On the use of delay timers and latches for efficient alarm design

Alarm systems are an important complement to good control systems design. However alarm systems have not received the attention that they deserve and as a result they do not necessarily operate in the most reliable and efficient manner. Delay timers and latches are often used in the process industry to reduce alarm chatter and to minimize nuisance alarms. Effect of varying the size of on-delay, off-delay timers and latches on the accuracy of detection is discussed from a theoretical view point by modeling them using Markov chains. The performance of delay timers is compared to that of the latches. A design procedure for selecting the length of on-delay and off-delay timers based on historical alarm event data and the corresponding 'Return To Normal' (RTN) information is introduced and illustrated using a real industrial case study.