Wednesday, June 24, 2026

With the tube failure overpressure scenario deemed applicable and decision as to the geometry of the break made, an appropriate required relief rate calculation model can be chosen.¹

Steady-state calculation methodology. These calculation models require the determination of the pressure on the upstream side of the break, P₁. Typically, this is taken as the maximum possible operating pressure on the high-pressure side of the exchanger. If a maximum possible operating pressure cannot be established, then the design pressure is a convenient basis. The calculation models also require the determination of the pressure on the downstream side of the break, P₂ (normally the pressure at maximum allowable accumulation of the protected equipment). Depending on the geometry of the break, the flow is then computed for the given fluid using the established single- and two-phase flow-calculation methods for the various types of fluids and openings, such as Orifice flow.

If the guillotine break is selected for design, the most conservative approach is to treat the broken tube ends as orifices. The flow through the break is then calculated as the flow through two orifices of the same diameter as the tube (see ISO 23251:2008 §5.19.3²). A lower but still conservative value results if the break is assumed to occur near a tubesheet. The more-realistic basis is then the flow through one short tube (modeled using either the orifice flow or the pipe flow calculations), plus the flow through the long end of the broken tube (modeled using the pipe flow calculations).

Additional considerations. Although the flow rates are usually determined from steady-state methods such as those described above, one case requires an unsteady-state model³). With a high-pressure gas or flashing liquid on the tube side and a low-pressure incompressible liquid on the shell side (e.g., cooling water), a tube can split open in about a millisecond. Gas flowing into the shell will pressurize the shell suddenly and force liquid into the rupture disk and associated piping. The resultant inertial acceleration of the liquid plays an important role in the process and involves unsteady-state phenomena that require a computer simulation; the consultation of a fluid dynamics specialist may be beneficial (see ISO 23251:2008 §5.19.3², Simpson³, Sumaria et al.⁴, Fowler et al.⁵, and the EI Guidelines on Tube Failure⁶). This is the shock-effect scenario noted previously in Structural failure / heat exchanger tube rupture.

The situation is particularly worrisome when the tube side of a heat exchanger contains a gas or lowly subcooled liquid above 1,000 psig and the shell side contains a low-pressure incompressible liquid; however, depending on the mechanical integrity of the heat exchanger, a sudden massive failure may not be credible even in this case. Otherwise, the only recourse may well be a new unit with higher shell-side design pressure.

One should consider the impact of a tube failure on piping and other components tied into the low-pressure side of the heat exchanger. Often that equipment will experience pressures as high as the low-pressure side of the heat exchanger, although an engineering analysis may show the low-pressure system has the capacity to handle the relief without overpressure. Even in cases where the low-pressure-side design pressure has been set equal to that of the high-pressure-side or the low-pressure system has sufficient capacity to prevent overpressure, tube failure could place such equipment at risk. Provisions should be made to prevent backflow of hazardous process fluids into utility distribution systems. A risk assessment can be useful in determining whether credit for flow out of the exchanger on the low-pressure side should not be taken; if such credit is not taken, the relief system is designed for the entire flow from the high-pressure side.

Blog series information. This blog is part of a series on the proposed updates to the CCPS Guidelines 2nd edition §3.3 Venting Requirements for Nonreacting Cases that were removed during final editing. See the general CCPS Guidelines for Pressure Relief and Effluent Handling 2nd Edition review for more information.

 

---

[1] AIChE Center for Chemical Process Safety. “CCPS Guidelines for Pressure Relief and Effluent Handling Systems”. 2nd Edition, 2017; New Jersey: John Wiley & Sons, Inc.

[2] ANSI/API Standard 521 / ISO 23251 (Identical), Petroleum and natural gas industries — Pressure-relieving and depressuring systems. 5th Edition, January 2007 (incl. Errata June 2007 and Addendum May 2008). American Petroleum Institute, Washington, DC.

[3] Simpson LL. “Tubing Rupture in Liquid-Filled Exchangers.” AIChE Loss Prevention Symposium, CEP Loss Prevention, 6, 1972, 92–98.

[4] Sumaria VH, Rovnak JA, Heitner I, Herbert RJ. “Model to Predict Transient Consequences of a Heat Exchanger Tube Rupture.” Proceedings — Refining Department, Vol. 55, American Petroleum Institute, Washington, DC, 1976, 631–654.

[5] Fowler DW, Herndon TR, Wahrmund RC. “An Analysis of Potential Overpressure of Heat Exchanger Shell Due to a Ruptured Tube.” ASME Petroleum Division Conference, September 22–25, 1968.

[6] Energy Institute (Great Britain). Guidelines for the Safe Design and Operation of Shell and Tube Heat Exchangers to Withstand the Impact of Tube Failure. London, Energy Institute, 2000 (updated 2015).

We used Anthropic Claude [Opus 4.8] to assist with editing and converting a Microsoft Word manuscript into WordPress formatting.