Sequence logic, process control

Technology advances in electronics such as process control instrumentation systems, computer capabilities, programmable logic controllers, and the use of independent PC s (personal computers) at field locations for special dedicated functions present new challenges to incident investigation. Some of the advances are so rapid that the team may not have the internal expertise to determine failure scenarios, sequences, and modes. The suppliers and manufacturers of these high-tech devices are sometimes the only source of credible information on failure modes of these devices. 

Programmable logic controllers PLCs) are widely used in batch process control to execute the desired binary logic operations and to implement the desired sequencing. The inputs to the PLC are a set of relay contacts representing various device states (for example, limit... 

Programmable logic controllers are used for the control and interlocking of processes where a sequence of operating steps has to be carried out such as, in batch processes, and in the start-up and shut down of continuous processes. 

Most processes operate more efficiently when functions must occur in a desired time sequence or at prescribed intervals of time. In the past, mechanical timers and logic relays were used. Now electronic logic and timing devices are used based on computer software programmable logic controllers. They lend themselves to easy set-up, reprogramming, and provide more accurate control. 

PLC (Programable Logic Controller)—Device used to control on-off process operations. The sequence of on-off executions and sometimes other control functions can be changed by programing instead of wiring changes. 

The central processing unit (CPU) controls the overall operation of the computer. It is made up of electronic registers and logic circuits that execute the simple logical and arithmetic operations of which the computer is capable. When these operations are executed in appropriate sequences, the computer can accomplish complex mathematical or data-processing functions. Moreover, if one provides the appropriate electronic interface, these simple operations can be used to control experimental systems, acquire data, or print results on a teletype printer, line printer, oscilloscope, or other peripheral device. 

---