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Use of Default WAF Limited to 12 or 16 Point Flow Traverses (12/17/99)

In response to a recent question concerning Method 2H default wall-affect factors (WAFs), EPA stated that the default WAFs can be applied to Method 1 traverses consisting of 12 or 16 points. EPA stated that applying default WAFs to Method 1 traverses of greater than 16 points might "overcorrect" for velocity decay. A copy of the WAF policy question/answer can be obtained from our FTP Library.

The justification for EPA limiting the use of the default WAF to 12 or 16 point flow traverses is that "the default WAF values specified in Method 2H (i.e., 0.9900 for brick and mortar stacks and 0.9950 for all other types of stacks) were derived [albeit, ultraconservatively] based on field data from 16-point Method 1 traverses." While this statement is true, a more flexible approach could also be suggested based on the field test data. The test data from both the EPA and EPRI flow studies overwelmingly show that the near-wall velocity decay tends to only be a significant factor within the first 6 to 8 inches from the wall. Beyond this near-wall region, certainly beyond 12 inches, the flow measurements are indicative of the bulk stack flow. Understanding this, one could propose that the default WAF could be extended to flow traverses consisting of more than 16 points if the nearest of those traverse were 12 or more inches from the stack wall. Permitting the extended use of the default WAF, would not penalize sources that would opt to use a greater number of traverse points to obtain a more precise characterization of the bulk stack flow. Disallowing the default WAF creates a disincentive to sources that might otherwise choose to measure additional traverse points.

However, as a practical matter, the wall effects measurements only take about an hour and usually result in greater corrections than the default factors. Utilities that are interested in the most precise wall correction factors should consider making the measurements.

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Last Revised: December 19, 1999