Let’s close this week with another great question and a great answer, this time involving operator alarm strategy development. A paraphrased version of the question was:
I am planning to implement the on-delay / off-delay and deadband alarm parameters. Is there a guideline re: the use of this delay – when would you use an on-delay and when would an off-delay make more sense? Is this parameter primarily used for switches and booleans with the deadband more commonly used for the analogs?
Emerson’s Cindy Scott, whom you may recall from earlier post on alarm design color considerations, shared some thoughts based upon an ISA-18.2 subcommittee technical report work in progress concerning basic alarm design. Cindy offered guidance based upon the actions the operators should take based on the alarm condition:
On-Delay and Off-Delay alarm timers provide an important way to reduce nuisance alarms and address different alarm conditions from alarm deadbands.
On-Delay and Off-Delay alarm timers are useful for any alarm that can quickly cycle on and off (e.g. fleeting or chattering alarms) or in cases where the amount of time an alarm limit must be exceeded is important for alarm annunciation. Both of these examples include analog alarms.
To decide between On-Delay and Off-Delay timers, consider if when an alarm limit is reached, should the operator be notified EVEN if the alarm condition immediately clears (e.g. operator action is required).
- If the operator action is required any time the alarm limit is reached, use Off-Delay timers to hold the alarm on and ensure that the alarm doesn’t repeatedly annunciate if the value bounces around the limit.
- If the operator action is NOT required any time the alarm limit is reached, use On-Delay timers to wait to alarm the operator so that the alarm only annunciates when the alarm limit has been exceeded for enough time to ensure the alarm is valid and repeatedly annunciates if the value bounces around the limit.
Every time the pump is turned on, the flow spikes (e.g. surges) and causes a temporary downstream high flow alarm, that clears as soon as the surge is over. Use an On-Delay timer, set to the typical time for the surge to clear, to eliminate this nuisance alarm.
A tank level alarm can repeatedly activate and clear due to foam in the tank. This could lead to a false alarm, where the level alarm sounds, but there is not a problem because it is being caused by foam. An On-Delay timer could be used such that the detected level would have to exceed the limit for the specified time before alerting the operator.
A filter could be damaged if the temperature exceeds a limit, even if it is only for a few seconds. A long Off-Delay time would ensure that the alarm is kept ON, even for temporary or transient temperature swings.
Another filter is only damaged if the temperature exceeds the limit for at least 1 minute. A 1-minute, On-Delay time ensures that the alarm only occurs when the filter must be inspected for damage.
Starting with the desired operator action in mind helps clarify how the alarm strategy should be designed and implemented.