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Rectangular Ducts Wall Effects Method (12/18/03)

EPA has released the optional rectangular duct wall effects test method, developed by RMB as part of an EPRI research project, as Conditional Test Method 41 (CTM-041) for use on rectangular stacks or ducts. This method is available for use with Part 75 Acid Rain and NOX Budget units.

CTM-041 may be used starting in the quarter that the written request is submitted. Before using the new method, a source must petition EPA. The Designated Representative (DR) or Authorized Account Representative (AAR) can submit a written request for use to cover all of the units for which the DR or AAR is responsible.

EPA anticipates approval of each request. The Agency, however, has reserved all sections in the original version of the method included in the EPRI final report that applied to ash build-up has stated that all approvals will be conditioned with the following language: "The rectangular duct cross-sectional area may not be adjusted to account for any build-up of particulate matter or other material in the bottom of the duct when calculating stack gas volumetric flow." While this does not allow a source to both use CTM-041 and take credit for any blockage in the duct due to ash build-up, this repesent a softening from the EPA's original position that the measurement-based approach in the method would be disallowed wherever an ash layer was present.

CTM-041 includes a number of improvements over the wall effects test method approach in EPA Reference Method 2H. CTM-041 establishes a one time test that may be performed at any load although at least three runs must be performed in comparison to the one run required by Method 2H. The wall effects adjustment factor (WAF) is also applied as a cross-sectional area adjustment that is subsequently applied both to any reference method as well as CEMS data. The conditional method includes procedures to determine unit specific default WAF and a reduced measurement-based approach based on the logarithmic-overlap law.

EPA requires units using CTM-041 to follow special EDR reporting instructions that were also released with this method. EPA recently updated MDC to incorporate rectangular duct wall effects. Copies of CTM-041, the reporting instructions, FAQs, and a spreadsheet that can be used to calculate the wall effects results are available at our FTP Library.

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Last Revised: March 31, 2005