Emergency Relief Sizing

A good understanding of the thermal safety of all materials is imperative when storing or producing materials by chemical reaction on a large scale. In case of incomplete knowledge on thermal hazards, as the history has shown, deviation from normal plant operation parameters could lead to dangerous consequences. Therefore, the correct design (including set pressure, sizing, routing, dump tank) of pressure relief devices (e.g. pressure relief valve and / or rupture disc) is of great importance to protect chemical reactors or storage tanks against overpressure. Often, pressure relief devices are the last preventive layer of protection. The protection principle is simple: Limit the vessel pressure below the failure point with emergency relief venting.

Pressure build-up in a system may occur because of a process interruption, mischarge, instrument failure, external fire or inadequate cooling. These scenarios can result in both vaporization due to exothermic runaway reactions and generation of gasses, which pose a serious threat to the safety of your plant, systems and people. To be resilient against potential vulnerabilities and threats, it is important to have a properly designed Emergency Relief System (ERS), and to ensure your process has robust passive safety.

In cases of a runaway reaction, abrupt increases of temperature and pressure can occur. The key parameters in relief vent sizing are the reaction rates at relief conditions, specifically the rate of temperature rise (dT/dt) and the rate of pressure rise (dP/dt).  Safe process design requires knowledge of these adiabatic chemical reaction rates, and also an understanding of the system character (whether gassy or vapor pressure driven, foamy or not foamy). These details should be obtained with an Adiabatic Calorimeter having low thermal inertia.

At Sigma-HSE, we help our customers in carrying out adiabatic calorimetry testing and in identifying the worst-case scenario required for vent sizing based on PHA & calorimetry results. Using the results, we perform Emergency Relief Designing including establishment of set pressure, RD/ PSV sizing, Discharge piping routing and Dump Tank sizing.

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