I have never seen this before, but, after reading the Appendix L example as described below, I can see the advantage.
UG-99(c) can be used to gain an advantage for a vessel calculated in an uncorroded condition when determining minimum metal temperature to be used only during the Hydro/Pneumatic testing as described in Appendix L-9.2
Also see Figure UCS-66.2 General Note 6
There are three kinds of people in this world; those who can count and those who can't.
The "calculated pressure" is what I was taught is the MAP (N&C) or Max Allowable Pressure (New and Cold). If you take the philosophy that a vessel should be stressed more in the shop than it ever will in the field then you have to use MAP as the test basis.
I'll throw out an example: Take a shell fabricated of 0.50" SA516-70 with a 0.25" corrosion allowance at say 200°F with a diameter such that the stress at MAWP is exactly 20 ksi. The MAWP will be half of the MAP since in the N&C condition the shell is twice as thick. At N&C if you pressurize to MAWP the shell will be stressed to 10 ksi. So if you test to 1.3*MAWP you'll stress the shell to 13 ksi. But at the end of life of the vessel, as the shell has corroded to its retirement thickness of 0.25", the stress in the shell will be 20 ksi. The 13 ksi stress at hydrotest in the shop is less than the 20 ksi stress the same vessel will see in the field.
On the other hand, a hydrotest pressure of 1.3*MAP will stress the shell to 26 ksi, thus more than the vessel will see as it approaches retirement.
Yes, most vessels will not be as extreme as my example, but hopefully it makes my point clear. Obviously as the new thickness to corroded thickness increases the situation get exaggerated. As an owner/operator, which would you rather have?
