Wednesday, June 24, 2026

The structural failure scenario is a special case of excess inflow.¹ The method of estimating the flow through the break depends on the service and the specific type of equipment. After establishing the flow rate through the break, the design relief flow rate is determined from consideration of the consequences of the flow of this fluid to the lower-pressure medium. Fluid from the high-pressure side may initially displace the low-pressure side fluid into the relief system. Mixtures of the two fluids can then flow from the system. The possibility of vaporization due to the pressure change, as well as from any intermixing of the fluids, should be considered. Relief requirements are then based on the most severe stage of the event.

The picture can be further complicated by shock effects from the sudden failure of a high-pressure tube in a low-pressure shell that is liquid-filled, or from rapid phase transition. Rapid phase transition may occur if the liquids are immiscible and if the temperature of the high temperature fluid is in the region of the critical temperature for the low temperature liquid. See ISO 23251:2008 §5.9² for a discussion of this problem; also see Porteous and Reid³ for information on this extreme case of rapid phase transition (sometimes termed “homogeneous nucleation”).

Sudden breaks in the wall of jacketed vessels or inner pipes of double pipe heat exchangers are not typically considered as credible events at pressures below the maximum allowable accumulation pressure (MAAP). Relief provisions should keep the pressure accumulation within allowable limits for all credible events, and thus prevent wall failure. Expert assistance is required if a break is credible due to some special mechanical configuration of the equipment.

Heat exchanger tube failure.. One common type of structural failure that is a basis for overpressure protection is the failure of a boundary (commonly a tube) within a heat exchanger. UG-133(d) of the ASME Code⁴ requires that “Heat exchangers and similar vessels shall be protected with a relieving device of sufficient capacity to avoid overpressure in case of an internal failure.” This Code mandate is usually applied to shell-and-tube heat exchangers, but has left the determination of a credible cause of overpressure and the definition of internal failure to the designer. In the course of providing this overpressure protection, the designer specifies the type and size of the failure (e.g., perforation, crack, or broken tube), the duration of the event, whether the tube-side and/or shell-side fluid vents through the relief device, the choice of a reclosing versus a nonreclosing pressure relief device, and its location.

Although there are no well-defined circumstances where total tube failure can occur, ISO 23251:2008 §5.19.2² states that

…loss of containment of the low-pressure side to atmosphere is unlikely to result from a tube rupture where the pressure in the low-pressure side (including upstream and downstream systems) during the tube rupture does not exceed the corrected hydrotest pressure (see 3.21 and 4.3.2). The user may choose a pressure other than the corrected hydrotest pressure, given that a proper detailed mechanical analysis is performed showing that a loss of containment is unlikely. The use of maximum possible system pressure instead of design pressure may be considered as the design pressure of the high-pressure side on a case-by-case basis…

Note that this was previously known as the “two-thirds rule” when the ASME Code required that the hydrotest pressure be 1.5 times the design pressure. The wording in the standard was changed to reference the hydrotest pressure as a result of the ASME Code adopting higher allowable stresses for many construction materials and reducing the required hydrotest pressure for these cases to 1.3 times the design pressure.

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.

 

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[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] Porteous WM, Reid RC. “Light Hydrocarbon Vapor Explosions.” Chemical Engineering Progress, 72(5), 1976, 83–89.

[4] ASME BPVC. Boiler and Pressure Vessel Code, Section VIII, Division 1, Pressure Vessels. American Society of Mechanical Engineers. 2007, New York, NY. www.asme.org.

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