Reference method for measuring releases of particulate from stationary sources: method B

Method B: Determination of Stack Gas Velocity and Volumetric Flow Rate

• 2.1 Applicability
• 2.2 Principle
• 2.3 Apparatus
• 2.4 Procedure
• 2.5 Calculations
• 2.6 Nomenclature

2.1 Applicability

This method is not applicable when one or more of the following conditions exist:

• cyclonic or reverse flow has been confirmed (refer to Method A);
• gas velocity pressures are outside the normal operating range of the velocity measuring device; or
• velocity pressures fluctuate more than 20% of the average at each traverse point.

2.2 Principle

The average gas velocity in a stack or duct is determined from the gas density and from the measurement of velocity pressure with an S-type (Stausscheibe or reverse type) pitot tube. A standard picot tube may be used where plugging of the tube openings due to particulate matter and/or moisture is not likely to occur. Stack gas volumetric flow rate is determined from measurements of stack gas velocity, temperature, absolute pressure, dry gas composition, moisture content, and stack diameter.

2.3 Apparatus

The following items are required:

S-type pitot tube. The S-type pitot tube should be constructed according to the specifications shown in Figure B-1. The face-opening planes shall be perpendicular to the tube transverse axis and parallel to the longitudinal axis of the tube. Both legs shall be equal in length and the centrelines of the tubes shall be coincident when viewed from the sides. In other words, P_a=P_b and 1.05 D_t ≤ P ≤ 1.50 D_t where D_t is between 0.48 and 0.95 cm (3/16 to 3/8 in). Slight misalignments or deviations from the specifications in Figure B-1 are permitted, provided the deviations are recorded when the pitot tube is calibrated.

Figure B-1: Design Specifications for an S-type Pitot Tube

Standard pitot tube. A standard pitot tube may be used where particulate or condensates do not plug the static and pressure impact openings. The specifications of a standard pitot are shown in Figure B-2.

Figure B-2: Design Specifications for a Standard Pitot Tube

Probe assembly. In many cases an S-type picot tube will be used in conjunction with a thermocouple and sampling probe. These assemblies should be constructed according to Figure B-3 to minimize the aerodynamic interference among the components of the assembly. The pitot tube coefficient for this assembly shall be determined by applying the calibration procedures described in Method F.

Figure B-3: Configuration of Pitot Tube, Sampling Nozzle, and Thermocouple

In the probe assembly, the tip of the temperature sensor must not contact any metal. The tip of the nozzle must extend at least two nozzle diameters below the tip of the upstream leg of the S-type picot tube.

Differential pressure indicator. The device, such as an inclined manometer, must be capable of measuring the pitot tube velocity pressure to within 0.1 mm (0.005 in) water (H₂0) on the 0 to 25 mm (0 to 1 in) H₂O scale, and 1 mm (0.05 in) H₂O on the 25 to 250 mm (1 to 10 in) H₂O scale. The device must be calibrated against a primary standard before the test.

Stack Temperature Sensor. A calibrated thermocouple or other suitable temperature sensor capable of measuring the stack temperature to within 1.5% of the minimum absolute stack temperature. To minimize aerodynamic interaction, the thermocouple must be attached to the pitot tube as shown in Figure B-4. When stack temperatures exceeding 370°C (700°F) are encountered, appropriate shielding and aspiration shall be provided for the thermocouple to avoid heat radiation effects.

Figure B-4: S-type Pitot Tube and Manometer Assembly

Barometer. A barometer capable of measuring atmospheric pressure to within 2.5 mm Hg (0.1 in Hg). The device must be calibrated against a primary standard before being used. Alternatively, the uncorrected atmospheric pressure provided by the local weather office may be used with an adjustment for the elevation of the sampling site.

Gas density determination equipment. The density of the stack gas shall be determined using Method C (dry molecular weight) and Method D (moisture content).

2.4 Procedure

Set up the apparatus as shown in Figure B-4 using an S-type picot tube or probe assembly that has been calibrated according to Method F. The pitot tube must be used in the stack or duct in exactly the same configuration as it was calibrated in the wind tunnel.

Leak check the pitot tube and lines by blowing into the pitot impact opening until 76 mm (3 in) H₂O is registered on the differential pressure indicator such as an inclined manometer. Close off the impact opening. The pressure shall remain constant for at least 15 seconds. Similarly, leak check the static pressure side by applying a suction on the static opening until at least 76 mm (3 in) H₂0 is obtained on the manometer.

Level and zero the manometer. Since the manometer level and zero may drift due to vibrations and temperature changes, periodic checks must be made during the traverse. Record all relevant data in Figure B-5.

Figure B-5: Velocity Traverse Data Sheet

Measure the barometric pressure and record on the Velocity Traverse Data Sheet (Table B-1).

Measure the static pressure in the stack. Align the S-type pitot tube face-opening planes parallel to the gas flow. Seal the openings between the port and the pitot tube. Connect only one leg of the pitot tube to the manometer and vent the other side of the manometer to the atmosphere. Record the static pressure on the Velocity Traverse Data Sheet.

Measure the velocity pressure and temperature at each of the traverse points. Ensure that the face-opening planes of the S-type pitot tube are maintained perpendicular to the longitudinal axis of the stack or duct. The differential pressure gauge must be compatible with the range of measured Δp values. Record the velocity pressure and temperature at each point (Table B-1). When the velocity pressure fluctuates, an average velocity pressure must be recorded. However, if the velocity pressures fluctuate more than 20% around the estimated average, then the measurement is considered to be unacceptable. Determine the stack gas molecular weight and moisture content using Method C and Method D, respectively.

2.5 Calculations

Absolute stack gas pressure at the sampling site is calculated using Equation B-1.

Actual stack gas velocity at each traverse point is calculated using Equation B-2.

The average stack gas volumetric flow rate on a dry basis at reference conditions is calculated using Equation B-3.

2.6 Nomenclature

A_s

inside cross-sectional area of the stack or duct, m ²

B_wo

proportion by volume of water vapour in the stack gas, dimensionless

C_p

pitot tube coefficient, dimensionless

M_s

molecular weight of stack gas on a wet basis, kg/kmol

Δ p

stack gas velocity pressure, kPa

Δ P_s

stack static pressure at each traverse point, kPa

P_bar

barometric pressure at sampling site, kPa

P_ref

reference pressure, 101.3 kPa

P_s

absolute stack gas pressure, kPa

Q_s

volumetric stack gas flow rate on a dry basis at reference temperature and pressure conditions, m ³/h

T_ref

reference temperature, 298 K

T_s

stack gas temperature at each traverse point, K

(T_s)_avg

arithmetic average of the stack gas temperatures, K

U_s

stack gas velocity at each traverse point, m/s

(U_s)_avg

arithmetic average of the stack gas velocities, m/s

128.95

dimensional constant (m/s) [(kg/kmol/K)] ^½

3600

conversion factor, seconds per hour, s/h