Many Process Operators install a dust monitor in their stack or emission point to monitor dust emissions, either for regulatory purposes, corporate responsibilities (ISO 14001) or for local regulatory demand. However in many cases the monitor only gives an indication of high dust levels based around an emission limit value or a pre-determined warning alarm level. This is quite normal, but does it give the “whole picture?”
When operating a Baghouse, the dust levels are a direct function of the efficiency of the filter system. This efficiency is in itself a function of the performance of the filter media and the cleaning system used to keep the media working correctly.
Most bagfilters use a reverse jet cleaning system where a high pressure pulse is used to blow-back into the filter bags to remove the inbuilt dust “cake”. It is at this time of “pulsing” that dust is instantly created and becomes part of the emission. This dust pulse has a very short duration but can have a very high level compared to the normal emissions when the bagfilter is not being cleaned.
The size, shape, duration and presence of these cleaning pulses offer an insight into the efficiency of the fabric media being used in the bagfilter. This “visible” information offers instant on-line indication of:
- Fabric media performance/deterioration – determines filter life and increased emissions
- Leaking or broken bags – leaking bags are typically a pre-cursor to broken bags, helps determine maintenance intervals.
- Correct reverse jet pulse system operation – indication of faulty timer/sequencer/ PLC driver
- Sticking or non-opening pulse valves – identifies faulty pulse valves for repair.
And of course helps to keep emission below pre-set levels and avoid unwanted dust excursions.
All this from a Dust Monitor? Well “YES” if the monitor has the ability to display the real-time fast moving dust pulses created from reverse jet cleaning. Most Dust Monitors do not do this, however, most PCME Dust Monitors do offer this “graphical” display of the cleaning pulses, either as standard (from PCME’s extensive range of advanced control units) or as an add-on option of a “Filter Display Module”.
It should be noted that trying to use the 4-20mA output to manage the same job is problematic as the 4-20mA is often not dynamic enough (to visualise the fast moving dust pulses) and is often averaged to coincide with emission data and not filter performance.
Alternatively, real-time data can be displayed on a PC screen using PCME’s “Dust-Tools” software. By using PC software, more data analysis can be undertaken, showing long term trends of filter performance, media wear rates and help schedule filter maintenance intervals more precisely against actual media wear rather than typical regular timings regardless of actual filter media performance. This capability has been proven by site operators to reduce costs associated with bag replacement costs and reduction of labour costs.
Being able to visually identify the performance of the filter, especially over time to track trends greatly helps the filter maintenance teams identify problems with the bagfilters before they become major, and by default keeping emission levels at reduced levels.
“Seeing is therefore believing”, if you know what to look for!
Figure 1: (Click on images to enlarge)
As the chamber goes “off-line” to clean, the isolating valve closes. Typically there would not be any dust passing the sensor and no pulse information from the cleaning cycle, however, the peaks seen during the offline cleaning sequence are a result of the valve after the chamber not sealing fully.
If the valve was working correctly, then no emission peaks would be measured by the dust sensor.
The large peak after the off-line cleaning sequence is due to the valve re-opening and letting out all the built-up dust in the chamber.
The slow rise in the dust levels after the cleaning sequence indicates deteriorating bags in the chamber. This is further verified by the height of the peaks identified during the off-line mode, ironically from a leaking isolating valve.
Schematic of 8–chamber (25 rows per chamber) baghouse, with dust sensors monitoring the emission from each chamber. The chambers are cleaned both “off-line” and “on-line”, determined by the operator based on load and filter media performance.
The on-line cleaning sequence creates one peak per row which is shown in the graph. Some of the peaks are much higher than the others (1 and 21) indicating a broken bag in that row. Other bag rows also show “leaking bags” which are a pre-cursor to broken bags. The graphical visualisation of all the rows being cleaned brings to life the performance of each individual row of bags facilitating predictive action by maintenance teams, reduction in filter down-time, fewer bag replacements and ultimately lower emissions.
For further information on how PCME’s products may help you manage your bag filters more effectively and how to analyse your emission data, please contact PCME Ltd: