Stack Gas Velocity Taking into Account Velocity Decay Near the Stack Wall – USEPA Method 2H

USEPA Method 2H is used in conjunction with USEPA Methods 2, 2F, and 2G to account for velocity decay near the wall in circular stacks and ducts.  A wall effects adjustment factor is determined and is used to adjust the average stack gas velocity obtained under USEPA Method 2, 2F, or 2G to take into account velocity decay near the stack or duct wall. 
 
The method contains two possible procedures:

• a calculational approach which derives an adjustment factor from velocity measurements
• a default procedure which assigns a generic adjustment factor based on the construction of the stack or duct.

The calculational procedure derives a wall effects adjustment factor from velocity measurements taken using USEPA Method 2, 2F, or 2G at 16 (or more) traverse points specified under USEPA Method 1 and a total of eight (or more) wall effects traverse points specified under this method.  The calculational procedure based on velocity measurements is not applicable for horizontal circular ducts where build-up of particulate matter or other material in the bottom of the duct is present.
 
When the calculational procedure is conducted as part of a relative accuracy test audit (RATA) or other multiple-run test procedure, the wall effects adjustment factor derived from a single traverse (i.e., single RATA run) is applied to all runs of the same RATA without repeating the wall effects measurements.  Alternatively, wall effects adjustment factors are derived for several traverses and an average wall effects adjustment factor applied to all runs of the same RATA.
 
A default wall effects adjustment factor of 0.9900 for brick and mortar stacks and 0.9950 for all other types of stacks and ducts is used without taking wall effects measurements in a stack or duct.
 
This method is not applicable for testing stacks and ducts less than 3.3 ft (1.0 m) in diameter.