SOURCE TESTING ASSOCIATION I Annual Guide 2022
The installation of dust monitors over recent years has at many sites been problematic. The monitors have been installed and ranged with little idea of monitoring location requirements, how the process operates and the expected concentrations. It is not uncommon for an analyser to be installed with a large range to cover that very high peak that may or may not occur when the typical dust concentration is extremely low. This results in an analyser indicating zero even when the dust level is increasing and will not indicate until the increase is substantial.
The introduction of BS EN 14181 (Stationary source emissions - Quality assurance of automated measuring systems) and supplemented by BS EN 13284-2 (Quality assurance of automated measuring systems) gave a method of comparing components in the stack gas with SRM tests for both gaseous and particulate matter. Whilst this method has been reasonably successful for gaseous compounds where the analysers, both SRM and Continuous Emission Monitors (CEMs) can be challenged with test gases of known concentration and tolerance before running parallel tests to determine a calibration function, the same is not possible for dust.
Dust within a stack has many characteristics such as size, shape and colour, each of these characteristics will cause the dust monitor response to act differently. Calibrating a dust monitor using the standard BS EN 14181 requires a series of parallel tests over consecutive days with stable operating conditions and ideally with differing operating loads across the test days. In an ideal world this will give a series of parallel data points that can be used to establish a calibration function as defi ned in BS EN 14181. Where dust is present, variable and reactive with dust monitor response, a test house should be able to form a correlation between the manual extractive tests and the analyser output. If the data is successfully tested against the BS EN 14181 criteria a calibration function and valid calibration range can be established.
Plant operating today with modern bag fi lter abatement will often have an emission limit value (ELV) of 10 – 20mg/m³. With a well maintained system the concentration within the duct at actual conditions is often sub 1mg/m³. With an installed dust monitor of the correct type, certifi ed range and instrument range where there is little to no dust, the analyser response is going to be minimal. The outdated equipment used for particulate matter SRMs even with extended test periods will most likely show a random set of results of varying low values. In this instance it is most probable that the modern laser type dust monitor is the most reliable way to indicate an increase or change in dust concentration. What is unknown when the dust monitor output changes is what the change means.
Attempts to calibrate particulate monitors according to the requirements of BS ISO 10155 and BS EN 14181 have in some circumstances demonstrated only that the particulate matter is very low and that no calibration is possible. The option then, in agreement with the regulatory authority, is to identify the instrument monitor as indicative.
Most indicative monitors tend to be sited downstream of bag fi lter abatement plant. Under normal operating conditions the dust concentration is very low - typically well under 5 mg/m3 - and the dust monitor is characterised to respond to fi ne dust. If, as the bag fi lter deteriorates, the bag fi lter continues to pass fi ne dust, the instrument would respond accordingly. However, bag fi lter failure can be catastrophic allowing large particles to pass, to which any monitor, indicative or quantitative, would respond very differently.
Should conditions change and the instrument response reaches 15-20% of monitor output, the actual concentration within the duct could be much greater than zero. Greater confi dence in this approach can be gained if additional manual isokinetic tests are undertaken at a slightly elevated monitor response to establish that at this elevated response the dust concentration within the duct is still compliant. The monitor response should at all times be recorded and plotted on a proven control chart that clearly indicates response levels when action must be taken.
We must remember that even a sophisticated particulate CEM measures changes of light as a reaction to the varying particulate levels and nothing more. We fi nd ourselves spending more and more time, measuring less and less….!
Find out more about the Source Testing Association at:
www.s-t-a.org
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