Dwyer DPM - Manual (Page 29)

Particulate Monitoring Systems
Installation & Operating Manual
Document No. 210-1003-M Page 22 ©2007
6.8 Alarm Levels for Fabric Filter Applications
For fabric filter applications it is recommended to set two alarm levels either using the internal alarms
and or using the optional 4-20mA output signal that is sent back to a PLC or other recording system.
One alarm should be set based on the average base line reading and another alarm should set based on
the peak readings following cleaning cycles.
Normally, Alarm #1 is used for detecting sustained increases in the base line reading. For example a
baghouse that has new, highly-efficient filters may have an average baseline reading of 10-20pA. It
would then be recommend to set the baseline alarm at 30-50pA with an alarm delay time that was
long enough so that cleaning cycle peaks did not activate the baseline alarm. Each application can be
different (for example much higher readings are possible with larger, older baghouses) and each plant
may have different operating demands in terms of how sensitive the alarms should be set. It is,
therefore, recommended to initially set the alarm as low as possible and to trend and data log the
readings over time before finalizing the settings. Correlations to stack test data can also be
incorporated to correlate the output to actual mass concentration to set more quantitative alarms.
Normally, Alarm #2 is set to detect changes in the peak readings caused by the filter cleaning cycles.
Recall that as filters just begin to tear or become porous, the momentary puffs of particulate emissions
that normally occur just after a cleaning cycle will increase in peak height and duration (peak width).
Essentially the cleaning cycle amplifies the existence of small tears. Thus, setting an alarm to detect
changes in the peak emissions is often referred to as a Pre-Visible Alarm as it is the best, and most
reliable, approach to detecting emissions before become visible. When a sustained increase in the
baseline level occurs, particulate emissions will likely be visible and the filters should be changed
immediately. Where as, when only the peak emissions have increased, emission will likely not be
visible and there likely would be time to schedule changing the filters (i.e. early warning).
Telephone or on-site assistance is available to provide suggestions in setting alarm levels.