Operating industrial plants involves numerous dynamic and constantly changing tasks. To assist operators to safely navigate these, alarms are normally configured against the normal operating state of different pieces of equipment. However, in practice equipment may have several normal, but differing, operating states. According to best practices, all such normal operating states should not trigger any alarm.
Alarms should only be triggered in abnormal or unexpected events. Yet in the case of simple configurations, other normal states can trigger alarms; flooding the panel and making it easy to miss necessary and high priority alarms. Rendering alarm systems a nuisance, a hindrance or a distraction, rather than useful tools.
In order to account for other expected conditions, state-based alarming has to be implemented. Such alarm methodologies produce dynamic alarm configurations based on specific process and equipment conditions. Multiple alarm set points and priority settings are configured for relevant alarms and enabled based on a plant’s state. Whenever the plant state changes, the system automatically adjusts its alarm settings to predetermined values that are appropriate for the new state.
Process states can be generally classified into 3 categories:
Planned shutdown: when a unit, equipment or area is shut down for maintenance. Alarms should either be suppressed or limits moved to lower values to detect when isolations have failed.
Operating modes: in batch processes, particular conditions may occur in instances when feed conditions change, like start up or with redundant equipment. These are normal conditions that are expected throughout a plant and alarm limits should be adjusted accordingly to account for the new state.
Consequential alarms: Such alarms can result from equipment failure that triggers a multitude of alarms down and upstream; the excess alarms should be suppressed and focus should be given to the equipment trip alarm.
When configuring the trigger conditions for each state it is important to take these factors into consideration:
· Use 2 to 3 reliable sensors to trigger a change in state
· Can this alarm be safely suppressed without creating a hazard in the process
· Use sensors that do not have multiple reasons for failure