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:
- Minimize: 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: 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: Eliminating problems by design 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:
- 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 by design, location or transportation of equipment so that the worst possible condition produces less danger, e.g. gravity will take a leak to a safe place, the use of bunds.
- Avoiding knock-on effects;
- Making incorrect assembly impossible;
- Making status clear;
- Ease of control;
- Software and management procedures.
The opportunity to adopt an inherently safer design is ideal at the research and conceptual design stages; such opportunity decreases and the project cost increases if changes are made during the subsequent design stages. Once a conceptual design is completed, the other safety strategies should be applied along with the inherently safer design concept. However, in this case, the project cost would significantly increase to have the same risk level at the same reliability relative to if ISD was adopted during the conceptual design stage. 
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.
After the Bhopal disaster in 1984, the US state of New Jersey adopted the Toxic Catastrophe Prevention Act(TCPA) from 1985. In 2003 its rules were revised to include inherently safer technologies (IST). In 2005, the New Jersey Domestic Security Preparedness Task Force established a new “Best Practices Standards” program, in which it required chemical facilities to conduct inherently safer technologies (IST) reviews. In 2008, the TCPA program was expanded to require all TCPA facilities to conduct IST reviews on both new and existing processes. The State of New Jersey created its own definition of IST for regulatory purposes and stretched the definition of IST to include passive, active, and procedural controls.
Under Executive Order 13650 the U.S. Environmental Protection Agency (EPA) has been considering a proposal to “nationalize” the New Jersey inherently safer technologies program, inviting comments until end of October 2014. The American Chemistry Council lists disadvantages.
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. However all of these are much more complex than the Dow F & E Index.
Notes and references
-  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
- Kletz, T.A., (1978) Chemistry and Industry pp, 287–292 “What You Don’t Have, Can’t Leak”
- Kletz, T.A., (1984) Cheaper, Safer Plants or Wealth and Safety at Work –Notes on Inherently Safer and Simpler Plants IChemE Rugby, UK
- Kletz, T. A., (1991) Plant Design for Safety – A User-Friendly Approach, Hemisphere, New York
- Center for Chemical Process Safety and the American Institute of Chemical Engineers, Final Report: Definition for Inherently Safer Technology in Production, Transportation, Storage, and Use (July 2010) 1-54.https://www.aiche.org/sites/default/files/docs/embedded-pdf/ist_final_definition_report.pdf[permanent dead link]
- Khan, F. I.; Amoyette, P. R. (2003). "How to make inherent safety practice a reality". Canadian Journal of Chemical Engineering. 81: 2–16. doi:10.1002/cjce.5450810101.
- Kletz originally used the term intensification, which is understood by chemical engineers to involve smaller equipment with the same product throughput
- Kletz originally used the word attenuation
- Park, Sunhwa; Xu,Sheng; Rogers,William; Pasman,Hans; El-Halwagi,Mahmoud. (2020). "Incorporating inherent safety during the conceptual process design stage: A literature review". Journal of Loss Prevention in the Process Industries. 63: 104040. doi:10.1016/j.jlp.2019.104040.
- Federal Register: May 9, 2008 (Volume 73, Number 91) 10 CFR Part 50 Regulation of Nuclear Power Plants; Draft Statement of Policy
- Health and Safety Executive, UK (April 2008). "The Safety Report Assessment Manual" (PDF). p. 4. Archived from the original (PDF) on 2006-11-02. CS1 maint: discouraged parameter (link)
- 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 978-92-828-1451-2. Archived from the original on 2008-05-11. CS1 maint: discouraged parameter (link)
- Sawyer, R.; et al. (2007). "Regulating Inherent Safety (conference abstract)". American Institute of Chemical Engineers.
- Communications Director (11 July 2012). "CSB Releases New Safety Video on Inherently Safer Design and Technology: "Inherently Safer: The Future of Risk Reduction" Examines how Industry Can Eliminate or Reduce Hazards". US Chemical Safety Board. Retrieved 31 October 2014. CS1 maint: discouraged parameter (link)
- 40 N.J.R. 2254(a), May 5, 2008
- Wikisource:Executive Order 13650
- William J. Erny (April 2014). (PDF). The American Chemistry Council https://web.archive.org/web/20140703133023/http://www.americanchemistry.com/Policy/Security/Presidents-Executive-Order-13650/ACC-Written-Comments-on-New-Jerseys-Inherent-Safety-Technology-Assessment-Program.pdf. Archived from the original on 2014-07-03. Missing or empty
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- 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
- 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
- Center for Chemical Process Safety (2009) Inherently Safer Chemical Processes: A Life Cycle Approach 2nd edn Wiley ISBN 978-0471-77892-9
- 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