks for the proof reading!

The material we intend to use is mild steel.

It all depends on the diameter of the pressure vessel. The stress that the vessel will see is called circufrential or hoop stress and is relativly easy to work out. The volume of the pressure vessel is not needed in this calculation, although it is needed if you need to find the strain then therefore deflection. The equation you need to use for the hoop stress is:

σ = P · r / t

where,

σ = Stress (MPa)

P = Pressure (N/m^2)

t = Wall Thickness (m)

You can transpose this equation to fine the wall thickness,

Just a quick note if your not familiar with the conversion, 1 Bar = 100,000 N/m^2

The Stress of the material obiousily needs to be lower than the materials yield strength.

Although if the diameter of the vessel is high then an alternative set of equations will need to be used call lames equations and these are far more complex.

Hope this has helped.

Glen,

Hopefully you are asking this question out of general interest, that's fine... However I'd suggest that if you have to ask this question then you REALLY should not be thinking of actually designing and manufacturing a pressure vessel... It would be very dangerous and in most countries illegal to put such a vessel into service without complying with local codes, standards and regulations.

Steve

Glen,

In a previous employment we made pressure vessels. In the US the insurance company had to inspect each vessel twice. Once at tack up, once after final weld. the later test required filling the vessel with water and pressurizing it.

The welders had to be certified. The metal had to have certification.

Work with your metal supplier to get yield strength and certification papers.

The insurance company may be able to supply you with all of the calculations that will be needed.

I agree with the other posters here that advise caution, even at 8 bar you are dealing with a lot of energy here.

Also a quick scribble reveals that if you use 316 stainless and make it into a tube of 1 metre diameter, it will need to be about 13 metres long and 2.5mm thick. I don't think this will stand up by itself! so probably some support features needed also.

Drew

Hi Glen, Though I am not an engineer and would not like to hurt other members on this site who would be having far more experience and engineering knowledge than me, I would only like to suggest the use of simple basic physics that the lower we go the pressure increases e.g. DAMS - are built on such principles. One should also consider the fact that the lower part of the vessel will be affected faster that the upper part because in most cases the vessel will not be completely filled with the media. It may also be that the input is from the top of the vessel which means that when the vessel is being filled a particular portion will have to sustain far pressure at the time of filling + the normal operating pressure. As far as welding of the vessel is concerned, I would only suggest a two welding proceedure - inside & outside as it should erase all possibilities of leakage. Again this also depends on how much money your client is willing to spend. I feel that u must first consider whether u want to design a horizontal or vertical vessel and then accordingly you will have to take the thickness.

consider the fact that the lower part of the vessel will be affected faster that the upper part because in most cases the vessel will not be completely filled with the media.

Depends on the media. If it is a gas or steam, all parts are affected equally.

GO ASME , API OR CSC

OR GO TO ANY STRENGTH OF MATERIAL TEXT & LOOK FOR PRESSURE VESELS & YOU WILL THE FORMULA NEEDED..