Introduction
During the late 1990’s, the US EPA developed three new methods to account for these different
variables in flow measurement. These new methods allow the tester to account for velocity drop-off
near the stack or duct wall and the yaw and pitch angles of flow. The primary users of the new
methods will be owners and operators of utility units subject to the Acid Rain Program under Title IV
of the Clean Air Act. Flow rates must be used to determine the units’ SO
2
and NO
X
mass emissions
and heat inputs. The XC-3D55 was designed to be utilized with Methods 2, 2F, 2G and 2H.
US EPA Method 2G allows Type S probes and 3–D probes (DAT and spherical) to be rotated into the
flow to measure total velocity pressure and yaw angle. The yaw angle is used to calculate ‘‘near-
axial’’ velocity from total velocity.
Method 2F allows 3–D probes to be used to measure total velocity, yaw angles, and pitch angle
pressure. Pitch angle pressure is used with a calibration curve to determine pitch angle. Yaw and
pitch angles are used to calculate axial velocity from total velocity.
Method 2H provides a procedure for accounting for wall effects by using either a default wall effects
adjustment factor or one derived from near wall measurements. The wall effects adjustment factor is
used with the Method 2-, 2G- or 2F-calculated velocity to derive a wall effects adjusted velocity.
Volumetric flow rate is calculated by multiplying the average flue gas velocity by the stack or duct
cross-sectional area. Yaw and pitch characterize the extent ot which flue gas is not flowing straight
out of a stack or duct. From the standpoint of a tester facing a vertical stack, a yaw angle is
represented by flow movement to the left or right of the stack centerline. The pitch angle is
represented by flow movement toward or way from the tester.
Some amount of yaw and pitch angle and wall effects are almost always present in utility stacks or
ducts. Flow disturbances produce flue gas flow that swirls and /or bounces off stack or duct walls.
Determining axial velocity without accounting for the drop-off near the stack or duct wall can result in
overstating the actual axial velocity. Thus, when enough yaw, pitch or wall effects are present,
Method 2 can overstate the measured flue gas velocities and thus, volumetric flow, because it only
allows the total velocity to be measured and does not account for yaw angles, pitch angles, or wall
effects. If the test method overstates flow rate, a flow rate monitor calibrated using the test method
may also overstate flow rate and result in overstated sulfur dioxide emissions and heat input.
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