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PT Notes

Pitfalls in Addressing Chemical Reactivity Hazards in PHA

PT Notes is a series of topical technical notes on process safety provided periodically by Primatech for your benefit. Please feel free to provide feedback.

Several pitfalls await process hazard analysis (PHA) teams that are inexperienced in addressing chemical reactivity hazards. Knowledge of possible pitfalls and how to overcome them will help in their avoidance. Various pitfalls are identified here together with ways to address them.

Understanding of chemical kinetics and thermodynamics

The behavior of chemically reacting materials can be complex and highly dependent on the circumstances under which reaction takes place. Consequently, a knowledgeable chemist should be part of the PHA team where complex or unusual chemical reactivity hazards may be present.

Inadequate process design

Inadequate designs, safety systems, and control systems have been identified as a source of reactivity incidents. A useful pre-cursor to a PHA study, is a review of the process by designers with assistance from process chemists to ensure that the process is properly designed to address chemical reactivity hazards before the PHA study is begun. The PHA team can then focus on how deviations from the design intent can produce reactivity hazard scenarios.

Issues with procedures

Both inadequate operating procedures and failure to follow procedures have been identified as a source of reactivity incidents. Procedures may not specify safe operating limits for addressing chemical reactivity hazards. A review of procedures should be conducted prior to embarking on the PHA study to ensure they address chemical reactivity hazards. PHA teams should not be burdened by identifying problems that should have been addressed by others.

Chemical reactivity ratings

The National Fire Protection Association's standard NFPA 704 rates chemical reactivity. However, NFPA ratings address inherent instability only, not reactivity with other chemical substances, except with water, nor chemical behavior under non-ambient conditions. This issue should be addressed with PHA team members when they are briefed on chemical reactivity hazards to ensure they are aware of the broader meaning of chemical reactivity.

Chemicals with low reactivity ratings

Such chemicals can be very reactive under some conditions, for example, in combination with other chemicals, or at temperatures that are reached only as a result of deviations from normal operating conditions. Where such chemical reactivity hazards may be present in a process, the PHA team should be briefed on how they may arise and a chemist should be part of the team.

Completeness of safety data sheets (SDSs)

SDSs for raw materials may not identify all hazards which may be encountered when they are mixed with other chemicals or contact other materials. This issue should be addressed during the chemical reactivity hazards screening for a process and a chemical interaction matrix for the process should be provided to the PHA team.

Sources of chemical reactivity hazards

Chemical reactivity hazards may derive not only from raw materials but also from intermediates, products, and by-products of chemical processes. The personnel performing the chemical reactivity hazards screening for a process must search for such hazards and communicate them to the PHA team.

Development of chemical reactivity hazards

Some chemical reactivity hazards form slowly over time, for example, the development of peroxides in stored chemicals. The personnel performing the chemical reactivity hazards screening for a process must search for such hazards and communicate them to the PHA team.

Combinations of causes

Reactivity incidents may be caused by combinations of causes. Identification of hazard scenarios involving multiple failures is challenging for PHA teams but studies should address them to the extent feasible.

Importance of runaway reactions

A common but incorrect assumption is that chemical reactivity hazards in processes are dominated by runaway reactions in reactor vessels. A study of actual incidents found that only 35% of the incidents were attributable to runaway reactions and reaction vessels accounted for only 25% of the equipment involved. Such reactivity incident data should be included in briefings of PHA teams to ensure they do not focus unduly on the types of reactivity hazard scenarios that they perceive to be the most important when data are available that contradict their perceptions.

More information on addressing chemical reactivity hazards in PHA can be found in the article:

Consider Chemical Reactivity in Process Hazard Analysis, Chemical Engineering Progress, Vol. 111 (1), pages 25 - 31, January 2015.

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