Silence the noise: fixing chattering and fleeting alarms
There are few things more annoying to operators than chattering and fleeting alarms. Chattering alarms have short durations. They occur, then quickly clear, then reoccur and clear, happening up to several times a minute. A few chattering alarms can ruin an alarm system's performance. Initial criteria for identifying chattering alarms are an alarm that occurs and clears three or more times within a minute.
A fleeting alarm is also a short duration alarm. It appears then clears almost immediately (often within a few seconds), without the operator having to do anything. It does not immediately repeat. (If it did, it would be a chattering alarm). It is still a distracting and pointless interruption for the operator and on most systems requires acknowledgment before it's removed from the alarm listing screens.
Both analog values (e.g., flows, pressures) and digital (on-off) signals, such as from switches or binary sensors, can and do chatter. You can address fleeting alarms using the same methods as chattering alarms, with minor differences. Chattering and fleeting alarms are common. No one intentionally designed any alarm to have these behaviors, and you can fix all. Here's how.
Start with deadband
For chattering analog sensors, first look at the alarm deadband. What is deadband? On-off control is the most basic form of control. The system places a certain degree of deadband around the process setpoint. If the process variable is lower or higher than the deadband, the control action is turned fully on (or off)––like an air conditioner or home heater.
On-off control can be used on a pump to fill or empty a tank, turning on at 20% and off at 80%. But you would not want to use it for automobile cruise control, with the throttle cycling on and off every time the vehicle encounters a hill.
With on-off control, the process variable cycles around the setpoint and through the deadband. To achieve tighter control, reduce the deadband. The resulting side effect is that the frequency of the oscillations increases, which reduces the life of the final control element (i.e., relays, control valves, etc).
Deadband and alarms
Similar to deadband for setpoints in process control, all alarms on analog values should have an alarm deadband specified. All process signals have some degree of noise. As a process value passes through an alarm setpoint, noise or slight variations in the signal can cause multiple alarms if the deadband is too small.
A properly configured deadband that is larger than the normal signal noise helps prevent these repeated alarm events. For example, as a process value moves above a high alarm setpoint, it must move sufficiently back below that threshold before the alarm clears. This helps prevent small fluctuations around the setpoint from repeatedly triggering and clearing the alarm.
Most DCSs provide several options for configuring alarm deadband. Check the DCS documentation to configure it properly. Deadband should always be applied to analog signals before using the delay-time techniques discussed below.
Rigorous calculation is not usually necessary. As a starting point, consider a deadband of approximately 5% for flow and level, 2% for pressure and 1% for temperature. Trial and error can also be used: start with a small value, such as 1% to 2%, and increment based on the results.
You may think this is pretty basic information and everyone already knows it. We assure you, we have been solving this problem for decades and almost every system has hundreds of sensors with alarm deadbands set at zero. Check yours!
Delay time analysis for chattering and fleeting alarms
Deadband only applies to alarms on analog values. Often, worst-case chattering or fleeting alarms are associated with on-off signals such as pressure and level switches. (Don't get me started on the many reasons you should not be using such devices––that would be a different blog.) There is another powerful method to use for these that is probably already a capability of your DCS. This method applies to both analog and digital point types. The method requires a bit of explaining, but once understood, the technique itself is simple. And the results are well worth the effort.
There are two types of alarm delays available in many DCSs, namely the on-delay and the off-delay. (The off-delay is sometimes referred to as a "debounce timer.") Each delay setting is specified as a number of seconds, and applies only to the point specified (not "globally" for the entire DCS). Some point or alarm types may have either delay available, and some have only one of them. On and off delays work differently and have different implications when used. These settings provide powerful methods for fixing chattering and fleeting alarms. Here is exactly how they work.
Alarm on-delay
Use of the on-delay time parameter can prevent a short-duration alarm from ever being seen by the operator. This is particularly useful for fixing a fleeting alarm that does not usually repeat.
With an on-delay, the alarm does not immediately annunciate when the alarm condition occurs. Instead, the condition must remain continuously in the alarm state for the specified on-delay period before the alarm is presented to the operator. If the condition clears before that time expires, the alarm is never annunciated.
Choosing the on-delay time parameter correctly is important because even a valid alarm will not be presented to the operator immediately. This increases the overall time available for a proper response and could be a safety concern for some alarms.
Alarm off-delay
An off-delay can turn a string of repetitive, nuisance chattering alarms into a single, longer-duration alarm event with no initial delay. Unlike an on-delay, the alarm is annunciated immediately.
When the alarm condition clears, however, the return-to-normal state is not shown to the operator unless it remains clear for longer than the specified off-delay. If the alarm condition quickly returns, the operator sees a single sustained alarm rather than a series of short alarms that repeatedly occur and clear.
Using this technique, hundreds or thousands of nuisance chattering alarm occurrences can become a single, longer-duration alarm occurrence. The key is choosing a delay time that is greater than the normal time between recurring alarms.
The minor disadvantage to this technique also concerns the delay time. If the operator gets the alarm and takes a corrective action to eliminate it, they will not see a return-to-normal condition until after the delay time has expired, regardless if the action was immediately successful. In most cases, this is acceptable for off-delays of up to even a couple of minutes. The operator can see (for analogs) that the process value has moved below the alarm setpoint.
So, how do you choose the number of seconds of delay for each chattering or fleeting alarm? Guesswork is not advisable. Instead, use your alarm analysis software. To do this, you want to perform two frequency analyses on each chattering or fleeting alarm. These are analyses of the times-in-alarm (durations) and times-between-alarms (intervals).
Time-in-alarm and time-between-alarms
DCSs produce time-stamped event records of at least three things: the alarm occurrence itself, the return-to-normal event (created when the condition causing the alarm to occur has cleared) and the operator acknowledgement event (created when the operator hits the acknowledge key for the alarm). Consider the first two.
In your alarm analysis software, you will have recorded thousands of occurrences from your nuisance alarms. For each nuisance alarm, take each pair of alarm occurrences and return-to-normal events and then subtract the timestamps. The result is the time-in-alarm (duration) for the alarm occurrence. In a similar method, the difference between an alarm occurrence timestamp and the prior alarm clear event timestamp is the time-between-alarms (interval). Round all of the timestamps to the nearest second.
Analyzing these durations and intervals across thousands of occurrences can reveal clear patterns. For example, an alarm may repeatedly remain active for less than 10 seconds and recur less than 20 seconds after clearing. An alarm that appears, lasts only a few seconds and then clears on its own is unlikely to meet the basic criteria for an alarm: a condition requiring operator action to resolve.
In one analysis, 93% of all activations of a particular alarm lasted 15 seconds or less. These alarms did not return to normal because of responsive operator action. Instead, they indicated transient conditions that resolved on their own. Some occurrences, however, remained valid for several minutes.
This information can be used to evaluate potential on- and off -delay settings and determine the percentage of nuisance alarm occurrences that could be eliminated at different delay times. For this example, an on-delay of 30 seconds would eliminate more than 96% of the alarm occurrences, while an off-delay of one minute would eliminate approximately 72%.
This type of analysis makes it possible to identify the point of diminishing returns and select an appropriate delay rather than relying on guesswork. Off-delay will generally eliminate fewer alarm occurrences than an on-delay of the same duration, although available delay options may vary by control system
Alarm mechanic
Octave Tempo Control System Effectiveness (formerly PAS PlantState Integrity) is a comprehensive alarm management solution. Its alarm analysis module automates this entire delay-time analysis with the alarm mechanic feature. Any alarm in a typical analysis list (such as most frequent, chattering or fleeting) can be selected and a new analysis window opens. Thousands of past occurrences of that alarm are automatically analyzed to determine the effect of different on- and off-delay times. The analysis identifies the point of diminishing returns and provides the expected reduction in alarm for specific delay-time choices.
The alarm mechanic analysis can be saved as a PDF for documentation of management of change. This analysis makes it easier to identify, evaluate and resolve nuisance alarms.
This method uses actual occurrence data to determine the appropriate delay-time value. For new points, however, there is no historical data available to guide that decision. So, what should the defaults be?
Implementing on- or off-delay is different from implementing deadband. With deadband, the physics of the situation generally make a zero default inappropriate. For many alarm points, however, a zero on- or off-delay may be perfectly acceptable. Both EEMUA 191 and ISA-18.2 provide guidance and cautions around the use of alarm delay times.
The default on-delay should be zero and applied only when a specific problem has been identified. For Priority three alarms, typical starting points vary by signal type: approximately 10–15 seconds for flow and pressure and 20–30 seconds for temperature. Level alarms may warrant delays greater than 30 seconds, depending on the application. Priority 1 and 2 alarms should be evaluated individually before an on-delay is applied.
Off-delay can be used more broadly for Priority three alarms. A 15-second delay is a typical starting point for flow and pressure, while level and temperature may warrant 30–60 seconds. Longer delays may be appropriate in certain applications based on process behavior and engineering judgment. For Priority one and two alarms, off-delay settings should also be evaluated individually for acceptability.
For other signal types, delay times should be determined individually based on the characteristics of the alarm. In some cases, even a very short delay can eliminate fleeting alarms. Delay-time settings should reflect the process behavior, the alarm priority and the time available for an operator to respond.
Summary
There are widespread problems with frequent and repetitive nuisance alarms. Chattering and fleeting alarms are often the worst offenders. The methods for fixing them are well known and easy to apply. There is no reason to have a poorly performing alarm system filled with nuisance alarms.
Ready to tame your alarm system? Contact us — or dive deeper with our white paper: Making a big dent in nuisance alarms.
Review other Taming the Wild Alarm System topics in this 7 part blog series:
Part two - The most important alarm improvement technique in existence
Part three - Silence the noise: fixing chattering and fleeting alarms
Part five - What alarm rationalization really uncovers — and why it matters
Part seven - Beyond alarm management – doing more with a powerful tool