Quality and Quantity
The quantity of alarms that should be presented to the operator is somewhat dependent upon the level of performance that is being sought for the alarm system. EEMUA 191 describes five levels of alarm system performance. They range from Level 1 to Level 5 and are described as Overloaded, Reactive, Stable, Robust, and Predictive, respectively. The recommended goal based on industry publications is to set alarm system performance at Robust in lieu of Predictive. This is due to the fact that the investment in additional effort, additional software, and additional cost may not be worth the return.
With a goal of robust performance, the key performance indicators (KPIs) should line out as follows:
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Average alarms per day = 1,440
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Average standing alarms per shift = 9
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Peak alarms per 10-minute period = 100
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Average alarms per 10-minute period = 10
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Percentage of time alarm rates outside of target = 5%
As a point of comparison, if an alarm system’s performance goal is set at predictive, then the KPIs would line out as:
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Average alarms per day = 144
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Average standing alarms per shift = 9
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Peak alarms per 10-minute period = 10
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Average alarms per 10-minute period = 1
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Percentage of time alarm rates outside of target = 1%
You might observe that 1,440 alarms per day is more than expected for a high-performing alarm system. However, you may also realize that fully investing in a more predictive-based alarm system is not economically feasible. This is one point of disagreement in the EEMUA 191 document as well as other available documents (the desire to keep the number of alarms minimal but also recognizing the capital expenditure required to achieve greatness).
This is where some balance may need to come into the picture. Industry documentation, as well as the newly released standard for alarm management (ISA 18.2) recommends that 300 alarms per day be set as a maximum number of alarms per operator. The ISA standard recommended that 150 alarms per day be set as a manageable number of alarms per day per operator. Therefore, if a control room staffing plan includes two operators, then the maximum number of alarms per day should be approximately 300 or fewer. This level of performance could be stated as Robust+ or could also be addressed as transitory (Robust-Predictive). For this example of two control room operators, the KPIs would line out as:
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Average alarms per day = 300
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Average standing alarms per shift = 9
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Peak alarms per 10-minute period = 4
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Average alarms per 10-minute period = 2
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Percentage of time alarm rates outside of target = 2% to 3%
Steps to Improve Alarm System Performance
Based on the information available in the process controls industry, there are some clear and distinct measures that can be taken to improve alarm system performance within the power industry, including those that follow.
Design Each Alarm. Every alarm should have a defined purpose, required response, defined priority, and known consequences. If no operator action is required, it is not an alarm.
Reduce Clutter. The alarm system’s defined audible and visual cues need to be distinct to that alarm or that alarm’s priority alone.
Apply Alarm Suppression (Masking). Some areas of opportunity may not be known during preliminary design; however, this should not provide license to delay the evaluation of alarm suppression as a whole.
Coordination. Every control system within a facility needs to be coordinated in alarm system design approach. Priorities of alarms and defined operator action should be consistent in context and meaning between all plant control systems.
--Brandon Parker (parkerbs@bv.com) is plant automation systems section head at Black & Veatch in Overland Park, Kan.
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