Alarm floods are periods of alarm activity during which the alarm rate is greater than the operator can effectively manage (e.g., when the operator receives ≥10 alarms in 10 minutes). During a flood situation awareness is compromised and alarms are likely to be missed. In the eleven minutes prior to the explosion at the Milford Haven Refinery, the operators received 275 alarms including the Flare High Level alarm which was missed. Performing a thorough alarm rationalization and implementing dynamic suppression can minimize alarm floods, but may not eliminate them entirely. This article discusses Human Machine Interface (HMI) design practices that can help the operator maintain situation awareness during an alarm flood contributing to effective response to alarms.
In modern distributed control systems, the most commonly used alarm display mechanism is the alarm list (summary display) which presents alarms chronologically. During an alarm flood, alarm lists can be challenging to use for reasons such as:
- Alarms move down the list and off the page faster than operators can read them
- Operators must scroll or change pages to see all of the alarms
- Reading information on an individual alarm is difficult because the alarm is changing position within the display
- High priority alarms may scroll off the page (depending upon how the alarm list configuration)
- Does not support determining the relationship between multiple alarms (cause and effect).
Studies by the Abnormal Situation Management (ASM) Consortium and Center for Operator Performance (COP) have identified specific practices that can help operators respond more effectively during an alarm flood.
1. Create an Alarm System Overview that aggregates alarm status by sub-system. This display allows the Operator to determine which equipment area requires their attention. In the example below, a summary graphic shows the status of key equipment and aggregates the alarm status in each process area. From this graphic the operator would be able call up an alarm summary list filtered to show only the alarms from the specific equipment area, rather than a list containing all alarms.
2. Configure the alarm summary display to keep the highest priority alarms at the top. Research by the COP has shown that optimum performance during alarm floods can be achieved by keeping the highest priority alarms together at the top of the screen. This makes it easier for operators to focus on the most critical alarms (the desired behavior) and sacrifice response to low priority alarms (which is effective if an effective prioritization has been performed as part of alarm rationalization).
3. Provide Alarm Timeline Displays. The purpose of these displays is to present alarms in a temporal arrangement that allows for discerning patterns as well as cause and effect relationships. Effective timeline displays also incorporate the ability to display related process data in context (such as operator actions). In the example below, alarms are displayed according to slices of time (e.g., thirty seconds) along with associated control actions.
For more information check out the exida webinar “HMI Design for Optimum Operator Response to Alarms”.
“Trial finds that many operators prefer an alternative visualization approach”, Chemical Processing, March 2014.
”How many alarms can an operator handle”, Chemical Processing, Dec 2011
“Addressing Alarm Floods: Operator Interface Consideration”, Bullemer et al, ISA Automation Week, 2011
“Designing for Situation Awareness: An Approach to User-Centered Design”, Second Edition, Endsley and Jones, CRC Press
“Human Factors in Process Plant Operation”, Strobhar, Momentum Press