Hi,
I would like to know while calculationg Do/t for design under external pressure, "Do" and "t" should be the correoded values or required thickness?

My problem is that the "required thickness+ corrosion allowance" is less than the std. plate thickness so I am choosing the next higher thickness.
In this case if I subtract the corrosion allowance the thickness remaining is more than the required thickness.

I feel that both should be taken as required values to be on safer side?
Could anybody comment on this.
Arun

The rules of ASME Code paragraph UG-28 provide a method to determine the maximum allowable external pressure ("Pa") based on the given thickness. Of course, the given thickness should be taken in the corroded condition to determine the maximum pressure for operating condition.

Notice that the Code provides rules to determine the required thickness based on a given internal pressure but that the rules for external pressure are written differently. For external pressure the thickness is given (assumed) and the pressure capacity is found.

There is no direct algebraic method of finding thickness as a function of external pressure; ie: t(P). The only direct relationship is external pressure as function of thickness: P(t).

It is possible, but very difficult, to determine the required thickness for a given external pressure. The procedure to do so would involve iterative numerical solutions to solve for the required thickness for given pressure. This could be done "manually" using a computing device (slide rule, calculator, computer) and the right algorithms. But this would be extremely time-consuming (which is why ASME Code does not take this approach). However, some commercial software programs for design of vessels include this calculation and will report both the MAEP for the given nominal thickness (when corroded) and the thickness required for pressure.

Of course, the thickness required for pressure is a function of the given thickness. So if you are using software you might find "Oh, I have 12 mm plate but the required thickness is only 1.70 mm." But do not think that you can change to 1.70 mm plate! Because as you reduce the nominal thickness of the plate the allowable stress will decrease as well, leading to an increase in the required thickness. Of course, at some point the nominal and required thicknesses will be equal.