In an overpressure condition due to tube rupture, the tube, which is the high pressure side, is ruptured, resulting in its fluid pressurizing the shell (which is the low pressure side) and causing overpressure. According to my company guide for this scenario (based on "PRV Sizing for Exchanger Tube Rupture" article), the relieving rate is a function of pressure drop (among other factors). Pressure drop is defined as high side design pressure less the overpressure value of the low pressure side or the critical flow pressure, which ever is greater.
I wonder why the high side pressure should be the design pressure. It seems that tube rupture doesn't happen only when the tube side reaches its design pressure. Instead, the rupture can ostensibly happen at the tube flow's operating pressure, or others. (this is assuming the tube side operating pressure is higher than the shell side design pressure).
There may be a simple explanation for this. I would appreciate your input/explanation.