(19c) I’ve Identified Overpressure Caused by Tube Failure and Subsequent Chemical Reaction as a Credible Scenario during My PHA, Now What?

Identifying and then mitigating potential sources of overpressure is a crucial aspect of a robust safety program. API 521, “Guide for Pressure-Relieving and Depressuring Systems,” provides a convenient checklist defining common causes of overpressure for consideration. Potential scenarios include closed outlets, overfilling, exterior fire, chemical reaction, and heat transfer equipment failure. When focusing on the common scenario of heat transfer equipment failure, API 521 and other standards provide some guidance as to things to consider when assessing the consequences and mitigation strategies such as type and extent of internal failure, selection of a relieving device that reacts fast enough to prevent overpressurization, and selection of the optimal relief device location. When defining the credible scenario, API 521 recognizes that the extent of internal failure can vary from a pinhole leak to a complete tube rupture, and therefore in practice, a sharp break (or double-ended guillotine tube rupture) is often a conservative first point of consideration. Assessing the required relieving rate to protect the vessel from this scenario is further complicated when the inadvertent mixing of the two fluids results in a chemical reaction. The relief device on the low-pressure side will now need to relieve the overpressure from the mixing of the high-pressure fluid in addition to vapors and/or gases generated from the reaction. This paper provides an example of a double guillotine tube rupture where a water-reactive chlorine containing material is inadvertently mixed with cooling water. Guidance is provided on calculating the equivalent addition rate and resulting mixture composition following this scenario, as well as how to simulate the scenario using adiabatic calorimetry. The presented experimental temperature and pressure generation rates are then used to design an emergency relief system. Additional experimental studies are presented with their impact on the relief requirements considering a double guillotine tube rupture if the system was altered to induce a smaller pressure differential across the two fluid sides, and a pinhole leak with an equivalent diameter of 0.25”, both resulting in different leak rates and final sample compositions. Finally, brief commentary on other considerations when designing the pressure relief system is included.