Inherent Safety - Principles

Principles

The terminology of inherent safety has developed since 1991, with some slightly different words but the same intentions as Kletz. The 4 main methods for achieving inherently safer design are:

• Minimize: Reducing the amount of hazardous material present at any one time, e.g. by using smaller batches.
• Substitute: Replacing one material with another of less hazard, e.g. cleaning with water and detergent rather than a flammable solvent
• Moderate: Reducing the strength of an effect, e.g. having a cold liquid instead of a gas at high pressure, or using material in a dilute rather than concentrated form
• Simplify: Designing out problems rather than adding additional equipment or features to deal with them. Only fitting options and using complex procedures if they are really necessary.

Two further principles are used by some:

• Error Tolerance: Equipment and processes can be designed to be capable of withstanding possible faults or deviations from design. A very simple example is making piping and joints capable of withstanding the maximum possible pressure if outlets are closed.
• Limit Effects: Designing and locating equipment so that the worst possible condition gives less danger, e.g. gravity will take a leak to a safe place, the use of bunds.

In terms of making plants more user-friendly Kletz also added the following:

• Avoiding Knock-on Effects;
• Making Incorrect Assembly Impossible;
• Making Status Clear;
• Ease of Control;
• Software and management procedures.