STACK MONITORING OR DUST MEASUREMENT IN ISOKINETIC SAMPLING

By this method dust measurement or stack monitoring can be calculated in any straight ducts or ducts having laminar flow flow by developing isokinetic condition in equipment.

Standard Operating Procedure for Particulate Matter Determination

1. Pre-sample Activity

Weigh the properly conditioned thimble/filter and place it into the clean, air tight container. Designate appropriate label or ID No. to each thimble/filter container.

Stack Monitoring – Material and Methodology for Isokinetic Sampling Field activity starts with the collection of detail information’s from the industry about the products, raw materials, fuels, and stack dimensions.

2. Traverse Point Calculation

Calculate the traverse point and accordingly mark the distance from tip of the nozzle, on pitot tube and probe. Do not forget to add the collar length of port to the calculated traverses.

3. Composition of Flue Gases

Determine flue gas composition by orsat apparatus or multi gas analyzer. In case of Orsat analysis gas sample has to be collected in tedlar bag / non reactive bladder and allowed to cool before analysis. Gas analysis by multi gas analyser may be performed by direct insertion of sampling probe inside the stack and simultaneous estimation of all the components in pre-calibrated gas analyser. At least 3 observations should be taken for average percentile composition. Use gaseous composition data to calculate dry molecular weight of flue gas (M_d).

Determine the Dry molecular weight (M_d) by following equation.
M_d = 0.44 (%CO₂) + 0.32 (% O₂) +0.28(% N₂ + % CO) +……

4. Measure ambient temperature (ºC) and Barometric pressure in mm Hg

5. Check the leak in sampling instruments

The sampling train after having set up will be tested for leakage by plugging the inlet. The rotameter shall not give a reading beyond 5 lpm when the flow has been set 100 lpm. Also the dry gas meter should give a reading of less than 5 percent of the air flow.

6. Moisture Determination

Moisture can be determined by condenser method , in principle, involves extracting a sample of the stack gases through a filter for removal of the particulate matter, then through a condenser accumulating the condensate formed in process, and finally through a gas meter. The objective of the test is to collect and measure the volume of all the condensate formed at the condensing temperature from a measured amount of gas.

Calculation

Calculate equivalent vapour of condensate under sampling condition, m³
V_v = [ (V_c × 22.4) / (1000 × 18 ) ] × [ T_m / 273 ] × [ 760 / (P_bar - P_m) ]

Where:
V_v= Equivalent vapour of condensate under sampling condition, m³
V_c = Volume of condensate in condenser. ml
T_m=Absolute meter temperature, ºK
P_m=Suction at meter, mm mercury column
P_bar =Barometer pressure, mm mercury column
Calculate the moisture content of the gases using the following equation:
B_wo = [ Vv / (V_v + V_m)]
M = [ V_v / _{(V_v} + V_m)] × 100
B_wo= Proportion by volume of water vapour in stack gas.
M= Moisture in the flue gases, percent
V_v= Equivalent vapour volume of condensate under sampling condition.
V_v= Volume of gas sampled (m³)

7. Wet Molecular Weight (M_s Determination

This equation can be used to determine the molecular weight of the stack gas on a wet basis

_Ms = M_d (1 – B_wo ) + 18 B_wo
M_d = molecular weight of stack gas on dry basis, kg / kg –mole

8. Determine stack Gas velocity Pressure (ΔP) and Stack Temperature (T_s)

Check and adjust the upper miniscus of manometer fluid at zero.

Connect +ve and –ve end of the pitot tube in respective points.

Slowly insert the pitot and thermocouple upto the first traverse mark inside the stack. Keep the positive end towards the direction from which flue is coming. Hold it for stabilisation. Take the reading of fluid displacement in manometer and temperature.

Repeat the same in next traverse mark and so on.

Take average reading for Δ P and T_s

For measurement of static gas pressure pitot tube should be rotate by 90 Degree from the position of actual Δ P measurement. This would provide better accuracy

9. Determination stack Pressure (absolute stack gas pressure)

For the static pressure determination requires first to disconnect the positive end of the pitot tube then take the reading of velocity pressure at the traverse point in which the calculated average Δ P matches closely. For measurement of static gas pressure pitot tube Should be rotated by 90o from the position of actual Δ P measurement. This would provide better accuracy.

Calculate Ps
P_s = P_bar ± ( Δ P_s / 13.6 )
Where:
P_bar = Barometric pressure in mm mercury column
Δ P_s = Stack gas velocity pressure, mm water column
_Ps = Static pressure mm Hg column.
Density of Hg = 13.6

10. Stack Gas Velocity Determination (U_S)

Connect pitot tube to the stack for velocity determination, calculate the stack gas velocity at all the traverse point by using the following formula. Consider the density factor for correction of velocity pressure and Δ P_s to convert water column manometer.

U_s = K_p× C_p× ( Δ P ) ^1/2 × [T_s / (P_s × M_s) ] ^1/2

Where
U_s = Stack gas velocity, m/s
K_p = Constant
_Cp= type pitot tube coefficient.
T_s = absolute stack gas temperature, ºoK
ΔP = Stack gas velocity pressure, mm water column
P_s = Absolute stack gas pressure, mm Hg
M_s = Molecular weight of stack gas on wet basis, Kg / Kg –mole

11. Determination of Volumetric Flow rate / Discharge (U_S)

The following equation is used to calculate stack gas volumetric flow rate (m³/hr).
Q_s = 3600 (U_S) × A_S × A_S (1-B_WO) × [ T_ref / T_S ] × [ P_S /P_ref ]

Where
A_S = Area of the stack (duct), m₂
B_WO = Proportion by volume of water vapour in stack gas.
T_ref = 298 ºK
P_ref = 760 mm
T_S = Absolute stack gas temperature, ºK
P_S = Absolute stack gas pressure

12. Determination of Flow Nozzles

Select the nozzle size, in such away that sampling rate a meter shall not exceed 70 % of pump capacity in any case. Cross sectional area of nozzle (mm) for different diameter is as follow:

Rs = (U_S * An) * 60 * 1000
Where,
R_S = Sampling Rate at nozzle, LPM
U_S = Stack gas velocity, m/sec
An = Area of nozzle, m2
60 = Conversion Factor Seconds to Minute
1000 = Conversion Factor m³ to Litre

13. Determination of Sampling rate at gas meter

The meter for measuring the gas sample measures the gas at conditions of temperature, pressure and moisture content which are different than those in the flue. Therefore, calculate the sampling rate at the gas meter for each sampling points before starting the test and record on the log the required rate (Table 1).

Calculate the sampling rate at the gas meter as follows:

_Rm = Rs × (Tm / Ts) × [ (P_bar - Ps) / (P_bar - Pm) ] × [ Vm / (V_m + Vv) ]
Where,
Rm = Flow rate through meter, m³/s

Rs = Sampling Rate at nozzle, LPM
Tm = Temperature at metering condition, ºK
Ts = Absolute stack gas temperature, ºK
Ps = Absolute stack gas pressure, mm mercury column
Pbar = Barometer pressure, mm mercury column
Pm = (Pm1 – Pm0) / 2 Suction at meter, mm mercury column
Vm = Volume of gas sampled at meter conditions, m³
Vv = Equivalent vapour volume of condensate at meter conditions, m³
Note: Take initial reading of vacuum guage (Pm0) in mm Hg at the staring of
sampling and final vacuum pressure (Pm1) in mm Hg just before putting off the pump when sampling is complete. Calculate average difference in suction
pressure, referred as Pm

14. Start the test after the sampling rate has been calculated and train assembled and checked for leakages. When equipment is ready in all respect, record the initial dry gas meter reading and push the sampling probe carefully into the duct to the point nearest to the back wall. Take the sample appropriately as per the requirement and with all the necessary precaution.

15. Determination of volume of gas sampled

Calculate the volume of gas sampled using the following equation:

V_std = V_m × Y × [ (P_bar - P_m) / 760 ] × [ (273 + 25 ) / (T_m + 273) ]
Where,
T_m = Temperature of gas at dry meter condition, ºC
V_m = Volume of gas sampled at dry gas meter conditions, m³
(P _bar - P_m) = Actual pressure in sampling train, mm mercury column.
P_m = Static pressure in sampling train, mm mercury column
P_bar = Barometeric pressure in sampling train, mm mercury column.
Y = Calibration factor of dry gas meter.

16. Sample Recovery

After the sampler has cooled, brush down the dust on the side of the nozzle carefully into the thimble using a small brush remove the thimble and replace it in same labeled container. In the case of filter holder is kept outside during the sampling, the dust from the sampling probe before the filter holder should be brushed down into the filter.

17. Determination of dust concentration

Determine the mass of dust collected in the thimble by difference i.e weighing the thimble before and after the run. Dry the thimble in an oven for about 2 hours at 120 ºC prior to sampling. After sampling, cool, dry and again weigh the thimble along with dust maintaining the same condition as prior to sampling.

Calculate the dust concentration using the following equation:

E_m = [ (W₂ - W₁) × 1000 ] / V_std
Where,
E_m = Dust Concentration in mg /Nm3, ( 25 ºC, 760 mm Hg, dry basis)
V_std = Volume of dry gas through the meter (25 ºC, 760 mm Hg), Nm3
W₁ = Initial weight of filter paper in gram
W₂ = Final weight of filter paper in gram

18. Correction of result at 11% O₂

O₂correction is only carried out, if O₂ corrections > 11%. For O₂ < 11% no correction is allowed. It’s require to correct the value for the 11% O₂by following formula

E_s = [ (21 - O_s) / (21 - O_M] × E_M

Where,
E_s = calculated emission concentration at the standard percentage oxygen concentration
E_M = measured emission concentration
O_S = standard oxygen concentration
O_M = measured oxygen concentration

19. Determination of Emission Rate

Calculate the dust emission rate as follows:

Dust Emission rate (Kg/hr) = (E_m × Q_s) / 10⁶

Where,
Qs = Flue gas flow rate (25 ºC, 760 Hg mm Hg), Nm³ / hr.
Note: Report the concentration as corrected at 11 % O₂

_TABLE-1

_{Plant Name & Address:}

_{Date & Time:}

_{Ambient Temperature:}

_{Barometric Pressure (mmWG):}

_{Moisture in Flue gas (%):}

_{Flue gas composition:}

_{Filter No and weight (Initial as well as Final):}

Δ P = Stack Gas Velocity Pressure, (mm water column), Ts = Stack temperature (ºK),
Ps= Static pressure (mm water column), Us = Velocity of stack gas (m/s),
Qs = Volumetric Flow Rate/ Discharge, Rs = Flow at nozzle (LPM),
Pm = Vaccum Pressure Drop (mm mercury column),
Rm = Determination of sampling rate at gas meter. (LPM),
Vstd = Determination of volume of Gas Sampled

Other Required Information
1. Feed rate
2. The nature, composition and quantity of the material being incinerated during monitoring
3. installed and operating capacity of the incinerator
4. No. of sample Port
6. Internal Diameter of the stack
7. Nozzle size selected for sampling
8. Pitot tube constant
9. ID fan capacity
10. Pollution control equipment installed and its status

SN	Dia. of Nozzle (Inches)	Cross Sectional Area(m2)
1	5/8	1.9783 X 10^4
2	3/4	2.8487 X 10^4
3	1/2	1.2661 X 10^4
4	1/4	3.16531 X 10^5
5	1/8	7.9132 X 10^5

Thursday, 7 March 2019

STACK MONITORING OR DUST MEASUREMENT IN ISOKINETIC SAMPLING