Sawsan311
Chemical
- Jun 21, 2019
- 303
Dear All,
I would like to see your views about the depressurization criteria under pool fire scenario for vessels with thickness less than 1 in. We all know that API 521 section 4.6 and Annexure A.3 explains that the require to reduce the pressure to 50% of design pressure is based on the API fire test and analytical validation to reduce the tensile stress of carbon steel grade 516 think un-wetted plate with 1 in thickness such that it doesn't experience a stress higher than it UTS of 138 MPa when the temperature exceeds 649 C.
Now if we take 50% of that stress value we end up with 69 MPA which is 6.9 barg. Is this from where the criteria of:
- ''depressurizing to 50% of design pressure of 6.9 barg whichever is lower in 15 minutes '' comes from.
A- Do you also agree that for thin vessels and in absence of any stress based depressurization analysis, it is safer to dictate a final low pressure of 6.9 barg since this will conservatively lead to a sufficiently size restriction orifice in light of the fact that if we consider 50% of design pressure.. it would not be enough to mitigate the increase in the stress due to the low conduction heat transfer across the thin vessel.
B- OR : you agree that for thin vessels.. we would still depressurize to 50% of design pressure but in a shorter time with a typical rule of thumb of 3 min reduction in the time for every 5 mm reduction in thickness.
which criteria do you think would be more influential in ensuring a safe design.
thanks
regards,
I would like to see your views about the depressurization criteria under pool fire scenario for vessels with thickness less than 1 in. We all know that API 521 section 4.6 and Annexure A.3 explains that the require to reduce the pressure to 50% of design pressure is based on the API fire test and analytical validation to reduce the tensile stress of carbon steel grade 516 think un-wetted plate with 1 in thickness such that it doesn't experience a stress higher than it UTS of 138 MPa when the temperature exceeds 649 C.
Now if we take 50% of that stress value we end up with 69 MPA which is 6.9 barg. Is this from where the criteria of:
- ''depressurizing to 50% of design pressure of 6.9 barg whichever is lower in 15 minutes '' comes from.
A- Do you also agree that for thin vessels and in absence of any stress based depressurization analysis, it is safer to dictate a final low pressure of 6.9 barg since this will conservatively lead to a sufficiently size restriction orifice in light of the fact that if we consider 50% of design pressure.. it would not be enough to mitigate the increase in the stress due to the low conduction heat transfer across the thin vessel.
B- OR : you agree that for thin vessels.. we would still depressurize to 50% of design pressure but in a shorter time with a typical rule of thumb of 3 min reduction in the time for every 5 mm reduction in thickness.
which criteria do you think would be more influential in ensuring a safe design.
thanks
regards,
