Process control safety

The basic approach is to direct the system to the safest operating level relative to people or the environment when any emergency condition is detected, including power loss. An important concept of process control safety is to have adequate redundancy to reduce unwanted shutdowns and maintain an adequate level of certainty that a safe state will result if a real emergency does occur. As far as possible, instruments should be of the fail-safe type. 

The use of analytical instrumentation for continuous online measurement of industrial process streams for real-time information gained popularity in the late 1950s and early 1960s. This information has met various demands, the most notable of which include process control, safety, waste reduction, environmental emission monitoring and control, and product quality assurance. With the application of computers and electronics to analyzer technology in recent decades, online analyzers have become even more sophisticated and widely used. In many instances, the application of online analysis has become a necessity for survival in the face of the economic pressures of globalization in chemical manufacturing. 

Active. Using process controls, safety interlocks, and emergency shutdown systems to detect potentially hazardous process deviations and to take immediate corrective action. 

The four process control parameters are temperature, pressure, flow, and level. Modem process level detection systems are varied and ubiquitous in modem chemical plants there are thousands of processes requiring Hquid level indication and Hquid level control. From accumulators to wet wells, the need for level devices is based on the need for plant efficiency, safety, quaUty control, and data logging. Unfortunately, no single level measurement technology works rehably on all chemical plant appHcations. This fact has spawned a broad selection of level indication and control device technologies, each of which operates successfully on specific appHcations. 

In continuous processes, most process control applications rely on continuous measurements. In batch processes, many of the process control applications will utihze discrete as well as continuous measurements. In both types of processes, the safety interlocks and process interlocks rely largely on discrete measurements. 

Although microprocessor-based process controls enhance plant safety, their primaiy objective is efficient process operation. Manual... 

By attempting to maintain process conditions at or near their design values, the process controls so attempt to prevent abnormal conditions from developing within the process. Although process controls can be viewed as a protective layer, this is really a by-product and not the primaiy func tion. Where the objective of a function is specifically to reduce risk, the implementation is normally not within the process controls. Instead, the implementation is within a separate system specifically provided to reduce risk. This system is generally referred to as the safety interlock system. 

The purpose of the logic within the safety interlock system is veiy different from the logic within the process controls. Fortunately, the logic within the safety interlock system is normally much simpler than the logic within the process controls. This simplicity means that a hardwired implementation of the safety interlock system is usually an option. Should a programmable implementation be chosen, this simplicity means that latent defects in the software are less likely to be present. Most safety systems only have to do simple things, but they must do them very, very well. 

The difference in the nature of process controls and safety interlock systems leads to the conclusion that these two should be physically separated (see Fig. 8-89). That is, safety interlocks should not be piggy-backed onto a process-control system. Instead, the safety interlocks should be provided by equipment, either hard-wired or programmable, that is dedicated to the safety functions. As the process controls become more complex, faults are more likely. Separation means that faults within the process controls have no consequences in the safety interlock system. 

Modifications to the process controls are more frequent than modifications to the safety interlock system. Therefore, physically separating the safety interlock system from the process controls provides the following benefits ... 

The possibility of a change to the process controls leading to an unintentional change to the safety interlock system is eliminated. 

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