Prefabricated and stress relieved before erection

[Syahar1975]

API 650 states :

5.7.4.3 When the shell material is Group IV, IVA, V, or VI, all opening connections requiring reinforcement in a shell
plate or thickened insert plate more than 13 mm (1/2 in.) thick shall be prefabricated into the shell plate or thickened
insert plate, and the prefabricated assembly shall be thermally stress relieved within a temperature range of 600 °C to
650 °C (1100 °F to 1200 °F) for 1 hour per 25 mm (1 in.) of thickness prior to installation

Is there any reason why we can not do local stress relief after erection before hyrdotesting?

I mean why we can't stress relief on nozzles to shell fillet weld joint only by electrical heater.

Syah

 

[JStephen]

If you're asking "Does that meet the standard?", the answer is no- there have been some requests for interpretation on that particular point, answered to that effect. I'm not sure what the reasoning is in that, though.

 

[IFRs]

When doing PWHT in the shop the shell plate can freely expand as needed due to temperature changes. However after the plate is installed in the tank the situation becomes much more complicated and the thermal expansion / contraction is restrained by the adjacent shell plate and welds. These strains are accompanied by stresses which may affect the tank in ways that are not easily calculated or included in a cook-book type of document like API 650. Unless of course you can heat and cool the entire tank at the same time and rate, which is not practical. That is my understanding of the reasoning behind no field PWHT.

 

[Syahar1975]

Dear IFRs,

I did stress relief cone roof tank design for Amine product.

The tank is about 10 M by 8 M. We cover all surfaces of the tank by insulation about 50 mm thick.

We put a burner through 24" shell manhole

We did the same after a few years later for the same client in Indonesia for the same tank design.

We did not found any problem.


Syah

 

[IFRs]

I'm not saying it can't be done, just that API 650 does not sanction it and what I understand the reason to be. You may have to negotiate with the owner and note it on the name plate.