PWHT of API 650 Storage tanks

[Naildozer]

Why is there a requirement to Stress Relieve Nozzle inserts on a tank with certain groups of metal over certain thicknesses but not on the vertical or horizontal course joints?

 

[IFRs]

In part because as you are building the tank, the metal can shrink away from the butt weld without causing high internal stresses.

 

[JStephen]

You also get stress concentrations at nozzles, so stresses are higher there than at regular shell joints.

Probably some practical issues enter in as well. If it was cheap and easy to stress-relieve an entire tank, the standard might require that as well.