PT Notes
Chemical Reactivity and HAZOP Studies
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Chemical reactivity incidents are sudden events involving an uncontrolled chemical reaction with significant increases in temperature, pressure, and / or gas evolution that can cause serious harm to people, property, or the environment. Chemical reactivity hazards are present in many industrial processes. Unfortunately, chemical reactivity incidents occur regularly in the process industries, often with catastrophic consequences.
The identification of reactivity hazard scenarios is more challenging than for toxics, flammables and explosives because reactivity hazards are not as well recognized or understood and hazard scenarios involving them are more involved and harder to identify. Many accidents have occurred as a result of inadequate understanding of or protection against reactive chemicals. Thus, it is important that they be addressed properly.
The hazard and operability (HAZOP) study is the most commonly used process hazard analysis method. Unfortunately, the nature of HAZOP studies poses challenges in identifying a complete set of reactivity scenarios. The HAZOP method focuses on investigating deviations from design intent, such as “High Pressure” in a vessel for which the pressure should not exceed a certain value. The goal in a HAZOP study is to identify all aspects of design intent for which deviations may result in scenarios within the scope and objectives of the study.
There are three types of chemical reactivity hazards that must be considered:
- Chemical reactions subject to runaway
- Chemicals that are unstable
- Incompatible materials:
- Inadvertent mixing of chemicals
- Reaction of process chemicals with process materials
The consideration of reactivity in regular HAZOP studies tends to focus on runaway reactions. They can be addressed directly by using the general parameter, “Reaction”, with guide words to generate deviations such as “No Reaction”, “More Reaction” (Runaway), “Less Reaction”, “As Well As Reaction” (Side reactions), “Part Of Reaction” (Incomplete reaction), “Reverse Reaction”, and “Other Than Reaction” (Different reaction). However, some HAZOP practitioners focus on specific parameters and do not address general parameters. In such cases, runway reaction scenarios are more likely to be missed.
An alternative approach for identifying runaway reaction scenarios is to address them when the team considers deviations for specific parameters, such as “High Temperature” or “High Pressure”. In these cases the runaway reaction can be considered either as a cause or a consequence of the deviation. The danger in such a treatment is that the underlying causes of runaway reactions will not be addressed.
It is more difficult to address the other two types of chemical reactivity hazards using HAZOP because it is harder for HAZOP practitioners to visualize deviations to identify them. For unstable chemicals, the design intent is for no reaction to occur. This poses a difficulty in selecting a suitable guide word to combine with “No Reaction”. Use of the guide word, “No”, to produce the deviation, “Unintended Reaction”, creates a double negative which is confusing. The guide word, “Other Than”, could be used with “No Reaction” but the combination seems contrived to HAZOP team members and can cause confusion. The situation is not as clear cut as with deviations such as “No Flow”, “More Flow”, and “Reverse Flow”. Consequently, scenarios involving self-reacting chemicals may be overlooked.
While many self-reactive chemicals need heat to initiate reaction, in some cases, other triggers such as mechanical shock may be sufficient. Heat sensitive materials can be addressed using the deviations “High Temperature” or “More Heat”. However, it is important for the HAZOP team to be aware of those aspects of design intent for a node that impact reactivity. For example, while the need to control the temperature in a reactor to avoid a runway reaction should be obvious, the importance of controlling the temperature in a line carrying a reactive material may be less obvious, although just as important. Other triggers can be addressed using deviations such as “More Vibration” but the HAZOP team needs to be aware of such triggers. Moreover, such deviations are much less common that “No Flow”, “High Pressure”, “Low Level”, etc. and may be overlooked. Furthermore, causes of heating or other triggers may be challenging for HAZOP teams to identify because they may be less familiar with them. For example, heat can be generated from the heat of solution, the heat of adsorption, friction, or mechanical energy from an agitator or other piece of equipment. These issues pose challenges for addressing self-reactive chemicals in HAZOP studies.
For incompatible materials, possibly the best that can be done is to consider the combination of the guide word, “As Well As”, with the parameter, “Composition”, to generate the deviation, “As Well As Composition” which is used to consider the presence of contaminants and can identify the possible combination of two process chemicals in an inadvertent mixing scenario. Chemical incompatibilities can produce adverse consequences but they can be missed by HAZOP teams if the team is not sensitive to chemical reactivity issues because they are not being addressed directly. Thus, inadvertent mixing scenarios may be overlooked. Moreover, it is more of a stretch to use this deviation to identify reactions of process chemicals with process materials.
Study teams must recognize that the identification of the consequences of reactivity scenarios is more complex than for other types of major hazards. While a principal concern with chemical reactivity incidents is loss of containment of hazardous materials owing to reactivity excursions that produce overpressures, other effects also may be important including the generation of toxic gases and other hazardous materials, production of energetic projectiles from damaged process equipment, heat generation, fires, and explosions. Study teams should consider consequences that significantly impact receptors of concern.
The consideration of reactive chemical hazards in HAZOP studies is discussed in greater detail in the article:
Chemical Reactivity and Hazard and Operability (HAZOP) Studies, Loss Prevention Bulletin, Issue 244, August, 2015.
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