BPCS versus SIS

A safety instrumented system, like a basic process control system, is also composed of sensors, controUer(s), and final elements. Although much of the hardware appears to be similar, safety instrumented systems and basic process control systems differ very much in function. The primary function of a control loop is generally to maintain a process variable within prescribed limits. A safety instrumented system monitors a process variable and initiates action when required. 

Even with the described functional differences, when one reads the definition of a SIS one naturally asks a few questions. What is so special about a SIS What is different between a SIS and a BPCS Many aspects are the same. 

The commonality of a SIS and a BPCS has led many to treat the design process for each the same. Some even use their control equipment to perform the safety instrumented functions. 

The SIS operates with completely different dynamics, however. Remember that the process is designed to be safe under normal conditions and process designers work hard to rninirnize process hazards within the process itself. 

When the process design is successfully oriented toward safety, risk analysis teams will usually estimate residual hazardous situations that occur only once in many years (clearly low demand mode). The average time period between hazardous events is often estimated to be over 10 years. Thus, we have a situation where the SIS is activated only once every ten years or more. During normal operation it is static. A safety isolation valve may sit motionless for years Contrast this with the operation of a BPCS. The control signals are normally d)mamic with some signals moving considerably at all times. 

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The AIChE (American Institute of Chemical Engineers) CCPS (Center for Chemical Process Safety) Process Equipment Reliability Database (PERD) initiative has rigorously identified and documented failure modes for instrument loops, which encompass control, indication, alarm, and automatic protection (Ref. 3). An excerpt from those lists shows a comparison of failure modes applicable to BPCS versus SIS (Figure 2-2). The ability of operations personnel to detect these failure modes is also quite different. 

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