Hot tapping tank shell with high liquid level

[Soln]

A tank that needs hot tapping at multiple elevations has liquid level that results in hoop stress of 15,000 psi. The material is known toughness, but I'm still leery of welding on the shell with that high hoop stress. API 653 states that materials of unknown toughness shall limit hoop stress to 7,000 psi (9.14.1.1.b4), but doesn't say the same about materials with known toughness. I don't want to impose unnecessary restrictions on the project, but also don't want to ignore the hazard.

Can anyone talk with me about experience related to limiting the hoop stress at the location where hot tapping takes place? Why wouldn't API 653 impose the max hoop stress of 7,000 psi for all materials? That seems like the conservative thing to do, but as I said, the project will cost more, and take longer, and the customer may not have the ability to adjust the liquid level for each hot tap elevation.

 

[IFRs]

When toughness is a concern, it is brittle fracture that is to be avoided. The standard considers 7,000 psi to be safe regardless of the material's toughness. For known material of adequate toughness that will therefore not be subject to brittle fracture, there is no limitation on liquid level or shell stress.