Inherent safety

Inherent safety is a concept particularly used in the chemical and process industries. An inherently safe process has a low level of danger even if things go wrong. It is used in contrast to safe systems where a high degree of hazard is controlled by protective systems. It should not be confused with intrinsic safety which is a particular technology for electrical systems in potentially flammable atmospheres. 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.”^[1]

Origins

The concept of reducing rather than controlling hazards comes from Trevor Kletz in a 1978 article entitled “What You Don’t Have, Can’t Leak” on lessons from the Flixborough disaster,^[2] and the name ‘inherent safety’ from a book which was an expanded version of the article.^[3] A greatly revised and retitled 1991 version^[4] gave the techniques which are generally quoted. (Kletz 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, the word inherent was adopted. Intrinsic safety may be considered a special subset of inherent safety.)

Principles

The terminology of inherent safety has developed since 1991, with some slightly different words but the same intentions as Kletz. The 4 main methods for achieving inherently safer design are:^[5]

• Minimize:^[6] Reducing the amount of hazardous material present at any one time, e.g. by using smaller batches.
• Substitute: Replacing one material with another of less hazard, e.g. cleaning with water and detergent rather than a flammable solvent
• Moderate:^[7] Reducing the strength of an effect, e.g. having a cold liquid instead of a gas at high pressure, or using material in a dilute rather than concentrated form
• Simplify: Designing out problems rather than adding additional equipment or features to deal with them. Only fitting options and using complex procedures if they are really necessary.

Two further principles are used by some:^[5]

• Error Tolerance: Equipment and processes can be designed to be capable of withstanding possible faults or deviations from design. A very simple example is making piping and joints capable of withstanding the maximum possible pressure if outlets are closed.
• Limit Effects: Designing and locating equipment so that the worst possible condition gives less danger, e.g. gravity will take a leak to a safe place, the use of bunds.

In terms of making plants more user-friendly Kletz also added the following:^[4]

• Avoiding Knock-on Effects;
• Making Incorrect Assembly Impossible;
• Making Status Clear;
• Ease of Control;
• Software and management procedures.

Official status

Inherent safety has been recognised as a desirable principle by a number of national authorities, including the US Nuclear Regulatory Commission^[8] and the UK Health and Safety Executive (HSE). In assessing COMAH sites the HSE states “Major accident hazards should be avoided or reduced at source through the application of principles of inherent safety”.^[9] 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.”^[10] In the USA, Contra Costa County requires chemical plants and petroleum refineries to implement inherent safety reviews and make changes based on these reviews.^[11]

Quantification

The Dow Fire and Explosion Index is essentially a measure of inherent danger and is the most widely used quantification of inherent safety.^[5] A more specific index of inherently safe design has been proposed by Heikkilä,^[1] and variations of this have been published.^[12][13][14] However all of these are much more complex than the Dow F & E Index.

See also

• Intrinsic safety (occasionally confused terminology)
• Passively safe
• Fail-safe
• Safety engineering
• Generation IV reactor

Notes and references

Links and Further Reading

• Kletz, Trevor (1998) Process Plants: A Handbook for Inherently Safer Design CRC ISBN 1-56032-619-0
• Dow's Fire & Explosion Index Hazard Classification Guide, 7th Edition (1994)American Institute of Chemical Engineers (AIChE) ISBN 0-8169-0623-8
• Bollinger, R. Et al. (1996) Inherently Safer Chemical Processes: A Life Cycle Approach AIChE ISBN 978-0-8169-0703-8
• Howat, C. S. (2002) Introduction to Inherently Safer Chemical Processes
• Mansfield, D., Poulter, L., & Kletz, T., (1996) Improving Inherent Safety HMSO ISBN 0717613070
• Mary Kay O’Connor Process Safety Center (2002) Challenges in Implementing Inherent Safety Principles in New and Existing Chemical Processes
• M. Gentile (2004) Development of a Hierarchical Fuzzy Model for the Evaluation of Inherent Safety
• Safer Design Front Loading Safety in Design