Structural stability of unfired pressure vessel tmin for low pressure cylinders

[Aberswn1982]

I have come across a rule of thumb believed to be used for cylindrical pressure vessels that are designed to a low pressure and the hoop stress is not the driving factor. I am told (as a rule of thumb) this can be applied, something like :

(Diameter/650) + 1.8 mm OR (Diameter/1000) + 2.5 mm

I am aware of all the design considerations that are required for pressure vessel design and accept that there are a number of things wrong with using a ROT, however I am just interested in where this rule has come from? I heard it may be from a shell DEP? does any one know what DEP number exactly?

Many thanks

 

[brkmech1234]

Something like above formula appears in shell DEP or any other specification. This is the minimum thickness prescribed apart from the requirements by pressure.

A very simple equation is available for calculating the thickness of shell under internal pressure of as per UG 27(c) of ASME VIII Division 1.

t = PR( SE-0.6 P) + CA where

P is design pressure in MPa ,
S is allowable stress(138 MPa for SA 516 Gr 70)
E is joint effciency( 1 for full radiography, 0.85 for spot radiography, 0.7 for no radiography)
CA.. 3 mm for CS & 0 for SS
R... internal radious in mm

S for the other material ca be obtained by minimum (Fy/1.5, Fu/3.5)

ASME VIII Division 1 also madates minimum thick of 2.5 +CA


To conclude: when you have such a simple and beautiful equation, why to wander in an unknown zone. Only you need to put little effort to understand 5 terms extra which you could do in a couple of hours time, I reckon.

 

[Aberswn1982]

Hi Brkmech

Thanks for the reply, this is for the small number of occassions (one example that i can think of is a reboiler BS5500 vessel and is only designed to 4bar) where the hoop stress is low but there may be external/mechanical loading that results in the tmin requirements for structral stability is larger than the tmin required for hoop stress.

Small bore piping is another example that springs to mind, tmin based on hoop stress for a 1/2" pipe could be <0.5mm, but a greater wall thickness is actually required to support itself over a piping span - this is just an example, in reality we would use the piping engineer.

As I said, I am familiar with construction design codes used in my role for PVD and piping and every man and his dog knows about hoop stress. I was just after the particular Shell DEP for reference and out of interest.


thanks

 

[brkmech1234]

Well then fine