PWHT allowable stress 1

[fcs16]

I'm doing a simulation of a PWHT and I'd like to know if ASME say something about allowable stress for the thermal stresses originating from the PWHT.

Thanks,
Francisco

 

[metengr]

ASME B&PV Code does not provide specific values for thermal stresses during PWHT only guidance for heating and cooling rates to avoid “harmful” thermal stresses. Thermal stresses can calculated based on the thermal diffusivity of the material and thermal gradients.

 

[fcs16]

Thanks for the answer. I already calculate the thermal stresses (using finite element). I want to verify these stresses against some criteria. Can I use the yield stress as an allowable stress in this situation? I saw you can use 2Sy for secondary membrane + bending. And for local membrane stress?

 

[metengr]

Yes.

 

[fcs16]

But for the local membrane stress, I use Sy or 1.5*Sy?

 

[TGS4]

You do understand that the purpose of the PWHT is to "relieve" the stresses? That is accomplished by increasing the temperature so that the yield strength at temperature is relatively low. You ARE going to see yielding. Have you included any of the residual stresses from forming/welding in your analysis?

The criteria in the Code (ASME Section VIII, Division 2, Part 5) is NOT appropriate for this type of analysis.

You need to think about how this thing could "fail". Then, protect against it. Forget about using ANY of the criteria listed above, unless you know/appreciate/understand WHY it's being used.

 

[fcs16]

Thanks for the reply.

Yes, I understand that PWHT is for stresses relieve. But no, I don't include any residual stresses from forming/welding in my analysis since I don't know how much is stress is on it.

I think the "system" will fail if there are "excessive" stresses above the yielding stress. But how much above the yielding?

I'm doing a linear elastic analysis. Since I'm using shell element type, I want to evaluate stresses in the TOP, BOTTOM and MIDDLE surfaces of the shell.

 

[TGS4]

Your analysis is doomed to fail. PWHT is an essentially plastic operation. Performing a linear elastic analysis on such an operation is unwise.

I think the "system" will fail if there are "excessive" stresses above the yielding stress. But how much above the yielding?

Please define failure in this case. I would recommend that you reference ASME Section VIII, Division 2, Part 5 when answering this. Excessive stresses above yield mean nothing, for example in a system where the resultant displacements are constrained. Are you really worried about plastic collapse? Or ratcheting? Or buckling?

Yes, I understand that PWHT is for stresses relieve. But no, I don't include any residual stresses from forming/welding in my analysis since I don't know how much is stress is on it.

Let me understand this correctly, you understand that there a phenomenon going on, yet are ignoring it, solely because you are ignorant of the magnitude? I understand that you are new here to eng-tips (welcome and enjoy the ride), but this is starting to sound really odd/dangerous. Why are you even looking at this using FEA in the first place? Are you trying to quantify the stresses resulting from a local PWHT?

I will also note that in addition to this being a highly plastic operation, you are also well into the creep regime. Short term creep can be a significant component. The resulting redistribution of stresses from creep deformation can dramatically alter the state of local stresses.

 

[fcs16]

Thanks for all the tips.
Actually I'm performing the FEA to quantify the stresses from the PWHT procedure. I first place I don't want residual stress near the welding (it's a nozzle's welding). So I'm looking in the ASME code to see if there is something about a allowable stress for the PWHT or for there stress.