Inherent Safety: Substitute
We continue our series of posts on the topic of Safety in Design. The first five posts in the series were:
Inherent Safety, and
The principles of inherent safety vary from organization to organization, but most descriptions of the topic are structured around the following five elements.
Eliminate
Substitute
Minimize
Moderate
Simplify
In this post we consider the second item in the above list: Substitute.
If hazardous materials can be replaced with safer alternatives then the consequences of a release would then be less serious. Typical questions to do with substitution are,
Can less volatile or flammable solvents be used?
Is it possible to replace powders or dusty solids with pellets?
Can aqueous solvents be used in preference to hydrocarbon solvents?
A classic example of substitution concerns the use of hydrogen fluoride (HF) as an alkylation catalyst in oil refineries. HF is a liquid within the process. However, if released it will form a very dangerous vapor cloud (its boiling point at atmospheric pressure is 19.5°C). An alternative alkylation catalyst is sulfuric acid. Although sulfuric acid is also a hazardous material, it cannot cause a catastrophic accident when released in the way that HF can.
Another example is the use a chemical additive as part of a reaction control system. The chemical additive can be supplied either in water solution or in hydrocarbon solvent. The water is non-flammable, whereas the solvent can burn. Therefore, with respect to fire, the first is inherently safe, the second is not
Substitution can also be used to make equipment and piping safer. For example, can changes to piping design remove the number of nozzles, bellows and expansion joints? Is it possible to use a safer material of construction?
There may also be ways of reducing transportation risk through substitution. For example, can barges be used in preference to tank cars, and tank cars in preference to tank trucks?
