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Inherently Safer Design

o Lower potential consequences of a release

A specific example for moderation is in the manufacture of ammonia. In the 1930s, pressures up to 600 bar were
needed. By the 1950s, pressure had been reduced to typically 300-350 bar. And by the 1980s, plants operating at pressures
of 100-150 bar were being built. Lower operating pressure was a result of improving the process. Lower pressure plants
are cheaper to build and operate, more efficient, and safer.

7.6 Simplify

Simplify is another strategy than may be effective in reducing hazards. Elimination of unnecessary complexity reduces
the likelihood of human performance failures. An example is the methyl acetate process developed by Eastman. An example
is shown in Figure 7-1 of a simplify inherently safer design approach, where fewer equipment items are needed within a
chemical process which may help reduce costs and reduce the hazards that must be managed.

Acetic Acid
Methanol Methyl
Catalyst
Acetate
Methyl
Acetate
Acetic Acid
Reactor Methanol
Recovery
Solvent
Recovery
Sulfuric
Acid
Splitter
Extractive
Distillaton
Water
Methanol
Decanter Reactor
Extractor Column
Impurity
Color Removal
Column Columns Heavies
Flash
Azeo Column
Column
Heavies
Water
Flash
Column
Water
Water Eastman Reactive Distillation Methyl
Traditional Methyl Acetate Process
Acetate Process

Figure 7-1 An example of a Simplify ISD

7.7 Inherently Safer Design and Risk Reduction
Although the focus of ISD is reduction in a scenario consequence severity, the concept may be applied to likelihood or
frequency as well. An ISD approach would make the Initiating Event (or condition needed for the event sequence to
continue) extremely difficult or unlikely so that multiple protective layers are not needed.

As an example: Consider a reactor containing a water reactive material that operates under vacuum during some
process steps, and vents to a process scrubber. Application of ISD may be to design the vent system with a seal leg such
that liquid from the scrubber cannot “back-up” into the reactor, as is shown in Figure 7-2.

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