Installation

The environmental engineer selects the specific points of the stack for sampling to ensure that the samples represent the material being discharged or controlled. The en-

Cross Section Pitot Tube

FIG. 5.11.3 Properly constructed type S pitot tube. (a) End view: face-opening planes perpendicular to transverse axis, (b) Top view: face-opening planes parallel to longitudinal axis, (c) Side view: both legs of equal length and center lines coincident; when viewed from both sides, baseline coefficient values of 0.84 can be assigned to pitot tubes constructed this way.

FIG. 5.11.2 Type S pitot tube manometer assembly.

FIG. 5.11.3 Properly constructed type S pitot tube. (a) End view: face-opening planes perpendicular to transverse axis, (b) Top view: face-opening planes parallel to longitudinal axis, (c) Side view: both legs of equal length and center lines coincident; when viewed from both sides, baseline coefficient values of 0.84 can be assigned to pitot tubes constructed this way.

FIG. 5.11.5 Minimum number of traverse points for particu-late traverses.

FIG. 5.11.4 Proper pitot tube with sampling probe nozzle configuration to prevent aerodynamic interference. (a) Bottom view: minimum pitot nozzle separation, (b) Side view: the impact pressure opening plane of the pitot tube located even with or above the nozzle entry plane so that the pitot tube does not interfere with gas flow streamlines approaching the nozzle.

gineer determines these points after examining the process or sources of emissions and their variation with time.

In general, the sampling point should be located at a distance equal to at least eight stack or duct diameters downstream and two diameters upstream from any source of flow disturbance, such as expansion, bend, contraction, valve, fitting, or visible flame. (Note: This eight-and-two-diameter criterion ensures the presence of stable, fully developed flow patterns at the test section.) For rectangular stacks, the equivalent diameter is calculated from the following equation:

Equivalent diameter = 2 (length X width)/(length + width)

Next, provisions must be made to traverse the stack. The number of traverse points is 12. If the eight-and-two-diameter criterion is not met, the required number of traverse points depends on the sampling point distance from the nearest upstream and downstream disturbances. Figure 5.11.5 shows how to determine this number.

The cross-sectional layout and location of traverse points are as follows:

1. For circular stacks, the traverse points should be located on two perpendicular diameters as shown in Figure 5.11.6 and Table 5.11.1.

2. For rectangular stacks, the cross section is divided into as many equal rectangular areas as traverse points so that the length-to-width ratio of the elemental area is between one and two. The traverse points are located at the cen-troid of each equal area as shown in Figure 5.11.6.

Radial Vane Flow Straightener
FIG. 5.11.6 Traverse point locations for velocity measurement or for multipoint sampling.
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