PWHT for head or whole vessel

[wcw1985]

Dear forum member,

I have a vessel which the thickness for
sa516 gr70
shell ID 660.4mm: 18mmt
Torispherical head: 25mmt

After calculation, my dish head elongation is about 24% which (>5% need to be pwht). So i need to do pwht, my question is can i just heat treat the head(consider as stress relief) only or i need to do pwht the whole vessel after completion of fabrication?

 

[metengr]

This is a process sequence question. If the vessel requires no PWHT because of exemptions in UCS-56, PWHT of the toris head can be performed separately versus the entire vessel. If the vessel requires a post-fabrication PWHT in accordance with UCS-56 thickness limits, PWHT for the head can be performed at the time the entire vessel is subject to PWHT.

 

[vesselfab]

can the head manufacturer hot form the head?

 

[wcw1985]

Thanks for reply.

Dear vesselfab,i will prepare a plate which cut to a certain diameter as per advised by the head manufacturer, the manufacturer does not do hot forming for head, they just help us to forming/pressing and flanging the plate i provided into a dish head.Is it written in code that if there are hot forming performed during the manufacturing of the dish head can waived the requirement of pwht for head? Do you mind point me out which code in asme section 8 shows it?

Dear metengr, thanks for your reply, after calculation, the vessel does not need to be completely pwht, just the head fiber elongation value are exceeded the pwht exempted value and is not entitled of the exemption of UCS-79 because of the UCS-79(d)(3) thickness > 16mmt . So from your reply i understand that dish head can be perform pwht separately from the body. Thanks.

 

[vesselfab]

ucs-79(d)

 

[wcw1985]

Dear vesselfab,

UCS-79(d) shows that all cold forming pressure parts requires heat treatment, i didn't see any statement stating that hot forming a head could waive the heat treatment process. Can you enlighten me? Thanks.

 

[vesselfab]

The code specifies that COLD formed parts with excessive elongation requires heat treatment.

It does not mention that hot formed parts need heat treatment.

This is why most head manufactures do a lot of hot forming.

 

[metengr]

and why... during hot forming strain is recovered because of thermal activation processes in steels, whereas, during cold forming strain is stored within the material and can result in harmful residual tensile stresses.

 

[wcw1985]

Dear vesselfab and metengr

Thanks so much for your explaination. I am very clear now. By the way i have few questions about the fiber elongation formula, is it correct to set Ro in UCS-79 formula to infinity when i forming a torispherical head using a flat plate? In what condition that we are not forming a dish head from a flat plate? What should we put the value of the Ro by then?