Internal failure of a heat exchanger is a potential causes of overpressure when the low pressure side of a shell and tube exchanger could be overpressured in the event of a tube failure that would allow the fluid from the high pressure side to enter the low pressure side. The secondary effects of tube rupture upon equipment connected to the low pressure side should also be evaluated, since the high pressure fluid could ultimately arrive at other equipment with relatively low design pressures. The following criteria are used to determine the applicability of the tube rupture causes of overpressure. The tube rupture contingency applies whenever either of the two following criteria hold:
a. The MAWP of the high pressure side of the exchanger is greater than 1.5 times the MAWP of the low pressure side.
b. The high side design pressure exceeds 1,000 psi.
If the MAWP of the high pressure side is less than 1.5 times that of the low pressure side, then the low pressure side generally will have been hydrostatically tested at a pressure at least that of the high pressure side fluid. It is therefore assumed that the high pressure side fluid will not be able to produce catastrophic damage when it enters the low pressure side. If the high pressure side is pressure-relieved below its MAWP, this relief device set pressure is often used in place of the high-side MAWP when determining the high to low side pressure ratio in criterion 1 above.
API Recommended Practice 521 notes that for double-pipe heat exchangers in which both the shell and the “tube” are constructed of standard schedule pipe, internal failure generally need not be considered a credible causes of overpressure. This is because the inner pipe is no more likely to rupture than any other pipe in the system. For “double-pipe” heat exchangers in which the inner conduit is actually fabricated of gauge tubing, API recommends the experienced application of engineering judgment in evaluating the credibility of internal failure as a cause of overpressure.
In order to evaluate the required relief flow rate, the assumption is normally made that the exchanger’s internal failure is a complete break across a single tube. The flow rate is then calculated for the flow from each side of the ruptured tube. This will involve the calculation of a liquid, gas, or flashing liquid discharging from the “high side” operating pressure to the “low side” relief pressure. Furthermore, consideration should be given to the possibility of vaporization of either fluid brought about by the improved heat transfer when the fluids come into direct contact. Even if liquid on the low pressure side does not evaporate, it is likely to be carried along with high-pressure vapor flowing through the rupture, creating a two-phase relief contingency. In this case, the flow of gas or vapor through the rupture would be single phase, but that the flow through the relief device would two-phase.
In certain instances where a very high pressure fluid enters the low pressure side of an exchanger, the rate at which pressure builds up in the exchanger may be faster than a pressure relief valve can open. In these cases, the installation of a rupture disk, which responds more rapidly to a pressure stimulus than does a typical pressure relief valve, is often preferred.