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Inherently Safer Design

The Active strategy includes controls, safety interlocks [Safety Instrumented Systems – SIS], and automatic shut-down
systems. These systems often contain multiple active elements such as:
 Sensor - detect hazardous condition
 Logic device - decide what to do
 Control element - implement action

An active strategy is intended to prevent incidents, or mitigate the consequences of incidents. An example would be a
high temperature safety instrumented system (SIS) measures the temperature in a reactor, and stops all feeds and applies
maximum cooling if it exceeds a specified value.

The Procedural strategy includes standard operating procedures, safety rules and standard procedures, emergency
response procedures, and training. An example would be that chlorine is supplied from 1-ton cylinders in a fully enclosed
building with a forced ventilation fan which maintains negative pressure inside the building and discharges to a tall stack.
The building contains chlorine gas detectors. If a high chlorine concentration alarm sounds, the operator is supposed to
start up a sodium hydroxide scrubber on the discharge air from the building. There is a written procedure, the operators are
trained on what to do, and audits confirm that all operators know and can perform the required procedure.
Generally, the preference for which strategy to use may be based on the order of robustness and reliability:
1. Inherent
2. Passive
3. Active
4. Procedural
There is a place and need for all of these strategies in a complete process safety program; especially when you consider
all the process hazards.

7.2 Principles of Inherently Safer Design

There are four principles or approaches within Inherently Safer Design – Substitute, Minimize, Moderate and Simplify.
Substitute - Replace material with a less hazardous substance.

Minimize - Replace material with a less hazardous substance.
Moderate - Use less hazardous conditions, a less hazardous form of material or facilities which minimize the impact
of a release.

Simplify - Design facilities which eliminate unnecessary complexity and make operating errors less likely.

7.3 Substitute

Example - Chlorine for Cooling Tower Water Treatment. The original design used five 1-ton cylinders per cooling tower,
manifolded, one hooked up at a time. Replacing chlorine with bleach solution in water eliminated downwind chlorine
exposure from a leak and provided better control for water treatment.
The desired substitutes, however, may change over time as we continue to learn of additional hazards. In the case of
refrigerants, the limited alternatives for substitution led to minimization. Many years ago (pre-1930), toxic or flammable
refrigerants were used such as: ammonia, light hydrocarbons, or sulfur dioxide. The quantities used were often several

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