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Define porthole3/27/2023 If necessary, an alternative location may be selected, at a position at least two stack or duct diameters downstream and a half diameter upstream from any flow disturbance. Sampling and/or velocity measurements are performed at a site located at least eight (8) stack or duct diameters downstream and two diameters upstream from any flow disturbance such as a bend, expansion, or contraction in the stack, or from a visible flame. It is suggested that the effective measurement path (1) be totally within the inner area bounded by a line 1.0 meter (3.3 ft) from the stack or duct wall, or (2) have at least 70 percent of the path within the inner 50 percent of the stack or duct cross-sectional area, or (3) be centrally located over any part of the centroidal area. These are the most stringent and require the longest length of straight stack or duct. Point CEMS: It is suggested that the measurement point be (1) no less than 1.0 meter (3.3 ft) from the stack or duct wall or (2) within or centrally located over the centroidal area of the stack or duct cross section.Here are what the regs call out to help insure a representative sample:ĬEMS: It is suggested that the measurement location be (1) at least two equivalent diameters downstream from the nearest control device, the point of pollutant generation, or other point at which a change in the pollutant concentration or emission rate may occur and (2) at least a half equivalent diameter upstream from the effluent exhaust or control device. ![]() Each device has its own placement criteria. Laminar flow is the goal rather than trying to take a reading where stratification or turbulence might exist which could give spurious readings. In order to get good readings from any stack monitoring equipment and pass a Relative Accuracy Test Audit (RATA), the location where the sample is taken needs to be as representative as possible to the total emissions of the affected facility. ![]() This is called out in Title 40, Appendix B to Part 60 – Performance Specification 1 (Opacity), Performance Spec 2 (CEMS), and Appendix A (Flow). The EPA spells out very clearly where ports for Continuous Emissions Monitoring System (CEMS) probes, Flow Probes and Continuous Opacity Monitoring (COMS) systems should be located on a stack or duct. This issue looks at the regulatory requirements for probe placement on a stack or duct for compliance systems.
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