Are Your Safeguards Fast Enough to Be Effective?

A number of design pitfalls must be avoided when allocating the process safety time during the specification of instrumented safeguards. A key attribute of all safeguards is that they must be functional against the loss event in question and one of the essential elements of functionality is speed. How to determine the response time that must be specified for a given setpoint is a common project question. A variety of engineering practices, from basic to advanced, may be used to provide justification for this required specification element. Among these are:

• Expert judgement
• Extrapolation
• Mass and energy balance calculations
• First principles process modelling

Key technical issues in this evaluation include hardware response lags, process response lags, application program delays, measurement error, and the design and safety margins that should be applied. Careful attention to these details are needed from the process engineering, instrumentation and controls design team members, for if an instrumented safeguard performs too slowly compared to the hazardous event, it provides no protection at all.