Inherent safety

In the chemical and process industries, a process has inherent safety if it has a low level of danger even if things go wrong. Inherent safety contrasts with other processes where a high degree of hazard is controlled by protective systems. As perfect safety cannot be achieved, common practice is to talk about inherently safer design. “An inherently safer design is one that avoids hazards instead of controlling them, particularly by reducing the amount of hazardous material and the number of hazardous operations in the plant.”

The concept of reducing rather than controlling hazards stems from British chemical engineer Trevor Kletz in a 1978 article entitled “What You Don’t Have, Can’t Leak” on lessons from the Flixborough disaster, and the name ‘inherent safety’ from a book which was an expanded version of the article. A greatly revised and retitled 1991 version mentioned the techniques which are generally quoted. (Kletz originally used the term intrinsically safe in 1978, but as this had already been used for the special case of electronic equipment in potentially flammable atmospheres, only the term inherent was adopted. Intrinsic safety may be considered a special subset of inherent safety.) In 2010 the American Institute of Chemical Engineers published its own definition of IST.

The terminology of inherent safety has developed since 1991, with some slightly different words but the same intentions as Kletz. The four main methods for achieving inherently safer design are:

Two further principles are used by some:

In terms of making plants more user-friendly Kletz added the following:

Inherent safety has been recognised as a desirable principle by a number of national authorities, including the US Nuclear Regulatory Commission and the UK Health and Safety Executive (HSE). In assessing COMAH (Control of Major Accident Hazards Regulations) sites the HSE states “Major accident hazards should be avoided or reduced at source through the application of principles of inherent safety”. The European Commission in its Guidance Document on the Seveso II Directive states “Hazards should be possibly avoided or reduced at source through the application of inherently safe practices.” In California, Contra Costa County requires chemical plants and petroleum refineries to implement inherent safety reviews and make changes based on these reviews. After a 2008 methyl isocyanate explosion at the Bayer CropScience chemical production plant in Institute, West Virginia, the US Chemical Safety Board commissioned a study by the National Academy of Sciences (NAS) how the concept of “Inherent Safety” could be applied, published in a report and video in 2012.

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