Inherent safety is usually divided into different elements, in rough order of priority. There is no standardized list, but the following five elements are representative. The preferred order in which they should be addressed is shown below.
The use of inherent safety principles not only improves safety, it is likely to reduce capital costs and the investment that has to be made in activities such as training and operating procedures.
EliminateThe only way of reducing risk to zero is to remove the hazard that creates that risk. (Safety can also be made perfect if no one is present. There is risk to the equipment, but not to the people.) Therefore inherent safety can best be achieved by totally removing the items that creates the hazard. ("If a tank's not there, it can't leak.")
Overton and King (2006) provide an example in which the liquefaction step in a chlorination process allowed for the elimination of at storage tank containing 750 tons of chlorine.
For example, if a pump is being used to transfer a liquid from one tank to another it may be found that there are other means of effecting the transfer that don't need a pump. Options may include gravity flow or the use of air at high pressure. (But the use of compressed air may create greater risk than that associated with having a pump.)
Another way of eliminating risk is to remove an activity altogether, and so remove the chance of that activity going awry.
The next step in addressing inherent safety is, where possible, to replace a hazardous material with one that is less hazardous. Thus the consequences of a release are fundamentally less dangerous. For example, water-based additive solutions are likely to be safer than hydrocarbon-based solutions because the latter are flammable.
In general, it is often found that the use of continuous reaction rather than batch processes will help minimize the amount of hazardous material that is present.
Referring to the pump example, once more, the risk associated with transferring the liquid can be reduced by using a smaller capacity pump and smaller piping to and from it. Then, if either the pump or the piping leaks, the consequences of the event are not so severe.
Moderation (attenuation) accepts that a certain condition exists but aims to reduce its impact. In the case of the pump that is transferring liquid from one tank to another it may be that the greatest risk occurs if the pump is blocked in while running and achieves dead-head pressure. In such a case the risk can be moderated by using a lower pressure pump curve.
The final step in achieving Inherent Safety is to reduce unnecessary complexity, make operating errors less likely and that are more forgiving of any errors that are made.
Referring to the pump example once more, it is possible that another vendor can provide the same piece of equipment but with many fewer moving parts - thus reducing both the chance of failure and the number of maintenance tasks to be carried out.
When to Use
The best time to apply the concepts of inherent safety is early in the design stages of a new-build process. However, many of the concepts - particularly those to do with removing or reducing equipment sizes - can be applied to facilities that are already in operation.
Unintended consequences can fall into one of three general categories.
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