(PV-Elite 4.10) I do not understand why the temperature used to determine the compressive allowable stress for the operating case of a vessel is the maximum of the internal and external temperature. Both internal and external design pressures do not act at the same time, so their temperatures must follow the same criteria.
This subject produces a significant difference in thicknesses.

Dear Carlos,

It is my firm belief that the intent of the ASME Code (UG-22) is to achieve vessel designs that will not fail under a variety of temperature/pressure/fatigue/mechanical loading conditions. A prime example is the design of heat exchanger tubesheets. While most exchangers have a pressure on both the channel and shell sides, this may lead one to believe that you merely have to design for the differential pressure. However, it is accepted that exchangers must be designed for loss of pressure or temperature on either side. This is in addition to vacuum, corrosion and other upset conditions. A brief review of Appendix AA paragraph AA-2.1 will confirm this.

So the question becomes, should the theoretical design of a process column be different than a heat exchanger (with regard to the question of which temperature to use for which case and upsets). The answer to that question is that it should not be for the most part. How can it be predicted with certainty that the vessel will always operate strictly within the design parameters. I don't think it can. Currently, what PVElite does is conservative and in our opinion meets the intent of the ASME Code.

As a result of our discussion of this topic earlier in the year, we have implemented a check box that segregates the calculation of compressive allowables based on load case and internal or external temperature. If the box is not checked (the default) the program will behave as in the previous version. This change has been implemented in the 4.2 version that is due to ship shortly.

Best Regards,

Scott Mayeux