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 ofhazardous operations in the plant.” [http://www.vtt.fi/inf/pdf/publications/1999/P384.pdf] 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]

Origins

The concept of reducing rather than controlling hazards comes from Trevor Kletz in an article entitled “What You Don’t Have, Can’t Leak” on lessons from the Flixborough DisasterKletz, T.A., (1978) "Chemistry and Industry" pp, 287–292 “What You Don’t Have, Can’t Leak”] ,and the name ‘inherent safety’ from a book which was an expanded version of the articleKletz, T.A., (1984) "Cheaper, Safer Plants or Wealth and Safety at Work –Notes on Inherently Safer and Simpler Plants" IChemE Rugby, UK] . A greatly revised and retitled 1991 versionKletz, T. A., (1991) "Plant Design for Safety – A User-Friendly Approach", Hemisphere, New York] gave the techniques which are generally quoted.

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: 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]
*Minimize: [Kletz originally used the term Intensification, which is understood by chemical engineers to involve smaller equipment with the same product throughput] Reducing the amount of hazardous material present at any one time
*Substitute: Replacing one material with another of less hazard, e.g. cleaning with water and detergent rather than a flammable solvent
*Moderate: [Kletz originally used the word Attenuation] 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.

2 further principles are used by some:
*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:
*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 [ Federal Register: May 9, 2008 (Volume 73, Number 91) 10 CFR Part 50 "Regulation of Nuclear Power Plants; Draft Statement of Policy"] 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”. [ [http://www.hse.gov.uk/comah/sram/s2-7.pdf HSE "The Safety Report Assessment Manual April 2008"] ] 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.” [ [http://mahbsrv.jrc.it/GuidanceDocs-SafetyReport.html] 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] In the USA, Contra Costa County requires chemical plants and petroleum refineries to implement inherent safety reviews and make changes based on these reviews. [ [http://aiche.confex.com/aiche/s07/preliminaryprogram/abstract_80862.htm Sawyer, R., et al (2007) Regulating Inherent Safety] (conference abstract)]

Quantification

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

Notes and References

Links and Further Reading

*Kletz, Trevor (1998) "Process Plants: A Handbook for Inherently Safer Design" CRC ISBN: 1560326190
*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 9780816907038
* [http://www.engr.ku.edu/~kuktl/lecture/cpe613/Inherent1.ppt Howat, C. S. (2002) "Introduction to Inherently Safer Chemical Processes"]
* [http://www.hse.gov.uk/research/othpdf/500-599/oth521.pdf Mansfield, D., Poulter, L., & Kletz, T., (1996) "Improving Inherent Safety"] HMSO ISBN 0-71716-1307-0
* [http://psc.che.tamu.edu/wp-content/uploads/whitepaper_inherentsafety1.pdf Mary Kay O’Connor Process Safety Center (2002) "Challenges in Implementing Inherent Safety Principles in New and Existing Chemical Processes"]
* [http://repository.tamu.edu/bitstream/handle/1969.1/1277/etd-tamu-2004B-CHEN-Gentile-2.pdf?sequence=1 M. Gentile (2004) "Development of a Hierarchical Fuzzy Model for the Evaluation of Inherent Safety"]

ee also

*Intrinsic safety (occasionally confused terminology)
*Passively safe
*Fail-safe
*Safety engineering