How to calculate the thickness of spherical crown head with a stiffener under external pressure 1

[mechengineer]

Hi Everyone,
How to calculate the thickness of spherical crown head with a stiffener under external pressure at convex side for a heat exchanger with a floating head?

The spherical crown thickness of floating head under external pressure (shell side), 53.2 kg/cm^2 at 399 deg.C. Shell ID=1100 mm and Radius of spherical crown=900 mm. The required thickness is about 72 mm (SA-516Gr.70) without stiffener by code calculation. I would like to add a stiffener (like a partition plate) to against the external pressure, whatever the thickness controlled by buckling or strength, however ASME Code has no a formula for such case. I would appreciate if any one shares the experience on such issue and provide a method/formula if possible. Thank you.

 

[SnTMan]

mechengineer, which Code, which type of head cover?

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

See Differential Pressure Design in UG-19. May be an alternative.

Regards
r6155

 

[cbPVme]

The buckling mode in a spherical dished head are not the same as a cylindrical shell. I'm not sure adding stiffeners is going to help you much for external pressure in this case. You are probably going to need an FEA to prove this design.

 

[DriveMeNuts]

When building an arch made of bricks, it supports its self with the help of gravity. If you to decide with good intentions to provide additional support by artificially providing upward support to the central keystone of the arch, the entire arch becomes unstable and collapses. By adding stiffener to a head you maybe essentially be doing the same thing. The stability of the head could not be relied upon once artificial support is added to it. My prediction of FEM design would be a head 5mm thick with a robust stiffening structure welded to it.

A flat head with stiffening structure would achieve the same thing using much simpler hand calculations. It would also be much cheaper to fabricate.

ASME VIII Div 2 Section 5.4.1.3 specifies that cylinders have a safety factor (or margin) against buckling under external pressure of 2.5x. For a head or sphere it is 16x due to the unpredictable and catastrophic way in which they flip inside out.
Therefore, I think using the cylinder design methods on a head is very non-conservative.

Using "ASME Code Case 2695 - Using ASME VIII Div 2 rules with Div 1" might get you a thinner head.

 

[SnTMan]

mechengineers' proposed construction is common in shell & tube heat exchangers, a floating head cover consisting of a dished head, bolting ring and one or more partition plates located on the concave (internal) side of the head. The following assumes the work is to Sec VIII, Div 1, which is not actually known.

Rules exist in Appendix 1-6 for calculation of the required thickness due to pressure on the convex (external) surface of the dished head. Per U-1(3), if such rules exist they must be followed. Therefore application of some other calculation or simulation technique to reduce the required thickness of a component is not permitted under this Code. It is permitted under 1-6(h) to account for continuity between the bolting ring and dished head, if such continuity exists, but this is not the same as stiffening the head.

Furthermore any such technique must meet with the approval of the Authorized Inspector, and perhaps with the purchaser and/or owner of the vessel. Doubtful.

Regards,

Mike





The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

With design temperature 399ºC all stiffening structure over hemispherical head shall be full penetration welds. NDE in these welds are difficult during fabrication and in-service inspection. PWHT is required before and after the floating head be machined.
This design is not a good idea, IMO
I suggest a plate without stiffeners

Regards
r6155