Human Machine Interface Alarms

The human interface should be suitable. Alarms may be presented either on annunciator panel, individual indicators, VDU screen, or programmable display device. Alarm displays may be color-coded according to their function. An example can be as follows:

Alarms Yellow
Pre-alarms Orange
Shutdown Red
Bypass Red

Alarms lists should be carefully designed to ensure that high priority alarms are readily identified, that low priority alarms are not overlooked, and that the list remains readable even during times of high alarm activity or with repeat alarms.

Alarms should be prioritized in terms of which alarms require the most urgent operator attention. Allowing alarms to be added to an alarm system without explicit priority criteria can result in differing priorities being assigned alarms that have the same consequences. A recommended breakdown of alarms into priorities for a process unit is: 14% High Priority, 44% Medium Priority, 42% Low Priority.

Alarms should be presented within the operators field of view, and use consistent presentation style (color, flash rate, naming convention).

Each alarm should provide sufficient operator information for the alarm condition, plant affected, action required, alarm priority, time of alarm and alarm status to be readily identified. Ambiguous or confusing alarm messages should be avoided, e.g. when a boiler feed water pump had tripped the message should be “BFW Pump Tripped” and not “BFW Pump Low Pressure”.

One of the most overlooked aspects of alarm system configuration is meshing the alarms with the displays. Making the wrong choices in meshing the 2 can have far reaching consequences and can slow the operator in reaching the problem area of the process.

Another point that should be taken into consideration when meshing alarms with the displays is how the alarm colors relate to others used in the displays.

Color coding used for the alarms should not conflict with those used in any other part of the display system. Color coding conflicts cause information processing delays as the operator has to decipher in what context the color is being used, e.g. “RED” for emergency vs. for pump/fan/motor status, valve closed, etc.

The visual display device may be augmented by audible warnings that should at a level considerably higher than the ambient noise at the signal frequency. Where there are multiple audible warnings, they should be designed so that they are readily distinguished from each other and from emergency alarm systems.

They should be designed to avoid distraction of the operator in high operator workload situations. Where both constant frequency and variable frequency (including pulsed or intermittent) signals are used, then the later should denote a higher level of danger or a more urgent need for intervention.

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