This is an interesting question that we hear in the field regarding bag leak detection systems. A good number of facilities have optical detectors (i.e. continuous opacity monitoring) installed on their dust collection systems. They often ask us whether or not they should consider replacing the opacity meter with the triboelectric unit or if they would see more benefits by using both in parallel.
Let us first consider the difference between the two technologies and their individual capabilities and then review why we recommend replacing the opacity detectors entirely with triboelectric technology. (That's actually a change specifically endorsed by USEPA at least for certain applications. Check out our timeline of BLDS technical evolution to learn about how Auburn led the development of technology that has improved the reliability and accuracy of monitoring.)
Differences Between Optical and Triboelectric Monitoring
The primary difference between the two technologies involves the method for detecting particles suspended in the passing airstream.
Optical detectors measure the transparency of the passing air as an indicator of particle concentrations. They do this by shooting a beam of light between two mirrors at opposite ends of a duct. As the light returns a portion of it is scattered by any particles within the airstream. They then produce a reading of opacity, or how much light pass through the given section of the air. Once an opacity reading is taken, a calculation is made to determine a mean particle concentration. (Note: Opacity does not correlate consistently across all industries…10% opacity in one application may indicate greater particle concentration than 10% in another application)
In contrast, triboelectric detectors measure the amount of current generated by dust particles as they pass (AC signal) or impact (DC signal) the insulated metal probe. By unifying the two signals and then processing them the triboelectric detector generates a signal output that directly measures particle concentrations.
Example of 20%, 40% and 80% opacity emissions.
Why Use Triboelectric Detectors Over Optical Methods
Triboelectric dust detectors have far greater detection sensitivity than optical technologies.
Opacity meters can only detect particle concentrations once they reach at least 10% opacity (equivalent of 0.0010 g/dscf). While in the past many facilities’ air permits allowed emissions of up to 20% opacity, this is no longer the case, especially in facilities covered by MACT standards that require emissions of 10% opacity or LOWER. This effectively means that by time the opacity meter registers anything the system has already exceeded its emissions limits. Additionally, opacity meters require careful calibration to operate reliably and are sensitive to changes in process or atmospheric conditions. Mirrors can become dirty, low light levels impair the readings and even simple misalignment can disturb accuracy. And, since the bottom line is always a consideration, they are expensive - often costing several tens of thousands of dollars for a single installation.
Triboelectric dust detection technology has none of these drawbacks. It requires no calibration, and is essentially blind to varying process and atmospheric conditions. Most importantly, triboelectric detectors are sensitive enough to detect particulate emissions down to 0.000002 g/dscf roughly 20 times more sensitive than the best opacity detectors. And they have no difficulty detecting small particles down to less than 1 micron in size and it has a detection response time of than than 1 second from detection to output. This allows triboelectric to observe gradual deterioration in system performance allowing preemptive maintenance to avoid reportable incidents and costly downtime. Tribo detectors are much more economical - a difference that allows for multiple points of installation within the same project budget. This provides invaluable diagnostic information to help maintenance quickly locate areas of deteriorating performance.
With its clear advantages, triboelectric technology offers lower acquisition cost, lower operating cost and greater accuracy than continuous opacity monitoring. The increased sensitivity allows for operators to track emissions trends as they rise and long before they reach reportable levels. That prevents unexpected system downtime, lower overall emissions and more careful process control. Additionally, with greater insight in to the realtime function of the dust collection system maintenance planners can better maintain and operate their collectors ensuring longer filter life and quick resolution when issues arise.
Any facilities currently using optical methods such as continuous opacity monitoring should consider switching over entirely to triboelectric detection systems. Trying to maintain both systems concurrently will result in no performance improvement and will fail to reduce maintenance workload.
Optical methods once provided a clear advantage to dust collector operators when no other detection technology existed beside smoke reading. However, with the advent of dual band triboelectric detection technology, continuous opacity monitoring systems no longer are state-of-the-art. USEPA has updated it's requirements under many demanding MACT standards to reflect a triboelectric requirement. With several key advantages, triboelectric systems can now provide additional capabilities on their own that optical methods cannot match.
Want to learn more about how triboelectric technology can help improve operation at your plant? Contact us today for a free consultation!