PT Notes
Inherent Safety Review
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.
Process risks often are addressed by adding safety systems to processes. However, such systems are subject to failure, and they increase the complexity of processes perversely making it more difficult to manage process safety. Best practice in addressing process hazards is to apply the hierarchy of controls. Inherent safety is at the top of the hierarchy.
Process risks can be managed using a variety of inherent safety approaches including:
Elimination: removing the hazard altogether, e.g. changing to a different process.
Substitution: replacing a hazardous material with a less hazardous one, e.g. using sodium hypochlorite instead of chlorine.
Abatement: reducing the quantity of hazardous material stored, used, or generated, e.g. reducing the inventory.
Attenuation: modifying a process to operate under less hazardous conditions, e.g. reducing the operating pressure.
Dilution: mixing a hazardous material with a less hazardous material, e.g. handling sulfuric acid rather than oleum.
Isolation: moving further away from sensitive receptors, e.g. relocating a work station.
Inherent safety measures can be categorized as first or second order:
First Order: A measure that eliminates a hazard, e.g. substituting a toxic chemical with an alternative chemical that can serve the same function but is non‑toxic.
Second Order: A measure that reduces the severity of a hazard or the likelihood of a release, without the use of add‑on safety devices, e.g. redesigning a high‑pressure, high‑temperature process to operate at ambient temperatures and pressures.
Of course, it is to be expected that it is easier to implement inherent safety measures for new processes. However, there are also ways to implement them for existing processes., for example:
- Increasing the pressure rating of blinds.
- Imposing design limits on pumps to restrict the amount of overpressure that is possible.
- Using thicker vessels.
- Using refrigerated materials under atmospheric pressure instead of under pressure.
- Using hot water or steam for heating instead of flammable oil.
- Piping in chlorine.
- Generating SO3 instead of using oleum.
- Replacing natural gas with nitrogen for blanketing.
- Replacing a large storage vessel with several smaller ones.
- Moving vaporizers (e.g. chlorine) closer to the liquid source.
The feasibility of adopting inherent safety approaches can be determined by conducting an inherent safety review as an adjunct to process hazard analysis (PHA) studies and revalidations. A useful mnemonic to assist in conducting reviews to help cover key approaches is MSMS or MS Squared for:
Minimize: Use smaller quantities of hazardous materials.
Substitute: Replace a hazardous material with one that is less hazardous.
Moderate: Use less severe operating conditions.
Simplify: Eliminate opportunities for errors by eliminating the need to add safeguards.
Further details on inherent safety can be found in: Process Plants: A Handbook for Inherently Safer Design, 2nd Edition, 2010 Trevor A. Kletz and Paul Amyotte.
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