Right-Size Your Safety Instrumented Systems Consider the cost versus value of adopting a higher safety integrity level

摘要

WHEN SOLVING a design challenge,most engineers like to build in a safety factor.For example,if structural calculations suggest 3/8-in.bolts will suffice for an application,designers often will specify 1/2-in.bolts to be"on the safe side."Here,the change incurs only slight added expense.However,in many cases,providing an extra margin can create significant costs.For instance,increasing the horsepower rating of a motor driving a pump by 20% means a more expensive motor but,worse,the cost of the extra electricity to run it over its lifetime.Let's extend this thought progression into another area:process safety.Is it possible to overdesign a safety instrumented system(SIS)or individual safety instrumented function(SIF)? Is it desirable to do so to build in the extra cushion of protection? Overdesign certainly is possible.However,engineers should carefully consider the cost,while ensuring provision of the required degree of safety.This article will not get into evaluating potential safety hazards or designing safety systems,which are extensively covered elsewhere.Instead,we will discuss ways of interpreting and implementing the findings of these evaluations.Before that,though,let's go over some basics of the design methodology,as detailed in IEC standard 61511.A process hazards analysis(PHA)leads to assignment of a particular SIF to a specific hazard,such as an overpressure incident in a given vessel.The criticality of an SIF,which depends on the likelihood or frequency of the incident occurring,is then multiplied by its degree of severity,e.g.,a small chemical spill versus a fire or explosion,to determine the safety integrity level(SIL)or risk reduction factor needed for the SIF.