Reduce Chattering MonAnL High Alarms in PCS 7 APL V9.0
For MonAnL in PCS 7 APL V9.0, use justified PV_Hyst hysteresis and, where appropriate, coming/going alarm delays to reduce repeated high-alarm episodes caused by a fluctuating value. Inspect Feature bit 8 before changing delay inputs: the high alarm uses PV_AH_DC/PV_AH_DG when separate delays are enabled, rather than the shared PV_A_DC/PV_A_DG pair. Confirm the actual alarm sequence from trends and events, and verify that genuine sustained excursions remain visible within the required response time.
Table of Contents
- Confirm That the Hit List Represents Chatter
- Method 1 — Apply Hysteresis
- Method 2 — Use the Correct Coming and Going Delays
- Signal Smoothing Is a Separate Decision
- Verification
- Limitations and Operational Precautions
- Engineer FAQ
- Technical References
- Related Articles
- Related Tags
Confirm That the Hit List Represents Chatter
A high hit-list count is a reason to investigate, not proof that the sensor itself is faulty. Correlate the individual incoming/outgoing alarm events with a process-value trend, the configured high limit and the block's active indication. Repeated threshold crossings can produce repeated alarm episodes when the configured hysteresis and delays permit the condition to clear and recur.
Distinguish genuine process excursions, measurement noise and configuration effects before suppressing messages. The reported count of 2539 alarms is an installation observation; it is not a manufacturer capacity or a guaranteed count for a particular waveform.
Method 1 — Apply Hysteresis
For MonAnL in APL V9.0, Siemens documents PV_Hyst as hysteresis for PV alarm, warning and tolerance limits.
- Record PV_AH_Lim, PV_Hyst, the process engineering units and the existing alarm philosophy.
- Review the observed fluctuation and determine a justified hysteresis in those physical units. Do not choose a universal percentage or treat hysteresis as a substitute for sensor repair.
- Configure PV_Hyst at the block or authorized faceplate location.
- In an approved test, drive the value into alarm and back through the recovery region, then inspect the alarm-state transitions.
Hysteresis separates the conditions for entering and clearing the alarm. Because this parameter applies to multiple limit categories, review the other configured limits as well. APL manual, p. 97 and MonAnL I/Os, p. 453
Method 2 — Use the Correct Coming and Going Delays
The correct high-alarm delay inputs depend on Feature bit 8 in the verified APL V9.0 MonAnL implementation.
With separate delays enabled, PV_A_DC/PV_A_DG serve the low-alarm side; their names do not include an extra L. Do not assume that changing PV_A_DC necessarily changes the high alarm in every instance.
- Inspect the installed block version and Feature bit 8.
- Select the appropriate incoming and outgoing parameters from the applicable configuration.
- Specify justified delay times in seconds. The documented default of 0 disables the respective delay.
- Test both a short excursion and a sustained alarm condition, then test clearing.
Incoming delay postpones alarm reporting; outgoing delay postpones clearing. Both can affect operational response and require process approval. APL manual, pp. 169, 197–200 and 441
Signal Smoothing Is a Separate Decision
APL V9.0 MonAnL also documents SmoothTi, a first-order process-value smoothing time; SmoothTi = 0 passes the value without that smoothing. Filtering changes signal dynamics, rather than merely separating alarm-entry and recovery conditions.
Do not introduce smoothing solely to improve the hit list. Review which value the actual application monitors and uses downstream, then evaluate the delayed response in an isolated test. No particular smoothing time or reduction in alarm count is established for this installation.
Verification
Use a consistent observation interval before and after the approved change. Compare alarm incoming/outgoing timestamps, the relevant PV trend, PV_AH_Act and the configured limit/delay state. Confirm that sustained high conditions still generate a timely alarm and that clearing behaves as intended.
Inspect all affected limit categories after changing shared hysteresis or shared delay settings. A lower hit-list count is useful only when the remaining alarms still provide the required process information; it is not sufficient proof by itself.
Limitations and Operational Precautions
Parameter names and Feature bit behavior here are verified for APL V9.0. Confirm the installed MonAnL library version before transferring the configuration to another release.
Retain existing values and a rollback record. Coordinate changes with the alarm philosophy and operations team, particularly where delayed annunciation affects operator response. Do not disable alarm messages or widen process limits merely to hide a frequent alarm, and investigate genuine process or measurement problems separately.
Engineer FAQ
Does every crossing necessarily produce a new alarm?
Not as a universal rule. The observed sequence depends on the configured hysteresis, coming/going delays and whether the alarm actually clears and recurs.
Which delay input controls a high alarm?
In APL V9.0 MonAnL, inspect Feature bit 8: shared mode uses PV_A_DC/PV_A_DG; separate mode uses PV_AH_DC/PV_AH_DG.
Technical References
- Siemens — Advanced Process Library V9.0 Function Manual, March 2017, A5E39147331-AA — limit hysteresis, p. 97; separate-delay Feature bit 8, p. 169; alarm delays, pp. 195–200; MonAnL behavior, pp. 441–443; I/Os, pp. 452–453.