PT Notes
PHA for Reactive Chemical Hazards
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Reactive chemical incidents are sudden events involving an uncontrolled chemical reaction with significant increases in temperature, pressure, and / or gas evolution. Unfortunately, they occur regularly in the process industries, often with catastrophic consequences. Inadequate understanding of and protection against chemical reactivity hazards have been significant contributors to incidents. Unfortunately, not much attention has been paid to the identification of reactivity hazard scenarios in process hazard analysis (PHA). A Chemical Safety Board study found that 60% of the reactivity incidents studied involved inadequate practices for identifying hazards and conducting PHA studies.
Addressing chemical reactivity hazards is more challenging than for the other major hazards of toxics, flammables and explosives because chemical reactivity hazards are not as well recognized or understood and hazard scenarios involving them are more involved and harder to identify. There are several types of chemical reactivity hazards that must be considered:
- Self-reacting chemicals.
- Runaway reactions.
- Incompatibilities:
- Inadvertent mixing of two or more process chemicals may lead to an unintended chemical reaction.
- Process chemicals may react with materials present in the process such as water, air, materials of construction, lubricating oils, utility fluids, etc.
Potential consequences of reactivity incidents include:
- Over-pressurization and rupture of closed containers and vessels.
- Physical explosions are possible.
- Projectiles can be generated that can cause injuries and property damage.
- Hazardous materials can be dispersed, possibly violently, producing toxic exposures, fires and explosions.
- Generation of gases, possibly toxic, or other hazardous materials.
- Generation of heat causing thermal burns and ignition of combustible materials.
- Fire without the need for an additional ignition source.
- Initiation of other chemical reactions.
Thus, the possible consequences of reactivity incidents are more diverse and complex than those for other types of major hazards. Consequences vary according to the type of chemical reactivity hazards present and the circumstances involved.
In addressing toxic, fire and explosion hazards, PHA studies focus on identifying hazard scenarios resulting from loss of containment and the release of toxic, flammable, or explosive materials. In contrast, uncontrolled chemical reactivity hazards often cause loss of containment producing various adverse effects including toxic exposures, fires and explosions. Thus, scenarios for chemical reactivity hazards are different in nature than those for the other types of major hazards. Scenarios for the hazards of toxicity, flammability and explosivity take the form:
Initiating event → Intermediate events → Loss of containment → Hazardous material release → Effects → Impacts on receptors.
In contrast, scenarios for chemical reactivity hazards take the form:
Initiating event → Reactivity excursion → Intermediate events → Effects → Impacts on receptors.
In the first scenario type, the hazard is realized after containment has failed and receptors are impacted directly by the realized hazard (toxic exposure, fires, explosions); whereas in the second scenario type the hazard is realized as the proximate result of the initiating event which results in effects, including possible loss of containment, that produce impacts on receptors. Thus, chemical reactivity hazards are closely connected to the initiating events for scenarios.
The Hazard and Operability (HAZOP) study is not well suited to the identification of scenarios for chemical reactivity hazards owing to these differences for reactivity scenarios. Also, HAZOP is susceptible to the incomplete consideration of design intent for a process which may cause reactivity scenarios to be missed by typical PHA teams. Approaches that directly identify the initiating events that cause chemical reactivity hazards to be realized, such as major hazards analysis (MHA), are preferred. MHA uses structured brainstorming to identify initiating events directly and is well-suited for identifying reactivity hazard scenarios.
It is possible to address chemical reactivity hazards in a separate PHA study as an adjunct to another study that addresses other types of major hazards. Thus, the HAZOP study method could be used to address toxic, flammable and explosive hazards and MHA could be used for chemical reactivity hazards . Alternatively, all major hazards could be addressed in a single study using a method that treats all major hazards equally well, for example, the MHA method. The advantages of a single study are that hazard scenarios are recorded in a one place and the analysis is more efficient. The advantages of separate studies are that different PHA methods can be used and specialty team members, such as chemists, may need to be present only for the chemical reactivity hazards study.
This topic is discussed in greater detail 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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