Figure 1300-8 shows a schematic for a triple circuit logic system. A triple circuit system can receive one, two, or three independent inputs for a given abnormal process condition (i.e., three high pressure shutdown switches set to trip at the same pressure) and can evaluate each input through independent microprocessors. The system then compares the results, and if any two out of the three circuits indicate that a shutdown should occur (voting logic) then the protective system activates a shutdown. No single component failure can cause a false shutdown. A triple circuit system with majority voting has distinct operational advantages over single and dual circuit systems, as follows:
• The individual circuits can be completely tested on stream without shutdown.
• At least two of the three parallel components must function to initiate a trip. The system is said to be fault tolerant.
• Online repair is permissible.
• Any component may be removed and replaced without risk of a spurious trip or of defeating the protective system. This assumes all components are triplicated.
• System failures and false alarms can be detected and annunciated without causing a shutdown and without losing the protection afforded by the system. Therefore, routine manual checking of the system becomes less critical.
Triple circuit systems are becoming more common in Company installations. For more information about operating company experience with this technology, contact the Monitoring and Control System Division (M&CS) of ChevronTexaco Energy Research and Technology Company (ERTC).