Designing Emergency Relief Systems for Runaway Reactions

This guidance applies to reactive systems subject to process- and fire-induced runaway reactions, tanks that store reactive materials, and two-phase flow of reactive materials.

Do you suffer from the ERS design ostrich syndrome? Most companies are well-equipped to perform emergency relief system (ERS) design for single-phase flow and non-reactive systems. Existing standards and recommended engineering practices developed by organizations such as the ASME, NFPA, API, and AIChE's CCPS enable a competent engineer with proper training to perform such calculations with a high degree of confidence and accuracy.

Reactive systems, however, are more complex, and may be susceptible to fire-induced and process-induced runaway reactions. The reactive characteristics of the material should be well understood by the ERS designer, including the material's potential to react with itself, decompose, rearrange or react with any contaminants present, such as water, air, rust, etc. All reactions that could occur should be identified and the kinetics of these reactions determined, either by experiment or through trusted literature sources. And, the characteristics of the vessel contents must be known so that two-phase flow methods may be utilized as appropriate.

The possibility for runaway reactions should be considered for all reactive chemicals. Keep in mind that while a material may not be reactive at normal operating conditions, it might be at relief conditions, depending on the setpoint of the relief device, the nature of the vessel contents (e.g., foamy vs. non-foamy), contamination, composition and flow regime.

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