As many of you will know, one of the most common form of nuisance to operators working industrial controls are repeating or chattering alarms. On a typical plant, repeating alarms may account for around 50% of the alarm annunciations. They are a problem because the operator will have to silence the alarm hooter and/or accept the alarm each time it occurs. This burden can overwhelm them and can possibly lead to operator fatigue. It could also cause them to ignore the alarms altogether, which has the potential to lead to serious safety and operability concerns.
A situation like this can be effectively managed using deadbands. The deadband will prevent an alarm from returning to normal until the alarm condition is cleared by a defined increment of percentage of the range. It is also a required function of control systems as per ISA-18.2 10.5.5.2 which states: “The control system shall provide the capability for implementing the deadband functionality.”
Fortunately, the deadband function can be found in most modern process control systems and following industry recommendations for alarm deadbands of various measurement types can prove to be the starting point to eliminate chatter. However, to get the best performance, the size of the deadband will need to be tuned to match the characteristics of the process signal.
EEMUA 191 recommended deadband settings
The use of deadbands is very effective for addressing chattering alarms, and to correct other alarm management problems such as alarm floods, nuisance alarms, and incorrect alarm priority, alarm rationalization is needed. Alarm rationalization is the process of reviewing each alarm in the system to confirm if it is required based on the consequence of inaction, what priority it should have, and what the operator response is.
For more information on alarm deadband and alarm rationalization you can check out our alarm management training courses.