Three Hazop Questions:

"What could go wrong?"

"How would we know it?"

"What could we do about it?"


Process Safety Management, within which the Hazard and Operability (HAZOP) discipline is a key component, has been unquestionably successful in reducing the incidence and mitigating the consequences of major accidents in all industries dealing with toxic, reactive, flammable and explosive substances. There has not been quite another Flixborough much less a Bhopal type incident since the widespread advent of these procedures. This means protecting the communities adjacent to such facilities as well as the workers within them.

For the current practitioner, there is an emphasis on this site to the founding purposes and the excellence observed in the initial work. This seems significant now when there have been fewer catastrophes on the front pages of the newspapers. And especially now with a new generation seeking to ensure safe operations of what are by their component nature, although providing essential goods to the societies we live in, potentially dangerous facilities.

For the many others in far-flung spheres of operation who are seeking to mitigate risks, especially against what Robert K. Merton {1}, {2} calls "unintended" or "unanticipated consequences", please consider the general principals applied so successfully in the Process Industries. Here, (unfortunately) real life and death have been on the line.


Clicking on this cropped image, will open the originating link in a new window

Flixborough 1974-2

Accident summary

"At about 16:53 hours on Saturday 1 June 1974 the Nypro (UK) site at Flixborough was severely damaged by a large explosion. Twenty-eight workers were killed and a further 36 suffered injuries. It is recognized that the number of casualties would have been more if the incident had occurred on a weekday, as the main office block was not occupied. Offsite consequences resulted in fifty-three reported injuries. Property in the surrounding area was damaged to a varying degree.

"Prior to the explosion, on 27 March 1974, it was discovered that a vertical crack in reactor No.5 was leaking cyclohexane. The plant was subsequently shutdown for an investigation. The investigation that followed identified a serious problem with the reactor and the decision was taken to remove it and install a bypass assembly to connect reactors No.4 and No.6 so that the plant could continue production.

"During the late afternoon on 1 June 1974 a 20 inch bypass system ruptured, which may have been caused by a fire on a nearby 8 inch pipe. This resulted in the escape of a large quantity of cyclohexane. The cyclohexane formed a flammable mixture and subsequently found a source of ignition. At about 16:53 hours there was a massive vapour cloud explosion which caused extensive damage and started numerous fires on the site.

"Eighteen fatalities occurred in the control room as a result of the windows shattering and the collapse of the roof. No one escaped from the control room. The fires burned for several days and after ten days those that still raged were hampering the rescue work." Citation from the HSE: {3}

"Its shock waves echoed all around Britain as neighbours of similar or older plants deluged politicians with fears about nearby installations. Within days, the government decided that a Public Inquiry was necessary to allay public concern." Citation from The Chemical Engineer (April 2005): {4}

recollections Personal recollections from some of those impacted by the Flixborough accident are collected here. "No claim is made for the veracity of these notes. They represent half-remembered, apocryphal experiences... because the sadness and the shock, if nothing else, were real and true."

Regulatory Implications

The Health and Safety at Work Act, 1974

"Great Britain has a tradition of health and safety regulation going back over 150 years. The recent system came into being in 1974 when the HSW Act set up new institutions and provided for the progressive revision and replacement of all health and safety law then existing." {5}

HSC Logo "A body of up to ten people... One of the present members.. represent(s) the public interest. HSC's primary function is... the health safety and welfare of people at work, and the public... including proposing new laws and standards, conducting research, providing information and advice, and controlling... dangerous substances." {5}
HSE Logo "Three people appointed by the Commission (and local government)... The executive's staff, approximately 4000, include inspectors, policy advisors, technologists and scientific and medical experts - collectively known as HSE." {5}

"The Health and Safety Commission met for the first time on October 1, 1974. HSC's annual report for 1977/78 states: 'Our overriding concern is... to stimulate awareness of the risks and encourage the joint participation of workers and management to eliminate them.' The new Act, which largely reflected recommendations of the 1972 Robens Report, introduced a broad goal-setting, non-prescriptive model, based on the view that 'those who create risk are best placed to manage it.'" {6}

Follow On Regulatory Implications

As a result of the Europen Union's "Seveso" Directive of 1982 (more on that, below) the HSC implemented the Control of Industrial Major Accident Hazards (CIMAH) legislation for the UK in 1984. And again, in 1999, the HSC made the regulation even more robust with Control of Major Accident Hazards (COMAH).

Tech Stuff

Comments on the facts of this case.

Flammable Logo with Cyclohexane Molecule First of all- cyclohexane {9}. While at Flixborough this molecule was one of the feed components for the production of nylon, the reader may more easily relate as it is also a component in gasoline - molecules selected because they ignite with air easily to produce lots of energy. With six carbon atoms, a boiling point of 178F, a low flash (-4F) and a wide flammable/explosive range (1.3% - 8.4%); it is smack in the middle of the 500 molecules we typically find in gasoline (Carbons: 3-12; Boiling Points: 86F - 428F) {10}. Potentially dangerous stuff!
40 Tonnes Storage Second of all - the quantities of it. Only 6% was converted with each pass - Trevor Kletz has likened this to a flight from London to New York where only 6% of the passengers disembark at each leg. That meant that 40 tonnes (88,000 pounds) was available to feed the fires... anecdotal evidence from some insurance sites suggests that 10,000 pounds of highly reactive/ flammable/ toxic material is generally found in incidents with a major impact on the community.

So...regardless of whether the initiating event was the rupture of the 20" line, built on staging without any engineering - a highly unusual, third-party-responsible, unlikely to be replicated event, or whether it was simply a loose flange on an 8" line, likely to occur anywhere... it was inherently unsafe to have so much of such highly flammable material without substantially more stringent safeguards in design, operation, maintenance, change control, start-up, human factors and overall management oversight.


While Flixborough involved the smaller company Nypro, not ICI, it may be noted that ICI {7}, the UK's largest and, at the time, one of the world's leading chemical companies, could easily have been considered worthy of the government's faith that 'those who create risk are best placed to manage it'. The basis for the HAZOP methodology was a progression from ICI's 1963/64 "critical examination" technique. "A Guide to Hazard and Operability Studies" was published by ICI and the Chemical Industries Association in 1977. The first HAZOP textbook would be written by an ICI employee, Trevor Kletz (also a participant in the Flixborough review team). {8} {11} The rub might be the many smaller, less sophisticated companies, presumptively less attentive towards safety who might therefore enjoy a hidden competetive edge at the expense of their workers and the public: a theme to be revisted herewith.

The following references will open in a new window:

{1} Merton's 1936 paper on "Unanticipated Consequences" Robert K. Merton (Meyer R. Schkolnick) was a brilliant scholar {A} and a wonderful mentor {B} to many during his thirty-eight years at Columbia University {C} in New York. He is also the father of Robert C. Merton {D}, {E} the nobel prize-winning force behind, among others, Black–Scholes derivatives pricing.
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{2} Wikipedia article on "Unintended Consequences" This Wikipedia article, as of July 2006, succinctly extracted from the Merton monograph, {1} above, five causes of unanticipated consequences (of course, given the nature of Wikipedia, this particular content may or may not reside still at this site):
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  1. Ignorance (It is impossible to anticipate everything).
  2. Error (Incomplete analysis of the problem, or following habits that worked in the past but may not apply to the current situation).
  3. Immediate interest which may override long-term interests... Common!
  4. Basic values may require or prohibit certain actions, even if the long-term result might be (un)favorable.
  5. Self-defeating prophecy ...more commonly "self fulfilling prohecy"... (Fear of some consequence drives people to find solutions, which may or may not have the desired overall effect).

{3} The UK Health and Safety Executive (HSE) Safety Report for "Flixborough (Nypro UK) Explosion 1st June 1974" It appears significant that the HSE acknowledges the 8" line as a potential initiating event as opposed to the generally acknowledged assessment that buckling of the 20" temporary line initiated the explosion. Also, the HSE site provides the following guidance within the heading of "Failings in Technical Measures": {A} Plant Modification / Change Procedures (HAZOP); {B}Design Codes - Pipework; {C} Maintenance Procedures (Recommissioning); {D} Plant Layout; {E} Control Room Design; {F} Operating Procedures; and {G} Inerting.
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{4} Flixborough Revisited by Jon Cox, with an emphasis on the 8" line rupture as the initiating event. Lot's of work done, here...and also here and here
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{5} HSEBooks: "The Health and Safety System in Great Britain" third edition: 2002 "This guide describes a health and safety structure which is integrated (across industry sectors), based on tripartism (the co-operations of Government and the social partners), and societal involvement (via consultation and engagement), and is aimed at delivering a proportionate, targeted and risk-based approach."
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{6} "HSE: 30 Years on and Looking Forward" c2004. "In some ways, the workforce of 2004 is unrecognisable from that of 30 years ago. Yet, the fundamental aspirations laid down in 1974 remain equally valid today"
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{7} "A very brief historical view of ICI , from its founding in 1926 from four chemical firms to become the UK's major chemical company". Although it is no longer the company it once was {a} it is still one of the top Chemical Companies in the world {b} {c}
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{8} "An overiew of Hazops from the , from the Department of Production and Quality Engineering: Norwegian University of Science and Technology, October 2005.
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{9} Safety (MSDS) data for cyclohexane listing it as "highly flammable" as well as reciting its physical properties. There is a great site with a 3D visualization here (deleted... may be infected with the FakeAlert-D virus)
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{10} Gasoline FAQ is an unreviewed source of the major comonents of gasoline.
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{11} Trevor Kletz's HAZOP and HAZAN textbook fourth edition.
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Created on ... July 21, 2006