## MAWP Problem in Sec. VIII Div.2

## MAWP Problem in Sec. VIII Div.2

(OP)

Dear All

I am using PV Elite software to model a pressure vessel based on ASME Sec.VIII Div. 2.

Design pressure: 93 barg

Design temperature: 88 C

Shell/Head material: SA-240 304

Shell ID: 3400 mm

Head type: Hemispherical

The problem is that with these conditions the vessel fails in the hydrotest calculations because the stresses are higher than allowable stress for hydrotest. As a normal procedure I increased the thickness of shell but again the vessel fails in hydrotest this time in the head region. Then after increasing head thickness the failure occurs on the shell and this process goes on.

Then I decided not to model the heads at all. This time the calculations were made on just a cylinder and the hydrotest fails again and by increasing the shell thickness this problem cannot be solved at all.

I think the root of this problem is that by increasing the thickness of the shell, the MAWP of the vessel increases too and this makes the problem like an endless loop.

What can I do to remove this problem?

Warm Regards

I am using PV Elite software to model a pressure vessel based on ASME Sec.VIII Div. 2.

Design pressure: 93 barg

Design temperature: 88 C

Shell/Head material: SA-240 304

Shell ID: 3400 mm

Head type: Hemispherical

The problem is that with these conditions the vessel fails in the hydrotest calculations because the stresses are higher than allowable stress for hydrotest. As a normal procedure I increased the thickness of shell but again the vessel fails in hydrotest this time in the head region. Then after increasing head thickness the failure occurs on the shell and this process goes on.

Then I decided not to model the heads at all. This time the calculations were made on just a cylinder and the hydrotest fails again and by increasing the shell thickness this problem cannot be solved at all.

I think the root of this problem is that by increasing the thickness of the shell, the MAWP of the vessel increases too and this makes the problem like an endless loop.

What can I do to remove this problem?

Warm Regards

## RE: MAWP Problem in Sec. VIII Div.2

## RE: MAWP Problem in Sec. VIII Div.2

## RE: MAWP Problem in Sec. VIII Div.2

Hope that helps!

## RE: MAWP Problem in Sec. VIII Div.2

## RE: MAWP Problem in Sec. VIII Div.2

P

_{T}=1.43*MAWPNow the allowable stress in div.2 is:

S=min{S

_{y}/1.5,S_{T}/2.4}For the cases that the allowable stress is limited by yield stress we have:

S=S

_{y}/1.5The MAWP that the software calculates is the pressure that makes the stresses in the part as high as allowable stress S. Now if this pressure is multiplied by a factor (1.43 in hydrotest) the stress will increase with the same factor. Therefore, the stresses for the hydrotest will be:

Hydrotest stress = 1.43*(stresses due to MAWP) = 1.43*S=1.43*S

_{y}/1.5=0.9533*S_{y}>0.95*S_{y}=Allowable stress in test conditionTherefore, if the allowable stress is limited by yield strength for a material, then the MAWP has to be set to a lower number. As far as I know stainless steel allowable stress is limited by the yield strength. Besides this situation is even worse if we want to use note G2 of table 5A in Sec.II-D and make the allowable more than S

_{y}/1.5.I do not know if what I have said is correct or if I am totally wrong. Can anybody help me understand?

Warm Regards

## RE: MAWP Problem in Sec. VIII Div.2

That 1.43 should actually be 1.425, which is 1.5/0.95.

We fixed this problem in the 2017 Edition - which should be out now for your perusal.

For the 2015 Edition and earlier, I would recommend, in your situation, to use 1.425 as is used in the 2017 Edition.

## RE: MAWP Problem in Sec. VIII Div.2

TGS4for your reply.I think this problem is still there for the materials which are subject to note G2 of table 5A section II-D. If you have corrected this in edition 2017 of the code and changed the 1.43 to 1.425 I think you should make another change. For the materials of note G2 this test pressure has to be:

P

_{T}=0.95*MAWP*(S_{y}/S)with this change if the ratio S

_{y}/S is 1.5 the test pressure is given by:P

_{T}=0.95*MAWP*1.5 = 1.425*MAWPAnd if the material is subject to note G2 of table 5A section II-D this value is less than 1.425

As an example for SA-240 304 in 100 C degrees temp. we have:

from table 5A II-D: S = 138 MPa

from table Y-1 II-D: S

_{y}= 170 MPaand the test pressure can be:

0.95*170/138*MAWP = 1.17 * MAWP

This factor 1.17 * MAWP is much lower than 1.425

Warm Regards

## RE: MAWP Problem in Sec. VIII Div.2

Paulettea, please submit a request directly through the ASME online tool. And then share in this discussion the ASME tracking number. I will personally follow up with that, as I was the Project Manager for the other test changes.