What are alarm, trip point, and alarm priority in DCS & PLC?

What is a process alarm in Control System?

An alarm system is created to govern the attention of plant operators to serious conditions of the existing status of the process plant.

Various types of alarms in DCS/PLC:

Types of alarms available in control system

1. Process Alarm
2. System Alarm
3. Safety Alarm
4. Shutdown Alarm

1. Process Alarm:

These are the alarms related to the process operating conditions

• These alarms inform about defects in existing process parameters.
• These alarms are associated to plant efficiency to avoid the cause of losses.
• This alarm is generally integrated into a control system such as DCS and uses the same sensors as system control.

2. System Alarm:

• These are the alarms related to the control system components like controller, IO Modules, Network equipments etc.,
• These Alarms help to detect equipment issues, but they will not affect the process operation.

3.Safety Alarms:

• These alarms create awareness of existing processes to plant operators informing them there may be the occurrence of any hazardous condition.
• Generally, these safety alarms must have high priority and device independence when they are engaged in protection against system malfunctions.
• In some cases, the security shutdown mechanism generates such warnings.

4. Shut down alarms:

• These alarms notify the plant operator that safety instrumentation systems have reached and triggered automatic shutdown conditions.
• There are some conditions resulting in the automatic start-up of such alarm systems.
• This device enables the standby generator to power up during supply failure.

What is the range of a transmitter?

The term “range” can be used to describe the upper and lower process values that an instrument can measure, such as flow, level, pressure, and temperature.

This scale is alternatively known as the “calibrated or calibration range.”

In the transmitter the lower range is mapped to 4 mA and upper range is mapped to 20 mA.

What is Lower Range Value (LRV) of a transmitter?

Smallest possible measurement an instrument is calibrated to measure is called Lower range value.

The LRV is alternatively called as Zero.

What is Upper Range Value (URV) of a transmitter?

Highest possible measurement an instrument is calibrated to measure is called Upper range value.

The URV is alternatively called as Span.

What is extended low range of a transmitter?

The extended low range is used to measure the lower value of the process variable in case the process variable goes below the instrument calibration range.

What is extended high Range of a transmitter?

The extended high range is used to measure the higher value of the process variable in case the process variable goes above the instrument calibration range.

Example Process Variable:

For example the process variable to be measured is level of a tank with 0 to 100%.

Then ,

The Range is 100

LRV is 0%

URV is 100%

Extended Low Range can be -5%

Extended High Range can be 105%

What is a deadband?

A deadband is a process variable region in which a change in the process variable results in no change in the control signal.

What is an alarm trip point/setpoint?

• A process signal is monitored and compared to a preset limit in order to generate an alarm. This preset value is referred as alarm trip point/setpoint.
• The alarm will go off in the event that the process signal goes above or below the alarm trip point.

Based on Process condition alarms are further classified to:

1. High High Alarm
2. High Alarm
3. Low Alarm
4. Low Low Alarm

The depiction of these alarm points are shown in below table:

Note: In above table LRV is 0% and URV is 100%

1. High-High (HH) Alarm:

A High-High alarm is triggered when the process value exceeds the High-High alarm trip point. The tag will remain in High-High alarm until it is acknowledged by operator and its value falls below the HH alarm trip point minus the deadband.

In above example if the level exceeds 95% High-High alarm gets triggered and it will stay until operator acknowledges the alarm and also the alarm falls below 90%, since the deadband considered here is 5%.

2. High (H) Alarm:

A High alarm is triggered when the process value exceeds the High alarm trip point. The tag will remain in High alarm until it is acknowledged by operator and its value falls below the H alarm trip point minus the deadband or exceeds HH alarm trip point, if configured.

In above example if the level exceeds 85% High alarm gets triggered and it will stay until operator acknowledges the alarm and also the alarm falls below 80% or the process value exceeds 95%.

3. Low (L) Alarm:

A Low alarm is triggered when the process value falls below the low alarm trip point. The tag will remain in low alarm until it is acknowledged by operator and its value exceeds the L alarm trip point plus the deadband or falls below LL alarm trip point, if configured.

In above example if the level falls below 30% low alarm gets triggered and it will stay until operator acknowledges the alarm and also the alarm exceeds 35% or the process value falls below 15%.

4. Low-Low (LL) Alarm:

A Low-Low alarm is triggered when the process value falls below the Low-Low alarm trip point. The tag will remain in Low-Low alarm until it is acknowledged by operator and its value exceeds the LL alarm trip point plus the deadband.

In above example if the level falls below 15% Low-Low alarm gets triggered and it will stay until operator acknowledges the alarm and also the alarm exceeds 20%, since the deadband considered here is 5%.

Why High-High & Low-Low alarm is configured?

High and Low alarms are configured to indicate the operator with potential process disturbance which can lead to process failure scenarios, so that operator can take immediate corrective action before the process value reaches High-High or Low-Low alarm trip points.

High-High and Low-Low alarms are configured to take immediate trip actions automatically through control system in a process plant to safely shutdown the process.

What is alarm priority?

• Prioritization of alarms is necessary for plant control room operators to distinguish alarms based on their priority to sequence actions and to avoid undesired process disturbances and process shutdowns.
• Overall prioritization helps to maintain plant safety with optimum productivity.
• Generally, an alarm means something which grabs the attention to alert the operator.

Classification of Alarms Systems based on priority:

A.   High Priority Alarms:

• As the name High Priority suggests, greater attention needs to be given to avoid the cause of serious damage.
• If the operator doesn’t respond to such alarms then it may lead to
  • Huge Loss to living beings such as humans or animals.
  • Affects the environmental conditions by polluting the ground, water, air, etc.
  • May damage costly equipment and cause huge economic losses

B. Medium Priority Alarms:

After High Priority Alarms, the next importance needs to be given to medium priority alarms.

But, these alarms are not critical like high-priority alarms, if neglected they may cause

• It lightly affects environmental conditions
• It affects or reduces the life span of an equipment
• Significant economic loss
• But there is no loss to human or animal

C. Low Priority Alarms:

These alarms don’t cause any hazard or damage to processes, plants, and to operators.

But if neglected it may cause

• Minor economic loss

D.  Journal Priority Alarms:

These alarms don’t generate any notification for the operator, but it will be logged in the event for recording purpose.

What is alarm shelving?

• Alarm shelving is an act of suppression of an alarm.
• Alarm shelving helps the operator suppress or shelve the nuisance alarm.
• Nuisance alarms distract the attention of an operator.
• To know about alarm shelving, let’s see various types of nuisance alarms that operators come across on day to day basis in a process plant.

A.   Chattering Alarms:

• These alarms frequently transit between alarm state and normal state in a minimum time period.
• These alarms come and go immediately so that the operator doesn’t get time to acknowledge or accept it.
• These alarms create more noise in the system.

B.   Fleeting Alarms:

• A fleeting alarm is another type of nuisance alarm that appears and disappears immediately.
• The time duration of these alarms is very short for the operator to act upon.
• The main difference between a fleeting alarm & chattering alarm is that a fleeting alarm will not repeat immediately.

C.   Stale alarms:

• These are the type of alarms that remains on operators’ screens for days, weeks, months, and even years.
• These alarms will not disappear even after corrective action is taken by the system operator.
• These alarms appear each and every time when the operator opens the alarm page.

What is alarm suppression?

• Alarm Suppression is defined as removing or abolishing of unnecessary alarms from the list.
• Alarm suppression can be classified as
  1. Jitter suppression: uses alarm continuity analysis to allow the device not to report the alarm if a fault is stable for a short time period.
  2. Correlation suppression: these use the alarm correlation rule to minimize the number of reported alarms

Classification of alarms Based on nature:

Based on nature, Alarms are categorized into:

Fault alarm:

1. A fault alarm is an alarm that is informed by the network management crew when the System has faults or exceptions.
2. After processing faults or exceptions, the alarms are cleared.

Event alarm:

1. An event alarm is an alarm produced to inform the transient state during the operation of the equipment.
2. Event alarms are initiated periodically. Event alarms indicate that the system is in an alarm state and, therefore, does not require handling.

Components and Properties of Alarm Management:

Alarm text:

1. An alarm text includes detailed information about that alarm.
2. The character formats assisted by the suitable operator panel are used for formatting an alarm text on a character basis.
3. The alarm text includes output boxes for present values of the labels or text lists.
4. The label image maintains the instantaneous value at the time the alarm status changes.

Alarm number:

The alarming number is used for managing the alarm internally.

Each alarm number is unique within the following alarm types such as :

1. Discrete alarms,
2. Analog alarms,
3. HMI system alarms,
4. Controller alarms within a CPU.

Alarm class:

1. The alarm class represents whether the alarm must be acknowledged or not.
2. The alarm class is used to determine the appearance of an alarm in an operator panel.
3. The alarm class directs where the corresponding alarm needs to be recorded.

Alarm group:

If an alarm belongs to a group of alarms, it can be established with another alarm of the same category within a single operation.

Procedure for Alarm management:

1. Management systems must ensure for operation, maintenance, and modification of alarms in a controlled manner.
2. The functionality of an alarm system should be evaluated and monitored to ensure effective operation during normal and abnormal conditions of the process plant.

Steps for alarm management system:

1. A design and guidance document should be prepared to define a plant standard that uses a best-practice alarm management methodology.
2. Analyze an alarm system to determine its strengths and weaknesses, and plan a practical solution for effective improvement.
3. About 50% of the total alarm load normally arises from a few alarms.
4. The methods to make them work correctly are documented are applied with minimum effort and maximum performance improvement.
5. A complete review of an alarm system ensures that each alarm meets the principles and philosophy of good alarm management.