Increased dust levels in bulk solids processing plants can create issues if they are not detected early. Issues like exposure to safety risks for employees, reduced process efficiency, material loss, annoyances to housekeeping and maintenance procedures, and damage to equipment like blowers and collectors. If the problem becomes big enough there could also be issues with neighbors and local regulations.
Dust Monitoring and the Triboelectric Effect
One way to monitor dust levels and prevent the mentioned issues from becoming major concerns is the use of a particle emission monitor to continuously monitor dust levels in the ducts. There are various types of dust level monitors (also known as broken bag detectors or particle emission monitors) available.
One of the more cost-effective, reliable and easy to operate varieties is the triboelectric-based monitor. The triboelectric effect is based on particles interacting with an electrically isolated sensing probe. When moving particles impact or even pass in close proximity to the probe, a small electrical charge is transferred from the particulate to the probe. Advanced monitors take this electrical signal and process it through a series of algorithms. The signal processing filters out other electrical charges, or electrical “noise,” which is not representative of the moving particles. These proprietary algorithms effectively differentiate undesirable signals from the desirable signals, resulting in reliable particle emissions detection. Triboelectric technology has been used for dust monitoring for many years and is an accepted technology by the United States Environmental Protection Agency (EPA) for dust particle emission detection. With the advanced algorithms to filter “noise” that are now offered by some of the providers, the triboelectric monitor is as reliable as ever.
In operation, dust particles flowing in the airstream of the duct collide with the monitor’s probe, creating an electrical charge. The dust monitor converts this charge to a particle emission signal and continuously monitors and analyzes the signal. When the particle emission signal exceeds a preset level, the relay output activates an alarm to alert the operator or sends a signal to a process controller. To prevent false signals, the electronics initiates a predetermined time delay. If the signal remains at that level or higher over the time delay’s duration, the relay output activates. If the dust levels decrease during the time delay period, indicating a false signal or possibly a brief high dust event like during the collector’s filter cleaning cycle, the time delay prevents the monitor from triggering the alert or sending a signal to a controller.
Detection and Trend Monitoring
A triboelectric dust monitor works well as a broken bag detector in the exhaust ducts of a filtration system. Some of the dust monitors are also available with a continuous trending feature that allows plant operators to identify changes/patterns in dust emission levels and optimize the facility’s filtration systems. The operator can also record the activity and provide reports with the trend monitor.
Triboelectric dust monitor with quick-connect mounting coupling
A company in the Midwest that processes furnace slag granules to produce high-quality slag cement was looking for a way to continuously monitor their filtration systems for early evidence of potential dust issues/events. They also needed to be able to provide the monitoring reports to their local regulators.
The company operates under strict guidelines set by the EPA and the states in which they operate. Their environmental program strives to go beyond just what is required and they work hard to try to meet expectations for clean, safe, and efficient plants for the local communities.
In order to process the slag, the material needs to be pneumatically conveyed (by pressure or vacuum) to the mill, then to the storage bins, and then to be filled into the railcars, tanker trucks, or other forms of transportation. Each of these conveying stages can create a lot of dust and filtration systems are incorporated at each step.
The company decided to go with Monitor Technologies’ DustTrend ES particle emission trending monitor. They located the sensors on the exhaust sides of their filter systems. The sensors provide analog signals to their PLC so that they can continuously monitor the dust levels in their filter systems, as well as capture the data and provide reports. The trend monitors have aided in achieving the goals of their environmental program, in addition to keeping their filtration systems running efficiently.
Dust emission trend monitor installed in exhaust area of dust collection system
Installation and Operation
The triboelectric monitor is usually installed on the desired duct with a quick-connect coupling. This makes it easier to install, as well as to remove for cleaning purposes.
Once the unit has been installed it can be calibrated. Some of the latest models of monitors provide operators the flexibility of automatic setup or customization of settings, depending on the particular application. The automatic setup usually consists of simply pushing the setup button during a normal “clean” condition in the duct. After approximately 15 to 20 minutes of analyzing the situation, the monitor will automatically set the alert signal ranges and time delays based on proven standards. However, for those applications that require custom alerts and/or time delays latest models include a configuration software that allows operators to input the specific settings they desire. The software can also provide a current “view” of the dust activity within the duct.
Software can provide “view” of dust activity in a duct.
Selection and Installation Considerations
There are some factors to consider in the selection of an appropriate dust monitor probe and in the installation location specific for your application. Working with a monitor supplier on the below factors can help ensure you receive the proper unit for your dust and process conditions:
- Will the monitor be used to provide alerts of increasing dust levels or used to view and capture data on dust level activity?
- What dust levels are you trying to detect? Are you looking at dust levels before or after the dust collection systems? Most of the time a dust monitor would be placed after the dust collection system. This would help detect issues with the collection system like leaking or broken filters and could help to determine optimal time to replace filters. A dust monitor can also be placed at some point in the process to help control product quality. If the dust conditions get too high while processing the product, it could indicate that the product is out of specifications.
- Conditions of the location (process and/or collector). Try to avoid or take appropriate care for areas with heavy vibration, high moisture, or extreme temperatures (process or ambient).
- Properties of the dust, including particle size, abrasiveness, corrosiveness, bulk density, conductivity, moisture content, etc.
- Duct shape, size, and material of construction
- Air volume through the duct
- Hazardous location conditions
A dust monitor can help prevent potential dust issues from becoming critical situations, assist in providing dust activity reports, and aid in increasing the overall efficiency of a filtration system, like increasing filter life and reducing equipment maintenance. The triboelectric-style dust monitors with advanced algorithms are reliable options to achieve your goals, especially if you take into consideration the available product features and installation location(s) that best match your application needs.