Process control system including a voter logic block with operational and maintenance override means

摘要

A process control system, or function block 94, comprises means N1, N2, N3 to receive signals from process plant indicating a process condition, means to produce a signal indicative of whether each signal from the plant meets a trip criterion, a voter logic block 100a which produces a trip signal as a tripped value or a normal value depending on how many signals from the plant do/do not meet trip criterion and means to prevent use of one of the inputs by the voter logic or to override a trip signal produced by the voter logic. Timers 110 may also be provided to track the elapsed time of an override and allow a trip signal to be delivered after a predetermined period. The system is said to provide a safe means of bypassing or overriding signals from field devices, during situations such as maintenance or testing, which could otherwise lead to a tripped state.