Allowable Stess for Zick analysis during PWHT

[abuanaselmasry]

Good day to you all.

I have one vertical vessel with big diameter (around 8 m) and high weight (around 350 tons). I'm currently designing the saddles for hydrotest and PWHT. The PWHT temperature is 610 °C. For SA-516 Gr.70 the maximum allowable stress is 12.9 MPa °C at 550 °C (considers also creep) and the yield is 158 MPa at 525 °C.

As per my 2 companies experiences, we had never considered this and never faced a problem. But, I'm worried about my current vessel since it is heavy and big in diameter. Shall I design the saddles for hydrotest only, and no need to worry about the PWHT, or do you see that some consideration due to high temperature shall be given?

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Regards,
M.Salaheldin

[saplanti]

I would suggest design saddles for both hydrotest and PWHT. But I would suggest to provide the necessary spider supports inside the vessel at the saddle locations (you may need the similar internal supports for other locations to keep the vessel within the tolerances expected) against the collepse and stress concentrations at above the horne regions especially at the PWHT temperature.