Utilitities and 2/3 rule

[GL431]

So often I come across heat exchangers in which the process side has a high design pressure and the utility side has a design pressure of 10 bar (150 psig). The utility side is often thermal oil or heating water. The process side invariably have operating pressures and therefore design pressures much above 3/2 utility side design pressures. Invariably the utility side is not protected against overpressure due to tube rupture in any logical way (there might be a pressure relief valve located on a head tank far away, but this is clearly not for the tube rupture case), although API 520/521 either in its current version or previous versions would require such overpressure protection. Am I missing something in the design of these systems? Trying to get in overpressure protection afterwards is a pain and very often the systems are just left as they are. (I have to add that this is in Europe - where I breathe and work. Is it perhaps that the tube rupture case is not stressed in the European guidelines? I work mainly with the API guidelines. It might be that utility system vendors do not know or follow these guidelines in the original build. Is the protection of these systems different in the States?)

 

[MMII]

I work in a european company and we apply the 2/3 rule. First of all take into account that actually the 2/3 rule has changed to "10/13 rule" (because ASME VIII code has changed and the hydrostatic test now is done at 130% of the design pressure instead to the 150% of the design pressure).

It is difficult to decide if is better to design for 10/13 rule or install a PSV to protect the low pressure side for the tube rupture case. If you design for 10/13, till which valve/equipment/pipe you design at this high pressure (10/13 x Pd high P side)? API says that the low pressure SYSTEM has to be design to 10/13xPdesign of the high pressure side. The question is, what is exactly the low P SYSTEM? because it is not only the exchanger, it should be at least till any valve that you have at that system.

So, what it has to do for each particular case is to think if it is worth or not to design the low pressure SYSTEM at 10/13 of the high design P side,or if it would be better to install a PSV for tube rupture case instead.

I hope all this helps you.