When an abnormal condition occurs, usually more than one alarms may be actuated. In a shutdown situation, one trip could cause a series of alarms to respond almost simultaneously.
In some situations, it might be desirable to determine which alarm was the first to respond. The order in which the alarms go off may affect the operating personnel’s perception as to what has occurred and subsequent action taken.
In the Standard Sequence, it is not possible to tell which alarm was initiated first. The First-Out Sequence, on the other hand, has the advantage of identifying the first initiating condition that caused the shutdown.
After the first-out system trips and is acknowledged by the operator, the alarm that tripped first stays flashing while the other alarms in the first-out sequence are indicated by a steady lighted window.
However, it should be noted that the first alarm to respond may not be the one that initiated the trip. This is because all of the process variables, equipment, and interlock components have different detection and response speed.
There are also pre-alarms that warn the operating personnel to the fact that there is a potentially dangerous trend developing and to give the operating personnel some time to take action to correct the problem before matters reach the point where the emergency shutdown is activated.
The type of alarm and its setting should be established so as to enable the operator to make the necessary assessment and take the required timely action. Settings should be documented and controlled in accordance with the alarm system management controls.
A fundamental reason for having an alarm should be to prompt an operator action (i.e. making a control change). Frequently there are alarms implemented that require no operator action, e.g. simply for status indication.
These alarms are “in” during normal operations. Although an argument can be made to justify the implementation of “monitoring” a value more closely (e.g. during a product switch in a processing system), this approach should be used with caution since it is an easy way to justify adding volumes of alarms.
Often in configuring alarm systems there is confusion over instrumentation redundancy (e.g. due to reliability problems) and information redundancy. This resulted in alarm conditions that have multiple alarms indicating the same situation, e.g. multiple level transmitters on a knock-out drum.
Also, alarms with improper set points may case alarm cycling, e.g. 2 alarms taking turns to activate one after the other.
Also Read :