Inherent Safety Trade-offs

Published on 22 Apr 2019 by Arun Patel

See Consideration for Inherently Safer Process Options for related information to Inherent Safety Trade-offs.

Chemical processes usually have many potential hazards, and a change which reduces one hazard may create a new one or increase the magnitude of a different existing hazard. It is essential that the process designer retain a broad overview of the process when considering alternatives that he/she remains aware of all hazards associated with each process option, and that appropriate tools are applied to choose the overall best option.

In many cases, the inherent safety advantages of one process are clear when compared with alternatives. One or more hazards may be significantly reduced, while others are unaffected or only marginally increased. For example, aqueous latex paints are clearly inherently safer than solvent based paints, although there are applications where the increased performance of solvent based paints justifies their use, with the appropriate layers of protection.

Unfortunately, many times it is not clear which of several process alternatives is inherently safer. Because nearly all chemical processes have a number of hazards associated with them, an alternative which reduces one hazard may increase a different hazard. For example, process A uses flammable materials of low toxicity; process B uses noncombustible materials, which are volatile and moderately toxic, and process C uses noncombustible and nontoxic materials but operates at high pressure. Which process is inherently safer? The answer to this question will depend on the specific details of the process options. In addressing these questions, it is essential that all hazards be identified and understood, including:

  • Acute toxicity
  • Chronic toxicity
  • Flammability
  • Reactivity
  • Instability
  • Extreme conditions (temperature or pressure)
  • Environmental hazards, including
    • Air pollution
    • Water pollution
    • Groundwater contamination
    • Waste disposal

The hazards associated with normal facility operations, such as normal stack emissions and fugitive emissions, as well as those resulting from specific incidents such as spills, leaks, fires and explosions should be considered.

It is also necessary to consider business and economic factors in making a process selection. These include:

  • Capital investment
  • Product quality
  • Total manufacturing costs
  • Operability of the facility
  • Demolition and future clean-up and disposal cost

Design strategies which result in an inherently safer design may also tend to improve process economics. For example, minimizing the size of equipment or simplifying a process by eliminating equipment will usually reduce capital investment and reduce operating costs. However, overall process economics are very complex and are impacted by many factors, and it may not always be true that an inherently safer process is also economically more attractive.

An inherently safer process offers greater safety potential, often at a lower cost. However, selection of an inherently safer technology does not guarantee that the actual implementation of that technology will result in a safer operation than an alternate process which is inherently safer. The traditional strategy of providing layers of protection for an inherently more hazardous process can be quite effective, although the expenditure of resources to install and maintain the layers of protection may be very large. In some cases the benefits of the inherently more hazardous technology will be sufficient to justify the costs needed to provide the layers of protection required to reduce the risk to a tolerable level. As an example, the inherent safety characteristics of air and automobile transportation are inherently safer for reasons such as:

  • The automobile, on the ground, will coast to a stop in case of engine failure, while the airplane will rapidly descend and may not be able to land safely.
  • The automobile travels at a lower speed.
  • The automobile contains a smaller inventory of passengers.
  • The control of an automobile is simpler (in two dimensions) compared to the airplane, which must be controlled in three dimensions.

However, the benefits of air transportation, primarily speed, make it an attractive alternative for longer trips. These benefits have justified the expenditure of large amounts of money for providing extensive layers of protection to overcome the inherent hazards of air travel. The result is that air travel, while inherently more hazardous, is in fact safer than automobile travel for long trips. Similar situations can be expected to occur in the energy industry in general.

However, even when we determine that the benefits of an inherently less safe technology justify its use, we should always continue to look for inherently safer alternatives. Technology continues to evolve and advance, and inherently safer alternatives which are not economically attractive today may be very attractive in the future. The development of new inherently safer technology offers the promise of more reliably and economically meeting process safety goals.

Below are some examples of inherent safety trade-offs:

  • Chlorofluorocarbon (CFC) refrigerants are inherently safer with respect to fire, explosion, and acute toxic hazards when compared to alternative refrigerants such as ammonia, propane, and sulfur dioxide. However, they are believed to cause long term environmental damage because of stratospheric ozone depletion.
  • Supercritical processing may use relatively nonhazardous materials such as water or carbon dioxide as reaction and extraction solvents, but supercritical processing requires high temperatures and pressures.
  • A solvent used in an exothermic reaction is nonvolatile, and moderately toxic. An alternative solvent is less toxic, but also has a much lower boiling point. There is a trade-off between toxic hazards and the potential for tempering the exotherm, but also generating pressure from boiling solvent in case of a runaway reaction.

See Inherent Safety Issues for related information to Inherent Safety Trade-offs.

Tags: inherent safe design safe process

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