Pressure Vessel Calcs

[bulldo]

Hello,

Im doing some simple calcs for the design of a subsea pressure housing. Basically a cylinder with two flat end plates. (Im using boiler/calculations to guide me since they are much more common!)

Now, how does the Ultimate tensile strength compare to the Ultimate compressive strength?? I assume I want compressive strength anyway to equate to the forces excerted by the water pressure on the external faces of the cylinder. (aluminium 6082 Ts = 295 MPa) but compressive srength is not mentioned on the data sheet?

i have
0.2% proof stress 255 MPa (???)
ultimate tensile strength 295 MPa
ultimate shear strength 175 MPa

For the end plates i assume i just carryout the calculations with the end area of the cylinder, area of the end plate and material shear strength??
cheers

 

[jte]

Sounds like a homework problem to me. If you are indeed using boiler calculations, then you have all of the info you need in your code books in front of you.

"Ultimate compressive strength" doesn't play a role in a buckling failure of the shell. The flat (why?) end plates will be in tension (membrane), bending, and shear and will have interesting interactions with the shell.

jt

 

[JStephen]

If this is something fairly small, you might just refer to Formulas For Stress and Strain, by Roark and Young. It includes, among other things, formulas for flat plates in bending, buckling loads of cylinders, etc. If it is small, you could probably make some reasonable assumptions that would simplify design- ignore interaction between shell and ends, for example.

If it is larger, IE, something people can be inside of, consider using hemispherical heads in lieu of flat ends, which would become unreasonably thick. Design of the heads and shell would be governed by buckling rather than yielding.

 

[bulldo]

Considering just the circular cross section of the cylinder.

In the boiler example I have, the stress within the cylinder wall is calculated according to the internal pressure exerted.
The wall thickness is specified so that the tensile strength within the walls is less than the ultimate tensile strength of the material.

Now in my case the pressure direction is reversed and the cylinder will buckle at failure rather than rupture!

Will the forces within the vessel wall be compressive rather than tensile????

 

[stanm]

Can I suggest the following:-

1) Use a design procedure for cylindrical shells under external pressure (e.g. ASME VIII Division 1 Section UG-28, PD5500 section 3.6).

2) If this is not suitable use another recognised method (e.g. Roark's Formulas for Stress & Strain, 6th Edition Table 32) but use safety factors to ensure that the calculated stresses will be lower than the allowable material stress.

3) Do not take internal pressure formula and try to adapt for external pressure. Some design codes state that the thickness of a cylindrical shell under external pressure shall be no less than the thickness required under an equivalent internal pressure. In this instance calculations for both external pressure and internal pressure should be carried out. Internal pressure calculations use formulae that are not suitable for evaluating suitability under external pressure. If the cylinder has additional stiffening rings then the calculation method becomes more complex again.

4) Suitability of a cylindrical shell under external pressure is normally a straightforward design process. If this is a legitimate query then you will probably be better served by passing it to an experienced pressure vessel engineer.

 

[GenB]

Boiler calcs (even if some boilers have external pressure parts) do not apply to your design.
Sec VIII will probably be the best and sec II-D has the external pressure charts which limits the materials,
look into vessels for human occupancy. sec VIII use to jhave them but now ASME gave them up to other agencies, I do not know much about.
ER

 

[arto]

Look in ASME PVHO for the human occupancy design

Sec. VIII Div. 2 has cylinder-flat head discontinuity stress analysis [Harvey P.V. Hdbk has an easier to follow section with example.] simple calc's - ha!