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# Analyzer Sampling

##### Stack Monitoring

Authored by: D. H. F. Liu , B. G. Lipták

# Instrument and Automation Engineers’ Handbook

Print publication date:  October  2016
Online publication date:  November  2016

Print ISBN: 9781498727686
eBook ISBN: 9781315370323

10.1201/9781315370323-4

Types of sample

#### Analyzer Sampling

 Types of sample Gas-containing particulates Standard design pressure Generally atmospheric or near atmospheric Standard design temperature −32°C to 815°C (−25°F to 1500°F) Sample velocity 120–3000 m (400–10,000 ft) per min Materials of construction 316 or 304 stainless steel for pitot tubes; 316 or 304 stainless, quartz, or Incoloy for sample probes Costs Probes only in 1–3 m (3–10 ft) lengths with glass, quartz, or stainless steel lining—from $1,300 to$2,500; $10,000 to$15,000 for a complete EPA particulate sampling system (Reference Method 5) Partial list of suppliers AMP, EPA Method Sampling System, (2015) http://www.ampcherokee.com/v/vspfiles/photos/C002.0002-2T.jpg Apex Instruments, Clean Air Europe: http://www.alpha.cleanaireurope.com/page.php?u=produit_ventes&idprod=235 Environmental Monitoring, http://www.em-monitor.com/IsokineticSamplers.html Inventys Inc. http://www.inventys.in/particle8.html Keika Ventures, http://www.keikaventures.com/productinfo.php?product_id=1174 Mirion Technologies (https://www.mirion.com/search-results/?q=stack+samplers) New Star Environmental, http://www.newstarenvironmental.com/product/nsm9096-mini-stack-sampler-a Sierra Monitor Corp. (http://www.sierramonitor.com/protect/all-fire-gas-products/gas-sensor-accessories) Teledyne Analytical Instruments, http://www.teledyne-api.com/manuals/07318b_602.pdf Thermo Scientific, Particulate Monitoring (2015) http://www.thermoscientific.com/en/products/particulate-monitoring.html

#### Introduction

Stack gas sampling has already been discussed in Chapter 1.2 in connection with Figures 1.2ae and 1.2af. In this chapter, the emphasis will be on particulate sampling by EPA Method 5 and in that connection, the topics of traverse point locations and pitot tube designs will be emphasized.

Some stack gas samplers (Figure 1.3a) are provided with microcomputer controls and perform the sampling automatically.

Fig. 1.3a   Isokinetic sampler. (Courtesy of Environmental Monitoring.)

#### The EPA Particulate Sampling System

A complete Environmental Protection Agency (EPA) particulate sampling system (Reference Method 5)*

EPA, Method 5—Determination of particulate matter emissions from stationary sources, (2011) http://www3.epa.gov/ttnemc01/promgate/m-05.pdf.

is comprised of four major subsystems:
1. A pitot tube probe or pitobe assembly used for temperature and velocity measurements and for sampling
2. A two-module sampling unit that consists of a separate heated compartment with provision for a filter assembly, and a separate ice-bath compartment for the impinger train and bubblers
3. An operating–control unit with a vacuum pump and a standard dry gas meter
4. An integrated, modular umbilical cord that connects the sample unit and pitobe to the control unit

Figure 1.3b is a schematic of an EPA particulate sampling train (Method 5). As shown in the figure, the system can be readily adapted for sampling sulfur dioxide (SO2), sulfur trioxide (SO3), and sulfuric acid (H2SO4) mist (Method 8).

#### Microprocessor-Controlled Stack Sampling

In these sampling packages, a microprocessor directs the automatic sampling method, which can be selected to follow U.S. EPA Method 5

EPA, Method 5—Determination of particulate matter emissions from stationary sources, (2011) http://www3.epa.gov/ttnemc01/promgate/m-05.pdf.

or other international methods specified by Verein Deutscher Ingenieure (VDI), British Standards Institution (BSI), or International Standards Organization (ISO). The microprocessor stores all measurements, reviews and diagnoses all inputs, controls the required parameters, calculates isokinetic conditions, and either reports the results in a printed form or transfers them to a floppy disk.

Besides the controller, such a package usually consists of a probe, a filter (hot) box, a cold box, a flexible sample line, glassware, a node box, and a monorail system. The probe is usually 0.9, 1.5, 2.1, or 3 m (3, 5, 7, or 10 ft) and made of stainless steel with a glass liner. Most probes are jacket heated and are provided with both a liner thermocouple and a stack temperature thermocouple.

This chapter will give a detailed description of each of the four subsystems: the pitot assembly, the heated and ice-bath compartments, and the control unit.

Fig. 1.3b   (a) EPA particulate sampling train (Method 5). (b) Sampling train adopted for SO2, SO3, and H2SO4 mist (Method 8).

#### Pitot Tube Assembly

The procurement of representative samples of particulates suspended in gas streams demands that the velocity at the entrance to the sampling probe be precisely equal to the stream velocity at that point. This is accomplished by regulating the rate of sample withdrawal so that the static pressure within the probe is equal to the static pressure in the fluid stream at the point of sampling.

A pitot tube of special design is used for such purposes with means for measuring the pertinent pressures. The pressure difference can be maintained at zero by automatically controlling the sample draw-off rate. Figure 1.3c shows a pitot tube manometer assembly for measuring stack gas velocity.

Fig. 1.3c   Type S pitot tube manometer assembly.

The Type S (Stauscheibe, or reverse) pitot tube consists of two opposing openings: one made to face upstream and the other downstream during the measurement. The pressure difference detected between the impact pressure (measured against the gas flow) and the static pressure is related to the stack velocity.

Type S Pitot and the Sampling Probe: Figure 1.3d illustrates the construction of the Type S pitot tube. The external tubing diameter is normally between 3/16 and 3/8 in. (4.8 and 9.5 mm). As can be seen, there is an equal distance from the base of each leg of the tube to its respective face-opening planes. This distance (PA and PB) is between 1.05 and 1.50 times the external tube diameter. The face openings of the pitot tube should be aligned as shown.

Fig. 1.3d   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 may be assigned to pitot tubes constructed this way.

Fig. 1.3e   Proper pitot tube with sampling probe nozzle configuration to prevent aerodynamic interference. (a) Bottom view: minimum pitot nozzle separation. (b) Side view: to prevent pitot tube from interfering with gas flow streamlines approaching the nozzle, the impact pressure-opening plane of the pitot tube shall be even with or above the nozzle entry plane.

Figure 1.3e shows the pitot tube in combination with the sampling probe. The relative placement of these components eliminates the major aerodynamic interference effects. The probe nozzle is of the bottom hook or elbow design. It is made of seamless 316 stainless steel or glass with a sharp, tapered leading edge. The angle of taper should be less than 30°, and the taper should be on the outside to preserve a constant internal diameter (ID).

For probe lining of either borosilicate or quartz glass, probe liners are used for stack temperatures up to approximately 482°C (900°F); quartz liners are used for temperatures between 482°C and 899°C (900°F and 1650°F). Although borosilicate or quartz glass probe linings are generally recommended, 316 stainless steel, Incoloy, or other corrosion-resistant metal may also be used.

Selecting the Sampling Point: The specific points of stack sampling are selected to ensure that the samples collected are representative of the material being discharged or controlled. These points are determined after examination of the process of the 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 criterion is adopted to ensure the presence of stable, fully developed flow patterns at the test section.) For rectangular stacks, the equivalent diameter is calculated from the following equation:

1.3.1

Traversing Point Locations: 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. This number may be determined by using Figure 1.3f.

Fig. 1.3f   Minimum number of traverse points for particulate traverses.

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 1.3g and Table 1.3a.
2. For rectangular stacks, the cross section is divided into as many equal rectangular areas as traverse points, such that the ratio of the length to width of the elemental area is between 1 and 2. The traverse points are to be located at the center of at least nine and preferably more equal areas, as shown in Figure 1.3g.

Pitot Tube Calculation Form: The velocity head at various traverse points is measured using the pitot tube assembly shown in Figure 1.3c. The gas samples are collected at a rate proportional to the stack gas velocity and analyzed for carbon monoxide (CO), carbon dioxide (CO2), and oxygen (O2).

The pitot tube is calibrated by measuring the velocity head at some point in the flowing gas stream with both the Type S pitot tube and a standard pitot tube with a known coefficient. Other data also needed for calculation of the volumetric flow are stack temperature, stack and barometric pressures, and wet-bulb and dry-bulb temperatures of the gas sample at each traverse.

Fig. 1.3g   Traverse point locations for velocity measurement or for multipoint sampling.

### Table 1.3a   Location of Traverse Points in Circular Stacksa

Traverse Point Number on a Diameter

Number of Traverse Points on a Diameter

2

4

6

8

10

12

14

16

18

20

22

24

1

14.6

6.7

4.4

3.2

2.6

2.1

1.8

1.6

1.4

1.3

1.1

1.1

2

85.4

25.0

14.6

10.5

8.2

6.7

5.7

4.9

4.4

3.9

3.5

3.2

3

75.6

29.6

19.4

14.6

11.8

9.9

8.5

7.5

6.7

6.0

5.5

4

93.3

70.4

32.3

22.6

17.7

14.6

12.5

10.9

9.7

8.7

7.9

5

85.4

67.7

34.2

25.0

20.1

16.9

14.6

12.9

11.6

10.5

6

95.6

80.6

65.8

35.6

26.9

22.0

18.8

16.5

14.6

13.2

7

89.5

77.4

64.4

36.6

28.3

23.6

20.4

18.0

16.1

8

96.8

85.4

75.0

63.4

37.5

29.6

25.0

21.8

19.4

9

91.8

82.3

73.1

62.5

38.2

30.6

26.2

23.0

10

97.4

88.2

79.9

71.7

61.8

38.8

31.5

27.2

11

93.3

85.4

78.0

70.4

61.2

39.3

32.3

12

97.9

90.1

83.1

76.4

69.4

60.7

39.8

13

94.3

87.5

81.2

75.0

68.5

60.2

14

98.4

91.5

85.4

79.6

73.8

67.7

15

95.1

89.1

83.5

78.2

72.8

16

98.4

92.5

87.1

82.0

77.0

17

95.6

90.3

85.4

80.6

18

98.6

93.3

88.4

83.9

19

96.1

91.3

86.8

20

98.7

94.0

89.5

21

96.5

92.1

22

98.9

94.5

23

96.8

24

99.9

Source: Courtesy of Clean Air.

Table 1.3b gives the equations for converting pitot tube readings into velocity and mass flow, and a typical data sheet for stack flow measurements.

Sampling Velocity for Particle Collection: Based on the range of velocity heads, a probe with a properly sized nozzle is selected to maintain isokinetic sampling of particulate matter. As shown in Figure 1.3h, a converging stream will be developed at the nozzle face if the sampling velocity is too high. Under this subisokinetic sampling condition, an excessive amount of lighter particles enters the probe.

Fig. 1.3h   Particle collection and sampling velocity.

Because of the inertia effect, the heavier particles, especially those in the range of 3 µm or greater, travel around the edge of the nozzle and are not collected. The result is a sample indicating an excessively high concentration of lighter particles, and the weight of the solid sample is in error on the low side. Conversely, portions of the gas stream approaching at a higher velocity are deflected if the sampling velocity is below that of the flowing gas stream.

Under this superisokinetic sampling condition, the lighter particles follow the deflected stream and are not collected, while the heavier particles, because of their inertia, continue into the probe. The result is a sampling indicating high concentration of heavier particles, and the weight of solid sample is in error on the high side.

Isokinetic Sampling: Isokinetic sampling requires the precise adjustment of the sampling rate with the aid of the pitot tube manometer readings and nomographs such as APTD-0576 and nomographs. If the pressure drop across the filter in the sampling unit becomes too high, making isokinetic sampling difficult to maintain, the filter may be replaced in the middle of a sample run.

To measure the concentration of particulate matter, the sampling time for each run should be at least 60 min, and the minimum volumetric rate of sampling should be 30 dry scfm (51 m3/hr).

#### Heated Compartment (Hot Box)

As shown in Figure 1.3b, the probe is connected to the heated compartment that contains the filter holder and other particulate-collecting devices, such as cyclone and flask. The filter holder is made of borosilicate glass, with a frit filter support and a silicone rubber gasket.

The compartment is insulated and equipped with a heating system capable of maintaining a temperature around the filter holder during sampling at 120°C ± 14°C (248°F ± 25°F), or such other temperature as specified by the EPA. The thermometer should measure temperature to within 3°C (5.4°F). The compartment should be provided with a circulating fan to minimize thermal gradients.

#### Ice-Bath Compartment (Cold Box)

The ice-bath compartment contains a number of impingers and bubblers. The system for determining stack gas moisture content consists of four impingers connected in series, as shown in Figure 1.3b. The first, third, and fourth impingers are of the Greenburg–Smith design.*

Impinger, Greenburg-Smith, https://us.vwr.com/store/catalog/product.jsp?product_id=9256289.

To reduce the pressure drop, the tips are removed and replaced with a 0.5 in. (12.5 mm) ID glass tube extending to 0.5 in. (12.5 mm) from the bottom of the flask.

The second impinger is of the Greenburg–Smith design with a standard tip. During sampling for particulates, the first and second impingers are filled with 100 mL (3.4 oz) of distilled and deionized water. The third impinger is left dry to separate entrained water. The last impinger is filled with 200–300 g (7–10.5 oz) of precisely weighed silica gel (6–16 mesh) that has been dried at 177°C (350°F) for 2 hr to completely remove any remaining water.

A thermometer capable of measuring temperature to within 1.1°C (2°F) is placed at the outlet of the last impinger for monitoring purposes. Crushed ice should be added during the run to maintain the temperature of the gas, leaving the last impinger at 16°C (60°F) or less.

#### Control Unit

As shown in Figure 1.3b, the control unit consists of the system’s vacuum pump, valves, switches, thermometers, and totalizing dry gas meter. This system is connected by a vacuum line to the last Greenburg–Smith impinger. The pump intake vacuum is monitored with a vacuum gauge just after the quick disconnect.

A bypass valve parallel with the vacuum pump provides fine control and permits recirculation of gases at a low sampling rate so that the pump motor is not overloaded. Downstream from the pump and bypass valve are thermometers, a dry gas meter, and calibrated orifice and inclined/vertical manometers.

The calibrated orifice and inclined manometer indicate the instantaneous sampling rate. The totalizing dry gas meter gives an integrated gas volume. The average of the two temperatures on each side of the dry gas meter gives the temperature at which the sample is collected. The addition of atmospheric pressure to orifice pressure gives meter pressure.

#### Automatic Sampling Trains

In the automatic sampling train (AST) packages (Figure 1.3i), a microprocessor stores all measurements, reviews and diagnoses all inputs, controls the required parameters, calculates isokinetic conditions, and either reports the results in a printed form or transfers them to a floppy disk.

The measured variables include the temperatures of the stack, probe liner, filter box, condenser outlet, and dry gas meter (Figure 1.3j). The pressures are detected by an absolute and a differential pressure transducer and are used to measure the pressure of the stack gas, the barometric pressure, and the velocity pressure of the stack gas. The normal capacity of the vacuum pump that draws the sample is 0.75 cfm (21 l/min), and the dry gas meter has an operating range of 0.1–1.5 cfm (2.8–42 l/min).

The node box provides the interface between the filter box and the cold box by measuring the temperatures in both. It measures and stores the temperature, pressure, and velocity in the stack. The monorail eliminates the need for bulky supports.

Precise measurements require that the thermometers be capable of measuring the temperature to within 3°C (5.4°F); the dry gas meter is inaccurate to within 2% of the volume; the barometer is inaccurate within 0.25 mmHg (torr) (0.035 kPa); and the manometer is inaccurate within 0.25 mmHg (torr) (0.035 kPa).

The umbilical cord is an integrated multiconductor assembly containing both pneumatic and electrical conductors. It connects the two-module sampling unit to the control unit, as well as the pitot tube stack velocity signals to the manometers or differential pressure gauges.

Fig. 1.3i   Automatic stack train. (Courtesy of Thermo Anderson.)

Fig. 1.3j   The components of an automatic stack train. (Courtesy of Thermo Andersen.)

#### Sampling for Gases and Vapors

Some commonly used components in stack sampling systems are illustrated in Figure 1.3k. If ball-and-socket joints and compression fittings are used, any arrangement of components is readily set up for field use. The stack sampling components are selected on the basis of the source to be sampled, the substances involved, and the data needed.

A summary of sampling procedure outlines was developed by industrial hygienists*

Ron, J.J., Environmental Calibration and Operation of Isokinetic Source-Sampling Equipment, (1972) http://nepis.epa.gov/Adobe/PDF/20013PMH.PDF.

for specific substances. The procedural outlines serve as a starting point in assembling a stack sampling system, after consideration has been given to the complications that might arise because of the presence of interfering substances in the gas samples.

#### Specification Forms

When specifying stack sampling systems, it is advisable to attach the Pitot Tube calculations that have been made for the system. To show these calculations, the form on the next page can be used.

The total sampling system can be specified by filling out the form Figure 1.2b, while the specification forms, found on the next 2 pages, ISA 20A1001 and ISA 20A1002 can be used to specify the combination of the analyzer and its sampling system. These forms are reproduced with the permission of the International Society of Automation.

Fig. 1.3k   Components of common sampling systems.

### Table 1.3b   Pitot Tube Calculation Sheet

#### Abbreviations

 APTD Air pollution technical data AST Automatic sampling train

#### Organizations

 BSI British Standards Institution ISO International Standards Organization VDI Verein Deutscher Ingenieure

#### Bibliography

AMP, EPA method sampling system, (2015) http://www.ampcherokee.com/v/vspfiles/photos/C002.0002-2T.jpg.
ASTM D6831-11, Standard test method for sampling and determining particulate matter in stack gases using an in-stack, inertial microbalance, (2011) http://www.astm.org/Standards/D6831.htm.
ASTM D3685/D3685M-13, Standard test methods for sampling and determination of particulate matter in stack gases, ASTM International, (2013) http://www.astm.org/Standards/D3685.htm.
ASTM D6331-14, Standard test method for determination of mass concentration of particulate matter from stationary sources at low concentrations (manual gravimetric method), (2014) http://www.astm.org/Standards/D6331.htm.
BACHARACH: Combustion analyzers, (2015) http://www.bacharach-inc.com/PDF/Instructions/24-9438.pdf.
Clean Air Engineering, Stack and source emission testing, (2014) http://www.cleanair.com/services/StackSourceTesting/index.htm.
Cooper, C. D. and Alley, F. C., Air pollution, (2002) http://www.amazon.com/Coopers-Pollution-Control-Edition-Hardcover/dp/B003WUI3I6.
EPA, current knowledge of particulate matter(PM) continuous emission monitoring, (2002) http://www3.epa.gov/ttnemc01/cem/pmcemsknowfinalrep.pdf.
EPA, method 5—Determination of particulate matter emissions from stationary sources, (2011) http://www3.epa.gov/ttnemc01/promgate/m-05.pdf.
Ron, J. J., Environmental calibration and operation of isokinetic source-sampling equipment, (1972) http://nepis.epa.gov/Adobe/PDF/20013PMH.PDF.
Sherman, R. E., Process analyzer sample-conditioning system technology, (2002) http://www.wiley.com/WileyCDA/WileyTitle/productCd-0471293644.html.
Teledyne Analytical Instruments, Particle measurement system, (2012) http://www.teledyne-api.com/manuals/07318b_602.pdf.
Thermo Scientific, Particulate monitoring, (2015) http://www.thermoscientific.com/en/products/particulate-monitoring.html.

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