Encyclopedia of Environmental Science and Engineering, Volume I and II

1100 STACK SAMPLING

12 inches (0.3 m) in diameter or 113 in^2 (0.071 m^2 ) cross-

sectional area. Sampling ports must not be within 2 duct

diameters downstream or half a diameter upstream from

any flow disturbance. Sampling points are then determined

by dividing the stack into equal area sections as shown in

Figures 2 and 3. A table is provided in the Method which

gives the percentage of the stack diameter from the inside

wall to each traverse point. For stacks greater than 24 inches

in diameter, no point should be closer than 1 inch from the

wall; for smaller stacks, no closer than 0.5 inch.

Once these criteria are met measurement of the direction

of flow is made to insure absence of significant cyclonic

flow. The angle of the flow is determined by rotating a

Type S pitot tube until a null or zero differential pressure is

observed on the manometer. The angle of the pitot tube with

the stack is then measured. This procedure is repeated for

each sampling point and the average of the absolute values

of the angles calculated. If the average angle is greater

than 20º, the sampling site is not acceptable and a new site

must be chosen, the stack extended, or straightening veins

installed.

A few unusual cases have been accounted for by the

method. If the duct diameter or size is smaller than that

required by Method 1, Method 1A can be used. For cases

where 2 equivalent stack diameters downstream or a half

diameter upstream are not available, a directional velocity

probe can be used to determine the absence of cyclonic flow,

as described in Method 1. If the average angle is less than

20º, then the sampling site is satisfactory; however a greater

number of points must be used.

Test Method 2

Test Method 2 is used to determine the average velocity in a

duct by measuring the differential pressure across a Type S

(Stausscheibe) pitot tube. The Type S pitot tube is preferable

to the standard pitot tube when there are particles that could

cause plugging of the small holes in the standard pitot tube.

Measurement sites for velocity determination are chosen as

described in Method 1, that is, required number of sites and

absence of cyclonic or swirling flow.

The type S and standard pitot tubes are shown in Figure 4.

When the Type S pitot tube has been correctly manufactured

and installed on a probe as shown in Figure 5, there is no

interference and calibration is not necessary. A pitot tube

constant of 0.84 is assumed. If the criteria for interferences

are not met, the method discusses the necessary calibration

procedures.

DUCT DIAMETERS DOWNSTREAM FROM FLOW DISTURBANCE* (DISTANCE B)

(^02)
10
20
30
40
50
0.5 1.0 1.5 2.0 2.5
(^345) 67 8 (^910)
20
24 OR 25a
8 OR 9a
16
12
STACK DIAMETER = 0.30 TO 0.61 m (12-24 in)
STACK DIAMETER > 0.61 m (24in)
FROM POINT OF ANY TYPE OF DISTURBANCE
(BEND, EXPANSION, CONTRACTION, ETC.)
aHIGHER NUMBER IS FOR RECTANGULAR STACKS OR DUCTS
DUCT DIAMETERS UPSTREAM FROM FLOW DISTURBANCE (DISTANCE A)
MINIMUM NUMBER OF TRAVERSE POINTS
DISTURBANCE
MEASUREMENT
SITE
DISTURBANCE
B
A
FIGURE 1 Minimum number of traverse points for particulate traverses.
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