pressure thickness design

[mielke]

I have a wall in a pressure vessel (1000F and 50psia) that we are designig. it is 15"x30" and we are comparing a flat plate and a hemispherical shell for the wall.

I have been reference to the following equations for determining the thickness of each of the above...
flat plate: t=d(ZCP/SE)
half pipe: t=PR/(2SE-0.2P)

where d is the shorter dimension
D is the longer dimension
Z is noncircular correction =3.4-2.4d/D
C is the joint factor (0.7)
R is the radius (7.5)
E is the joint efficiency (0.33)
P is the design pressure (50)
and using allowable stress of A516-70(1000F) S=12,000

in calculating it out i get that the flat plate is about 44x as thick as the half pipe! is this reasonable? am I using the equations correctly? any advice?

Thank you

 

[TGS4]

Sounds about right. Flat plates and pressure don't mix well.

Think about it - are your balloons spherical/cylindrical, or flat plates? For a given geometry and internal pressure, the sphere/cylinder is the most efficient use of material.

 

[mielke]

Is there any way to quantitate the effect of running a piece of angle along the corners of a flat plate to reinforce it?

 

[rb1957]

d'ya think the analysis is valid for a displacement of 44x thickness ?

i'm guessing you don't, 'cause you're asking.

well, it ain't !

i'm guessing your structure is a flat panel, without stiffeners. flat plates don't like pressure (as noted above). they will deflect and in-plane membrane tension (like "hoop stress" in a fuselage) will help the plate react the pressure. displacements may still be too large to be acceptable ... so break up the flat plate with stiffeners (angles/tees welded/fastened to the plate).

i'm guessing you don't have access to an FEM ... how about Roark ?

if this is the way you ant to go, we can help with equations/solutions. you can look this stuff up on-line as well.

 

[mielke]

the equation i had above is the only one i can find for flat plate (i get lost easily searching on the web.) i think that equation is ASME and probably pretty conservative if so. if anyone has any other references for flat plate thickness i'd appreciate it.

 

[rb1957]

google "moody rectangular plates" ... seriously, i did and got a hit on Roark's reference for rectangular plates with transverse pressure.

 

[JStephen]

The statement was that the flat plate thickness calculated out 44 times that of a sphere, not that the deflection was 44 times the thickness. For that size and pressure, calculated cylinder thickness would be very small.

 

[loafer09]

The two references you need are:
Timishenko’s “Theory of Plates and Shells”
Young’s “Roark’s Formulas for Stress and Strain”

 

[Cockroach]

You may wish to look at legacy threads on this subject in the forum. This is a very old issue, I remember providing closed form solution sets using Thick Wall Pressure Vessel, Von Mises-Hencky Theory, FEA using SolidWorks (I think 2004 in the day).

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada