The following questions was posed to us:
Current edition of section 5.5.3 of API-650 requires us to select the larger of
a) product design thickness plus corrosion allowance or
b) hydrotest thickness
2 - Note b to Table 5.1a requires us to use the larger of
a) product design stress or
b) hydrotest stress
in selecting the column to look at.
For a tank with
corrosion allowance = 1 mm
bottom shell course thickness = 45 mm,
product design stress for bottom shell course = 220 MPa
hydrotest stress for bottom shell course = 240 MPa :
item 2 above requires us to use the column for 240 MPa
bottom shell course thickness of 45 mm requires us to use the fifth line of
We end up with the 19 mm from among the tabulated values.
item 1 above requires us to use the larger of
a) 19 + 1 = 20 mm
b) 19 mm
If we could use the column for 220 MPa for the design conditions, then we would be comparing the value for design thickness as a) 16 + 1 with the same b) 19 mm for the hydrotest condition.
With the way it currently reads we always end up having to add the corrosion allowance to the
hydrotest thickness calculated.
In reviewing the way 5.5.3 reads you are correct. Since it specifies that you use the higher of the two stresses in determining which column you use and then must use the thickness shown plus corrosion allowance for the design condition, you will always be using the higher required thickness and then adding the corrosion allowance to that. It would seem that we should be using the column that corresponds to the stress in the shell for whichever condition we are evaluating and use the whichever thickness is greater. Since this is not how the standard currently reads, I have submitted an inquiry to the API committee for review. It will take time to get a response and even longer if they decide to address the issue and make a change in the standard.