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News Release from: ABB Automation Tech (Instrumentation + Automation) | Subject: Inherent safety
Edited by the Engineeringtalk Editorial
Team on 23 July 2002
Designing safer chemical processes
Establishing a culture of inherent safety can help in meeting society's desire for better industry major accident safety performance, says ABB's safety consultant, Kevin Fitzgerald.
The explosion in September 2001 that killed 30 people and destroyed a fertiliser plant at Toulouse in France has once again demonstrated that industrial operations can impact local populations in the worst possible way As society becomes more concerned about industrial hazards it demands guarantees that such accidents will never happen again
This article was originally published on Engineeringtalk on 17 Jul 2008 at 8.00am (UK)
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Unfortunately, as any safety professional will tell you, unless a hazard can be completely removed, the potential for incidents to happen will always remain.
Process engineers and chemists are routinely faced with scaling-up processes from the laboratory to plant, and ensuring that the risks from the plant to operating personnel and to the general public are acceptable.
"Acceptable" levels of risk, couched in terms of the risk of death to members of the general public, are set down in industry and government guidance (eg United Kingdom Health and Safety Executive Report "Reducing risks, protecting people", 2001).
The factors influencing the scale of the risk are the extent of the hazard (the consequences of a possible incident) and the likelihood (how often it can happen).
Risks have traditionally been reduced by focusing on the likelihood with which accidents can happen, and introducing additional equipment and management procedures to minimise the frequency.
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"For example, if we needed to use 10 tonnes of hazardous material which could potentially react and generate very high pressures in process equipment (and so potentially harm people and the plant), we might use high-reliability equipment and detailed operating procedures to help ensure that the material was not overheated.
We might also fit high-integrity trip and alarm systems to warn of any deviations from normal process conditions, and fit pressure relief systems to reduce the possibility of damage to the reactor, and so on.
Although these measures would all help to reduce the likelihood of an accident, the possibility would however remain", says ABB's safety consultant, Kevin Fitzgerald.
An alternative approach to risk reduction is to focus on reducing the consequences of potential incidents.
This is the fundamental idea behind inherent safety.
Chemical processes can be made safer through the application of inherent safety principles such as substitution, minimisation, moderation and simplification.
Application of these principles means that we try to: substitute hazardous materials with less hazardous ones; minimise the amount of hazardous material in use in a process (the process inventory); moderate process conditions to make the consequences of an accident less severe; and simplify processes and their operation.
Fitzgerald goes on to explain, "For the reactor example given, application of these principles might mean making changes to the process chemistry, reducing the size of the reactor (or quantities of reactants) so that the scale of any incident would be smaller, or designing the system to ensure that there could never be enough heat available to overheat the material".
"It is widely acknowledged that use of these inherent safety principles can make a process and plant cheaper to build and operate, as well as helping to reduce the risks to people and the environment.
Because of the benefits that can be gained from application of inherent safety principles, companies such as Rohm and Haas and Dow have placed particular emphasis on it, and ICI has made consideration of inherent safety a requirement within company management systems.
There is also pressure from industry regulatory authorities to consider Inherent Safety formally in the design process.
In the UK, under the COMAH legislation (the UK's implementation of the "Seveso II" directive), there is a requirement to demonstrate that a hierarchical approach to process design, including inherent safety, has been taken.
In the USA, the Environmental Protection Agency recognises and encourages prevention of incidents through passive or inherently safer process technologies, and legislation reflects this".
For the principles of inherent safety to have greatest impact, they need to be applied as early as possible during the project cycle.
Flexibility is greatest in the early stages of the design process, and changes can be made easily and cheaply.
Once a project team is committed to a design, and investment made, major change becomes much harder.
Consistent and timely application of the ideas has however proved difficult to achieve.
In contrast to HAZOPs (hazard and operability studies), which are generally "owned" and driven by a project manager accountable for delivery of a physical asset, the ownership of and accountability for the early stages of product development can be less clear.
In the absence of clear ownership there is the risk of a business becoming committed to a process which is not as attractive with regard to inherent safety as it might have been if the safety aspects had been actively managed at the outset.
There is therefore a need to build awareness of the principles, particularly amongst development chemists and development engineers.
The common-sense nature of inherent safety can itself make consistent application of the principles difficult.
Toolkits or a standard approach can help companies apply the ideas in a structured and consistent way project after project, and ensure that the benefits achieved are not dependent on the insights of one or two people, but can be delivered by different project teams.
The reasons behind the recent incident at Toulouse are still unclear.
But, the history of the process industries is marked by major accidents which have served to drive process safety development and industry regulation.
In 1974, at Flixborough, near Scunthorpe, a chemical plant released a flammable vapour cloud, which blew up killing 28 people on the site.
In 1976, at Seveso, outside Milan, a mixture of toxic dioxin was released from a reactor creating the most serious environmental disaster in Europe up to that time.
And, at Bhopal, in 1984, the release of a toxic cloud from a plant killed more than 2000 people in the town.
The human consequences of Flixborough, Seveso and Bhopal have been described many times, but there is some inherent safety learning from these incidents.
The accident at Flixborough would have been less severe had the onsite inventory been less (inherent safety principle: minimise).
The Seveso reactor could have been designed so as to make it impossible to initiate the exotherm which led to the formation of dioxins (inherent safety principle: moderate).
The release at Bhopal was of an intermediate material, which need not have been stored in the large quantities which were there, and which need not have been produced at all if another available chemical route had been used (inherent safety principles: eliminate and substitute).
"If we start during the initial research phase of a project, we can build inherent safety in to our plants from the outset and reduce the need for "bolt-on" safety measures to reduce risk to acceptable levels.
Although it can be more challenging, there are also numerous examples of inherent safety being successful applied to existing plant.
Establishing a culture of inherent safety and providing some tools makes it easier to apply the principles consistently, and can help us in meeting society's desire for better industry major accident safety performance", concludes Fitzgerald.
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