826 PARTICULATE EMISSIONS
Very often more points are required if the flow is highly non-
uniform or if the sampling point is near an elbow or other
flow disturbance. Figure 5 shows the EPA adjustment for
flow nonuniformity.
Velocity Measurement Velocity measurements in dusty
gases are made with a type S (special or staubscheibe) pitot
tube, shown in Figure 6, and a draft gage manometer. Gas
velocity is given byVC gh (^2) LLrr/g
where
V = gas velocity
C = pitot tube calibration coefficient. This would be
1.0 for an ideal pitot tube, but type S tubes deviate con-
siderably.
g = acceleration of gravity
h L = liquid height differential in manometer
L = density of manometer liquid
g = gas density.
It is necessary to measure the temperature and the pres-
sure of the gas stream and estimate or measure its molecular
weight in order to calculate density.
Gas Analysis For precise work gas composition is needed
for three reasons (1) so that molecular weight and gas density
may be known for duct velocity calculations, (2) so that duct
flow rates at duct condition can be converted to standard-
ized conditions used for emission specifications. Standard
conditions are usually 70°F, 29.91 in. mercury barometric
pressure, moisture free basis with gas volume adjusted to
TABLE 4
Required traverse points
Code Duct sizes Number of points
EPA^8 2 ft dia. 12 minimum
More according to Figure 2 if near flow disturbance
WP^17 <2 ft^2
2–25 ft^2
25 ft^2
4
12
20 or more
APCD^14
and
ASTM^15
1–2 ft^2 (rectangular)
2–12 ft^2
12 ft^2
1–2 ft dia.
2–4 ft
4–6 ft
6 ft
4
6–24
24
12
16
20
24 or more
These numbers should be doubled where only 4–6 duct
diameters of straight duct are upstream.
ASME^16 <25 ft^2
25 ft^2
8–12
12–20
Double or triple these numbers for high nonuniform flow.
MINIMUM NUMBER OF TRAVERSE POINTS
NUMBER OF DUCT DIAMETERS DOWNSTREAM
(DISTANCE B)
DISTURBANCE
SAMPLING
DISTURBANCE
FROM POINT OF ANY TYPE OF
DISTURBANCE (BEND, EXPANSION, CONTRACTION, ETC.)
NUMBER OF DUCT DIAMETERS UPSTREAM*
(DISTANCE A)
SITE
A
B
(^0) 23 4 5 6 7 8 9 10
10
20
30
40
50
0.5 1.0 1.5 2.0 2.5
FIGURE 5 Sampling points required in vicinity of flow distur-
bance (EPA).
TUBLING ADAPTER
PIPE COUPLING
STAINLESS STEEL TUBLING
FIGURE 6 Type S Pitot tube for use in dusty gas
stream.
12% CO 2. Some codes differ from this, however. (3) For iso-
kinetic sampling moisture content at stack conditions must
be known in order to adjust for the fact that probe gas flow is
measured in a dry gas meter at ambient conditions.
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