MAWP Problem in Sec. VIII Div.2

[Paulettea]

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

 

[BenStewart]

In my experience this seldom occurs. I'm not sure about the inner workings of PV Elite so as part of your troubleshooting I'd recommend you investigate what it's doing, but have you read 2.2.2.1(1) in ASME VIII-2 on the specified design pressure and MAWP? There's a similar comment in UG-99 note 36 in ASME VIII-1.

 

[TGS4]

Under no circumstances should your component be limited to the allowable stress at the test condition. See 4.1.6.2.

 

[ChrisF90]

My suggestion would be to only do one iteration of thickness increasing. Set the MAWP to be equal to either the design pressure OR equal to the MAWP prior to increasing the shell thicknesses. This should resolve your problem. We have taken this approach several times with Div. 1 vessels. I don't believe Div. 2 prevents you from setting the MAWP. For your data report you would need to indicate the MAWP limited by design.

Hope that helps!

 

[jtseng123]

If your client's spec asking MAWP must be calculated, then you will need to ask for an exception in this case. Tail chasing is not unusual especially for SS unless you set the MAWP equal to design pressure. Allowable hydrotest stress can go up to 95% of the yield per Div. 2.

 

[Paulettea]

I investigated this issue more and I think I have found the root problem. In Div. 2 the test pressure is given by:
P[sub]T[/sub]=1.43*MAWP
Now the allowable stress in div.2 is:
S=min{S[sub]y[/sub]/1.5,S[sub]T[/sub]/2.4}
For the cases that the allowable stress is limited by yield stress we have:
S=S[sub]y[/sub]/1.5
The 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[sub]y[/sub]/1.5=0.9533*S[sub]y[/sub]>0.95*S[sub]y[/sub]=Allowable stress in test condition
Therefore, 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[sub]y[/sub]/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

 

[TGS4]

Indeed, you have detected a flaw in the rounding of numbers. You are correct and the Code was wrong.

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.

 

[Paulettea]

Thank you TGS4 for 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[sub]T[/sub]=0.95*MAWP*(S[sub]y[/sub]/S)
with this change if the ratio S[sub]y[/sub]/S is 1.5 the test pressure is given by:

P[sub]T[/sub]=0.95*MAWP*1.5 = 1.425*MAWP

And 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[sub]y[/sub]= 170 MPa

and 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

 

[TGS4]

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.