Annual Guide 2022 I SOURCE TESTING ASSOCIATION
Advancement in this type of technology led to the development of back scattered laser devices which allow for a much larger measuring path to be used, and has proved a major advancement in measuring of low dust concentrations in large ducts. Back scattered laser devices are again mounted on one side of the duct and have the ability to cope with higher temperatures as the measuring device is mounted on the outside of the duct wall. These laser devices as with all dust monitors still have a requirement to be characterised with comparison to a manual isokinetic test.
The original dust monitors were based on transmission of light across a duct and this was in the main expressed as opacity. Opacity monitors installed on plant were in many cases no more than a car lamp bulb and light receiver (single pass) and most of the time provided no useful information at all.
As dust monitors evolved the opacity values were expressed as transmission or a logarithmic expression of transmission know as extinction. The dust expressed as a logarithmic expression of transmission enabled the dust monitor response to be displayed linearly, particularly in the lower portions of the extinction range. With dust levels measured in the 10s of mg/m³ calibrations could be achieved with success. Higher levels of dust could be achieved by de-tuning of ESPs to give a range of concentrations from the plant that when regressed with normal operating concentrations, gave a calibration which when applied to the dust monitor output gave a reliable instrument response across a wide range of operating conditions. This type of calibration was adopted using the standard ISO 10155 (Stationary source emissions-Automated monitoring of mass concentrations of particles-Performance characteristics, test methods and specifi cations) and was used across different industries including power generation, cement industry, brick production, refi neries and many more.
As the emission limits for dust were reduced, instrument manufacturers were required to produce monitors that could reliably trend the reduced levels. Whilst transmission was the main technique used for measurement it was less successful at lower concentrations. A new technique was introduce in the form of the forward scattered laser. This technique used a laser to refl ect light off the dust particles, the refl ected light or stray light was measured by the analyser and converted into a measured response. These analysers were mounted from one side of the duct and would measure the concentration in a small section of the duct.
The advantage of this technology was that for the fi rst time that a true zero could be achieved on the stack, opposite to transmission where, as the dust levels rise in the measuring volume, the light produced increases giving a greater resolution in measurement. The response is still monitored against an isokinetic extractive test to relate the analyser output to a dust concentration for the whole duct. This type of analyser proved to be successful across a range of concentrations giving a reasonably linear response to the changes in concentration. An analyser of this type is particularly successful when used in smaller ducts i.e. less than 2m diameter.
The advancements in dust measuring analysers have enabled dust to be reliably monitored at very low concentration levels. Advancement in the Standard Reference Method (SRM) has been documented and updated in the latest versions of the Standards with the addition of many quality control features placed on the equipment used and the personnel undertaking the testing. The issue that is becoming apparent with manual isokinetic testing for dust, is that the equipment used is virtually unchanged since its introduction many years ago. There has been some basic changes such as the increased use of in- stack fi lters, fi lter conditioning, improved pump control, data collection of peripheral measurements, increased sample times, accredited companies and personnel. When these improvements are consistently implemented the end result should show improvement but it is still limited by the basic equipment used. It has to be considered that at present a good quality dust monitor can give the most reliable trend of the concentration within the duct and react to the changes in concentration.
The trend in itself is meaningless until it is related to a manual isokinetic test and if the manual test is not reliable the analyser data is compromised.
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