Strategy for Inherent Safety in Design and Operation

Pressure can increase as a result of a runaway. Gas can be generated as a normal reaction product, as a result of evaporation of low-boiling components in the reaction mass, or as a by-product produced during a runaway. Gas can be produced at very high rates at continued runaway conditions. Maximum 

Time-to-runaway, described later, is an important parameter in the development of adequate emergency measures. Short times require the use of well-defined operating modes or require completely automated controls. 

In most cases, however, the design will not be inherently safe so that measures and/or controls will have to be introduced into the design. Two types of safety measures are defined (1) preventive, and (2) defensive (protective or mitigating). 

A measure is preventive if it prevents the occurrence of a runaway, a decomposition, or a hazardous secondary reaction. The system conditions remain close to operating conditions. Excessive increases in temperatures and/or pressures are avoided. Preventive measures include feed rate control systems, interlocks to prevent the reaction from starting unless sufficient diluent is present or the cooling system is working, and tests for the presence of catalysts or unwanted impurities. Preventive measures are always to be preferred over protective or mitigating (defensive) measures. 

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