Detailed Design. This process consists of three main activities: basic alarm design, HMI design and advanced alarming (see Table 1).
Basic alarm design involves using information contained in the master alarm database to plan and configure the system. Poor configuration practices are a leading cause of alarming issues — following ISA-18.2 recommendations will help prevent them. For example, proper use of dead bands and off-delays can go a long way to eliminate "chattering" alarms, i.e., ones caused by points that repeatedly transition between the alarm state and the normal state in a short time, that operators then ignore.
HMI design is all about presenting alarms in a way that enables operators to quickly detect a deviation, diagnose the problem, determine corrective action and then respond appropriately. Effective operator performance depends on proper use of color, text and patterns within the HMI. The goal is to clearly and uniquely indicate the state of the alarm (normal, unacknowledged, acknowledged, suppressed) while also providing functionality such as filtering and navigation links within alarm displays.
Advanced alarming addresses how to build in "smarts" to support the operator. To optimize operator performance, only present alarms when they are meaningful. Additional layers of logic, programming or modeling are added to the system to modify alarm attributes or suppression status during operation. This ensures alarms that are insignificant because of the state of equipment (e.g., redundant pump running) or plant (e.g., area shutdown for maintenance) aren't presented to the operator. One common example is suppressing a low flow alarm when it's triggered as a result of a pump trip. The operator must focus on the underlying cause, the trip, and not low flow. Another example is modifying alarm setpoints and priorities for different batch recipes. It's also possible to make relevant information like a standard operating procedure available to the operator in context via information linking.
What it means. Following the standard's design recommendations and requirements is key to creating an effective alarm system. This can preempt many potential issues that normally would surface during system operation. A good design prevents many nuisance alarms and ensures that needed alarms are clearly presented to the operator. Success depends on developing effective design procedures and documents — and having the discipline to adhere to them.
Implementation.When putting the alarm system or an individual alarm into operation, testing and training are key activities. Alarms classified as "highly managed alarms" (such as safety alarms) require extra attention, including creating a documented alarm response procedure. You must revisit this phase as new instrumentation and alarms are added or process changes are implemented.
What it means. Alarm systems shouldn't be in put into service without proper training for operators and maintenance personnel. Operators must be comfortable with the system and learn to rely on it.
Operation. This section of the standard describes the appropriate use of alarm-handling tools like shelving by operators and necessary documentation to help them do their job. Alarm response procedures should include information such as setpoint (limit), potential causes and consequences of an alarm, recommended corrective action and allowable response time — which is information fleshed out during rationalization and documented in the master alarm database.