Configure limit alarms

Limit alarms monitor a process value (PV) by comparing it against four configurable setpoints: HiHi, Hi, Lo, and LoLo. When the PV moves beyond one of these limits, the corresponding alarm becomes active. An alarm is raised only when the PV exceeds a limit. Merely matching the limit exactly does not activate the alarm.

Each limit of the four limits are enabled and configured independently. Configurable parameters include:

• The specific numeric limit

• A priority level

• An alarm message for the operator to see at runtime

  

HiHi and LoLo alarm limits are intended as critical safety thresholds. Hi and Lo limits can be used to notify operators that a value is of potential concern and to provide an early warning.

Limit alarms also support value and time deadbands. The deadbands , which can be used to prevent nuisance alarms, such as those that may occur when a process variable oscillates slightly around a setpoint.

Alarm deadband

The alarm deadband applies only when determining whether an active alarm can reset to normal. It has no effect on alarm activation.

• An alarm activates immediately when the PV exceeds its configured limit.

• After activation, the PV must move back into the normal range beyond the deadband buffer before the alarm resets.

Reset Condition

• Hi and HiHi alarms:

  The PV must fall below the configured limit minus the deadband value.

• Lo and LoLo alarms:

  The PV must exceed the configured limit plus the deadband value.

Time deadband

The time deadband applies only to raising the alarm condition. It does not affect resetting the alarm to its normal state (RTN).

• When PV exceeds the limit, the time deadband starts a countdown. An alarm is raised when the time deadband expires if the PV remains out of normal range.

• Once the PV returns to the normal range, the alarm resets.

Example

• HiHi alarm limit is 90.

• Value deadband is 5 seconds.

Behavior:

• If PV exceeds 90, the time deadband starts counting down. At the end 5 seconds, provided the PV is still more than 90, the HiHi alarm is triggered and a HiHi alarm is raised.

• After PV falls below 85 (limit minus deadband value), the HiHi alarm resets (RTN).

The timestamp on the InAlarm attribute when a limit alarm becomes active or inactive is the most current timestamp of the corresponding PV value at the time of the event. If there is no timestamp associated with the alarmed value, the AppEngine timestamp is used instead.

Sample sequence of events

Using the parameters described above (HiHi alarm is set to 90, alarm deadband is 5, and time deadband is 5 seconds), the following sequence of events describes how a HiHi alarm could be triggered. This sequence of events assumes that no PV values, other than the ones described here, have been received.

1. PV is less than 90, so the HiHi alarm is inactive.

2. PV increases to 90.1. Because it has exceeded the limit, the time deadband starts counting down. HiHi alarm immediately activates (InAlarm = true) when the time deadband reaches 0.

3. PV decreases to 87. The HiHi alarm is still active because PV is still above the HiHi limit, minus the alarm deadband.

4. PV decreases to 84.9. The alarm deactivates because PV is now below the HiHi alarm setpoint minus the deadband. The time deadband does not affect RTN timing.