Reference method for measuring asbestos emissions from asbestos mining and milling operations: part II

Part I - Sampling - Method S-2

Sampling of Baghouse Systems in Asbestos Milling Operations

S-2.1 Scope

This method is employed where method S-1 cannot be applied and where access to the clean-air side of the baghouse is possible. The equipment and procedures required for sampling in baghouses by an open-face-filter method employing membrane filters are described. Handling of the samples after recovery is also described.

S-2.2 Apparatus

Filter Holder. A standard, 37-mm acrylic plastic filter holder with a filter support pad is required. Figure S-2-1 is an exploded schematic of this filter holder.

Figure S-2-1: Filter Unit - Exploded View

Filter Unit - Exploded View

Filters. A cellulose acetate membrane filter, 37 mm in diameter with 0.8 μm pore size, is used. When supported on the filter holder there is an effective filtering area of about 855 mm2. The effective filtration area is printed with a grid which divides it into eight, 45° angular sectors. Each sector is further divided into an inner (Ai) and outer (Ao) radial area. The ratio Ai:Ao is 1:3, (Figure S-2-1).

Pump. A portable vacuum pump capable of maintaining a sampling rate of 2 lpm for 4h or more is required. A bypass valve is connected across the vacuum pump to allow control of the sample flow rate.

Rotameter. A calibrated rotameter, accurate to within ± 2%, is required for adjustment of the flow rate through the filter unit.

Temperature Gauge. A thermometer is required which will measure ambient baghouse temperatures to within ± 1°C.

Barometer. A barometer capable of measuring absolute atmospheric pressure to within 2 mm Hg is required.

Cellulose Collars. These collars are used to seal the filter holders so that air cannot bypass the filter.

S-2.3 Procedures

S-2.3.1 Sampling Sites

For any particular baghouse system a minimum number of filter samples must be taken through the area of the baghouse to obtain a representative test result for that system. Thus, a valid test result for any baghouse system will be the arithmetic average of all the filter samples taken to yield a representative sampling of the air in that system.

The minimum number of filter samples to be taken is a function of the exhaust capacity of the system and the practicalities involved in sampling large capacity units. To ensure a representative sampling of the system with a reasonable number of filter samples, Equation S-2-1 is used to determine the minimum number of filter samples which must be taken to yield a valid test run.

Equation S-2-1
N = 0.22 × (CFM)0.25


When applying this equation, any fractional number must be rounded off to the next highest integer. When small-capacity baghouse units are encountered at least three filter samples must be taken to yield a valid test run, regardless of the result obtained using Equation S-2-1.

Locate the minimum number of filter samples as evenly as possible throughout the bagroom. Exact locations are to be agreed upon by the inspector and the person responsible for sampling. Prepare a schematic of the bagroom layout indicating locations of samples and fans with respect to the bags. For a valid test run, all samples should be taken simultaneously whenever possible. If restrictions on time or supply of equipment preclude simultaneous sampling, all samples must be taken within two days.

S-2.3.2 Preparation of Filter Unit

Using a pair of clean tweezers place a filter, with the printed side up, on the support pad of the holder. Replace the insert and upper cover of the holder, insert the plugs and then place a wet cellulose collar around the unit so that it seals the joint between the insert and lower cover of the holder. Allow the collars to dry and store the prepared units upright, in a suitable container, for transport to the sampling site.

S-2.3.3 Sampling

Before sampling, ensure that the pump batteries are fully charged and that the pump is operational. Using a suitable piece of tubing, attach the filter unit to the inlet of the pump. Remove the upper cover, exposing the filter surface. Adjust the pump withdrawal rate to 2 lpm using a previously calibrated rotameter, as shown in Figure S-2-2. Print the sample number on the filter unit and record all pertinent sampling data on the data sheet, shown in Figure S-2-3. Remove the calibration assembly and allow the pump to run for up to 4h depending on the concentration expected. The minimum sampling time is that required to achieve the optimum filter loading for analysis, 5 asbestos fibres per field. When sampling, the printed filter face is generally pointed downward. If the general flow of the air stream is known, the filter face should be placed parallel to the air flow. At the end of the sampling period check the flow rate as before and record this value. If the difference between initial and final rates is less that 0.3 lpm the sample is acceptable and the arithmetic average of the two values is used as the sampling rate. If the difference in values is greater than 0.3 lpm the sample is discarded and another taken in the same location within two days.

Figure S-2-2: Pump Calibration Assembly

Pump Calibration Assembly

Figure S-2-3: Data Sheet - Baghouse Sampling

Data Sheet - Baghouse Sampling

On completion of sampling, carefully remove the filter unit from the pump, replace the top cover and sealing plugs and place the sample in a container with the printed surface of the filter facing upward. When conveying the samples to the analytical site avoid jarring the units and ensure that they are maintained upright at all times.

S-2.3.4 Analysis

The filter samples are analysed according to Method A-1 in Part II. At least one of the eight sectors of the filter must be analysed, usually sector 1. Replace the rest of the filter in the holder, seal and store upright for future evaluation if necessary.

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