Inherent safety

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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. 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]


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.)


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[edit]

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]


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[edit]

Notes and references[edit]

  1. ^ a b [1] Heikkilä, Anna-Mari. Inherent safety in process plant design. An index-based approach. Espoo 1999, Technical Research Centre of Finland, VTT Publications 384. ISBN 951-38-5371-3
  2. ^ Kletz, T.A., (1978) Chemistry and Industry pp, 287–292 “What You Don’t Have, Can’t Leak”
  3. ^ Kletz, T.A., (1984) Cheaper, Safer Plants or Wealth and Safety at Work –Notes on Inherently Safer and Simpler Plants IChemE Rugby, UK
  4. ^ a b Kletz, T. A., (1991) Plant Design for Safety – A User-Friendly Approach, Hemisphere, New York
  5. ^ a b c e.g.Khan, F. I. & Amoyette, P. R., (2003) Canadian Journal of Chemical Engineering vol 81 pp 2-16 How to make inherent safety practice a reality
  6. ^ Kletz originally used the term Intensification, which is understood by chemical engineers to involve smaller equipment with the same product throughput
  7. ^ Kletz originally used the word Attenuation
  8. ^ Federal Register: May 9, 2008 (Volume 73, Number 91) 10 CFR Part 50 Regulation of Nuclear Power Plants; Draft Statement of Policy
  9. ^ HSE The Safety Report Assessment Manual April 2008
  10. ^ [2] Papadakis, G. A., & Amendola, A., (eds) (1997) Guidance on the Preparation of a Safety Report to meet the requirements of Council Directive 96/82/EC (Seveso II) ISBN 92-828-1451-3
  11. ^ Sawyer, R., et al. (2007) Regulating Inherent Safety (conference abstract)
  12. ^ Khan F.I., Husain T. and Abbasi S.A., 2002, Process Safety and Environmental Progress, 79(2): 65-80 Safety Weighted Hazard Index (SWeHI), a new user-friendly tool for swift yet comprehensive hazard identification and safety evaluation in chemical process industries
  13. ^ Gentile, M., Rogers, W. J., Mannan, M. S., (2004) AIChE Journal Vol 4 pp 959-968 Development of an inherent safety index based on fuzzy logic
  14. ^ Abedi, P., Shahriari, M. (2005) Central European Journal of Chemistry Vol 3, no 4, pp 756-779 Inherent safety evaluation in process plants – a comparison of methodologies

Links and Further Reading[edit]