Do I Need a BLDS If I Already Monitor Differential Pressure?

Posted by Justin Dechene on Jul 30, 2015 6:15:00 AM


Differential pressure monitors have been standard equipment on dust collection systems for decades. Differential pressure, or DP, was often regarded as the key metric for dust collector operation. However, some think it is the only data source required or even the only available. This is old thinking 30+ years out of date. 

Today, many modern dust collection systems also employ Bag Leak Detection Systems to provide operators and technicians the most relevant and valuable data possible. In addition, BLDSs are even more important than ever as many newer dust collection systems must operate under stricter emissions standards to comply with air permits issued by local, state or federal agencies. Newer air permits often explicitly or implicitly require the use of a BLDS in addition to differential pressure monitoring. 

For a modern dust collection system it is helpful to have both systems in place as they provide complimentary data and diagnostics.

How Differential Pressure Readings are Used

Dust collector differential pressure (also known as pressure drop, or Delta P) is the difference in vacuum pressure between the two sides of a dust collector. Its principal use is to determine the level of dust buildup on the filters, since more dust buildup means more resistance to airflow. DP can be used to diagnose a number of different issues within the system. For example, DP provides a reliable guide for triggering the cleaning cycles of the unit. Rapidly changing DP levels can also indicate the presence of upset conditions in the system or other problems. DP can be used to determine whether the filters have become blinded and need to be replaced.

While monitoring DP is important, on its own it cannot provide a 100% clear picture of what is happening inside the system, nor can it direct operators to the ultimate source of problems. Additionally, while in the past DP was often used to gauge collection efficiency (and thus emissions levels for air permits), DP readings cannot be relied on fully to confirm the system is meeting emissions standards. By the time leaking filters affect a noticeable change in DP the leak will be so severe that it will be a visible emission point.

While DP is a valuable data metric for operators to monitor, it clearly has its limits. Further, as emissions standards become stricter, DP is increasingly not a reliable guide for monitoring emissions compliance.

What is a Bag Leak Detection System and How is It Used on a Dust Collection System?

A triboelectric bag leak detection system is used to measure the concentration of dust particles in an airstream. It does this by using an electrically isolated metallic probe inserted into the ductwork and then measuring the signals generated when dust particles pass by or impact the probe. By means of the triboelectric effect, a BLDS is powerful enough to detect concentrations of sub micron-sized particles down to as little as 0.005 mg/m. Being able to monitor emissions levels this accurately in real-time is immensely valuable to system operators and maintenance personnel - it provides powerful predictive warning capability rather than the typical indicative alarm.

BLDSs are primarily used to carefully monitor the collection efficiency of a dust collector and to ensure compliance with air permits. Beyond this primary function, the same data can also be used in a number of ways. For example, data gathered from the BLDS can alert operators when a set of filters is beginning to leak (or even weakening - before a leak occurs) indicating the need to replace aged filter bags or for the need to upgrade the system to achieve greater collection efficiency. Sudden changes in emissions levels can indicate problems in the dust collector such as massive bag leaks from upset conditions.

Benefits of Using a BLDS in Conjunction with Differential Pressure

Monitoring differential pressure can only go so far. It can start operators and maintenance staff on the path to finding problems, but requires follow up action to confirm the cause of the readings. Rather than relying on just one data metric (DP), many benefits can be had by using BLDS along with DP readings.

Using the data obtained from a BLDS along with differential pressure enables system operators and maintenance personnel to gain a greater awareness of operating conditions in a dust collection system. It also aids them in identifying problems that DP alone cannot accurately diagnose.

Example: DP sensors indicate a DP of less than 1” in an fully operational baghouse. Judging by only the DP, a number of possibilities present themselves, such as: (1) The filters were not installed correctly and are now leaking. (2) The bags or unit structure have suffered a massive failure (fire, acid flash, etc.) and are now leaking on large scale. (3) The DP sensors are returning false readings.

If the only data available are the DP readings, technicians will need to manually investigate each of these possibilities in order to determine the source of the problem. This will involve a considerable commitment of time and resources, including checking the DP sensors and airlines, shutting down the unit and manually inspecting it and performing a dye leak test on the filters. Additionally, units operating at high temperatures, containing hazardous materials, or that require confined space safety measures will only increase the investment required to locate the problem.

Whereas the system equipped with a BLDS can eliminate several of these possibilities immediately. In the example cited above, plant personnel could analyze the emissions data from the BLDS recorded both before and after the low DP reading was first recognized. If the emissions data shows a sudden spike in total particulate emissions right after the DP readings first dropped, it would be reasonable to conclude that a sudden filter failure has occurred. Or if the emissions levels stayed relatively stable overtime despite the low DP it would mean that the system is operating normally and indicate that the DP sensors are providing false readings. By using the data from the BLDS in conjunction with the differential pressure readings, plant personnel would be able to quickly identify the problem and immediately begin planning to fix it without having to first manually investigate multiple different possible causes.

Further, when tribo probes are installed upstream from the main stack (e.g. baghouse and compartment level) the problem can be quickly located to save further time, and most importantly avoid downtime.

BLDS + Differential Pressure Monitoring = Best Results

While monitoring differential pressure in a dust collector is useful, it does have its limitations. Incorporating a triboelectric bag leak detection system will enable plant operators and maintenance personnel to realize savings in several areas:
  • Maintenance staff will be able to diagnose problems quicker and resolve them more efficiently, thereby saving time and resources.
  • Installing a BLDS can eliminate the need to manually monitor and record DP gauge readings
  • Operators will have a more accurate picture of how the dust collection system is operating. This will enable them to make better operating choices and avoid unscheduled system shutdowns by scheduling preventative maintenance better.
  • Staying in compliance with air permits will become easier and more reliable. Any issues effecting emissions levels can be immediately identified and repaired before they become problems that could result in fines or shutdowns.

For these reasons and more, many dust collector operators, manufactures and regulators have begun making bag leak detection systems an integral part of their systems in combination with differential pressure monitoring. The smart choice is to install a bag leak detection system and not rely only on differential pressure readings. Want more info?  Check out our prerecorded webinar on predictive monitoring.

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Topics: Baghouse Maintenance, Dust Detection