Prioritize and Group Your Alarms
Alarms must be designed so that they are useful and relevant to the operator based upon prioritization, grouping, and mode of operation of the plant. Furthermore, the design should be such that when an alarm is active, it tells the operator the level of priority and the functional system or process generating the alarm. This is achieved by spending more coordination time and effort in designing and developing consistent application of priorities, graphic coloration, and alarm audible alerts.
Alarm prioritization should be designed using a three- to five-level system. Three levels of priority (1-2-3 or high-medium-low) are the most popular, according to The Alarm Management Handbook by Bill Hollifield and Eddie Habibi.
Within the priority system, alarms should be distributed so that only a select few are designated "high" or "critical," thereby truly differentiating these alarms from lower-priority alarms. For example, if using a three-level system, alarms should be distributed so that 80% are low priority, 15% are medium priority, and only 5% are high priority.
Each level of priority should have its own unique audible tone and unique graphic color. Note that the color should not be repeated for use in any other function on the control system graphics.
Alarms should also be grouped by process. Any alarm that is active within a specified alarm group would cause that group’s visual alarm to activate, as necessary. Some suggested groupings include fuel handling and preparation, turbine, and generator and electrical auxiliaries.
In addition to grouping alarms by process, some alarms may be grouped by criticality of subsequent action. These alarm groups may be treated as first-out groups. First-out groups may include unit protection or a turbine trip summary. Alarm relevance should be built into the alarm design to the greatest extent practical.
Alarm relevance should take into account the operating mode of the equipment, system, and plant in order to provide more intelligent alarm masking. For example, an alarm that may be typically generated on a "main steam header temperature low" alert may not be relevant when the source of steam energy (boiler or heat-recovery steam generator) is offline. Instead of merely alarming steam temperature low, the relevance of low steam temperature should be evaluated, and an alarm should only be generated when it is of concern, such as when steam turbine stop valves are not closed.
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