<< Back to Dust Control Handbook for Minerals Processing


For problems with accessibility in using figures and illustrations in this document,
please contact the Directorate of Technical Support and Emergency Management at (202) 693-2300.

Reason For Testing


Testing is necessary to evaluate the effectiveness of dust control measures. In general, testing is performed to-
  • Determine if the system is operating as it was designed to operate
  • Evaluate the system's effectiveness in reducing dust concentrations and employee exposure to dust


Dust Collection System Testing

Testing a dust collection system primarily involves airflow measurement. It can provide data necessary to-
  • Evaluate whether the system is performing in accordance with the design
  • Set blast gates properly and adjust airflows
  • Identify maintenance needs
  • Determine the system's capacity for additional exhaust hoods
  • Design and operate future installations effectively
Test Procedure

Testing can be performed by obtaining airflow and pressure measurements at selected locations in the system. The following steps should be conducted:
  • Obtain original design drawings and calculations or prepare a sketch of the system to indicate size, length, and relative location of all dusts, fittings, and associated hardware and system components. Use the drawings as a guide in selecting airflow measuring points and identifying incorrect installation or poor design.
  • Measure the following:

    • - Air velocity and static pressure in each branch and main

      - Static and total pressure at the fan inlet and outlet

      - Differential pressure between the inlet and outlet of the dust collector
  • Analyze these measurements to find any changes from the original designs, such as a change in velocity in the branch or a change in the air volume exhausted through the hood.
Common problems that can reduce the performance of dust collection system and their remedies are shown in the troubleshooting chart on the following page.

Airflow Measurements

Airflow inside the ductwork is usually not uniform; therefore, it is necessary to obtain velocity pressure measurements at several points within the dust cross-section. The following procedure should be used to measure velocity pressure:
  • Divide the dust cross-section into equal areas. The suggested equal area grids for circular or rectangular ducts are illustrated.
  • Obtain velocity pressure measurements at the center of each of these areas.
  • Convert the velocity pressure measurements into air velocities using the following relationship:
  • V = 4005   ____
    VP
Troubleshooting Chart-Reduction in Dust Collection System Performance
 
Symptom Cause Remedy



Reduced air volume Ducts plugged Clean out ducts, check air velocities, and check design specifications.
  Reduced fan speed due to belt slippage. Check belt tension and adjust it according to manufacturer's recommendations.
  Wear or accumulations on rotor or fan casing that would obstruct airflow. Replace or clean the worn-out equipment; consult manufacturer to see if fan is correct for the application.
  Leakage in the ductwork due to loose clean-out doors, broken joints, or worn-out ductwork. Replace or repair leaking and worn-out sections of ductwork.

  Additional exhaust points added to the system Redesign or rebalance the system.
  Change of blast gates setting in branch lines. Reset the blast gates according to original design and lock them in place.
  Increased pressure drop across dust collector Refer to operation and maintenance instructions of the dust collector and check its operation.
Pitot Traverse Points in a Circular Duct
Pitot Traverse Points
in a Circular Duct

Reprinted by permission from the Committee on Industrial Ventalation, Lansing, MI, 18th Edition.
  • Average the air velocities to obtain an average velocity (ft/min).
  • Measure the inside duct diameter and calculate the cross-sectional area (ft2).
  • Multiply the duct cross-sectional area and average velocity to obtain airflow in cubic feet per minute.
Points to Note:
Pitot Traverse Points in a Rectangular Duct
Pitot Traverse Points
in a Rectangular Duct

Reprinted by permission from the Committee on Industrial Ventalation, Lansing, MI, 18th Edition.
  • The smaller the equal areas, the more accurate the measurement.
  • An approved method for obtaining velocity pressure is to make two traverses, at right angles to each other, with a probe (such as a Pitot tube) across the diameter of the duct.
  • Wherever possible, the traverses should be made at 8 or more duct diameters away from any major air disturbances, such as an elbow, hood, or branch entry.
  • Correction for air densities should be made when air is at nonstandard conditions. For example, it is necessary to correct for air densities when-
    • Static Tap Connections
      Static Tap Connections

      Reprinted by permission from the Committee on Industrial Ventalation, Lansing, MI, 18th Edition.

      - Moisture content is greater than 0.2 lb/lb of dry air

      - Temperature of the airstream varies more than 30 F from the standard temperature

      - Altitude is greater than 1,000 ft above mean sea level
  • Dust in the air may affect the instrument performance.
Static pressure measurements are made either by inserting a probe inside the ductwork or by holding a piece of tubing tightly against a small static pressure opening in the side of the duct with its other end connected to a pressure-measuring device. The characteristics of the static pressure openings in the ductwork are critical in measuring static pressure.

Ideal Location for Pitot Tube Measurement
Ideal Location for
Pitot Tube Measurement

Points to Note:
  • The static pressure openings should be-

    • - Flush with the inner surfaces of the duct wall

      - Drilled and not punched

      - Without burrs or projections n the inner surface
  • Pressure measurements should not be taken at the heel of an elbow or at other points where, due to excessive turbulence or change in air velocity, there is sudden expansion or contraction of ductwork.
Commonly used flow-measuring instruments are summarized in the table on the following page.



Wet Dust Suppression

Testing of wet dust suppression systems should begin with a study of the following:
  • Process flow diagram
  • Piping and instrumentation diagram
  • Specifications for each component of the system, such as the pump, compressor, and nozzles
Test Procedure

Illustration of Circuit
Illustration of Circuit

The system should be tested in the following sequence:

1. Measure Water Flow - The flow rate of water per transfer point should be checked to verify proper operation. It is strongly recommended that direct readout flowmeters, such as rotameters, be installed near the transfer point so the flow rate can be checked quickly.


Flow-Measuring Instruments
 
Instrument Characteristics   Advantages/Disadvantages


 
Pilot tube Consists of two concentric tubes: one detects total pressure in the airstream and the other detects static pressure. Used in conjunction with monometer or Aneroid type gauge.

  An extremely useful tool in flow measurement. Does not need calibration. Use in the field is limited at velocities greater than 600-800 ft/min.
U-tube manometer Simplest type of pressure gauge. Usually filled with oil or water, but can be used with other fluids such as alcohol, mercury, or kerosene. Usually calibrated in inches of water gauge.

  Can be used for either portable or staionary applications. Suitable for most static pressure measurements. Use is limited at velocities less than 1,000 ft/min (below .06 in wg).
Inclined manometer Improved version of the U-tube manometer. Many commercial versions equipped with a 10:1 slope, built-in level, leveling adjustment, and a means of adjusting scale to zero.   The single leg is tilted to obtain increased sensitivity and scale magnification. Accuracy dependent on slope of tube. Consequently, base must be firm enough to permit accurate leveling.

Swinging vane
anemometer
Direct readout instrument used to measure air velocities and static pressures in duct systems as well as in unrestricted areas. Consists of a meter, a measuring probe, range selectors, and connecting hoses. Velocity range is from 30 to 300 ft/min for a low-flow probe and from 100 to 10,000 ft/min for a Pitot probe. Static pressure ranges are from 0 to 1.0 in. wg and 0 to 10 in wg.   Can be operated under high temperatures (up to 700 F) and high static pressure (up to 10 in.wg). May require periodic calibration. Presence of dust, moisture, or corrosive material can affect accuracy. A plugged filter increases resistance and the amount of air passed to the swinging vane may change. Requires a larger diameter hole than the Pitot tube (approximately 5/8 in. diameter vs. 1/4 in. to 3/8 in. for the Pitot tube).

Heated wire
anemometer
Operates on principle that resistance of a wire varies with temperature and degree of temperature change is proportional to velocity of air passing over the wire. Direct readout instrument. Has short response time (less than 1 minute). Is portable. velocity range is from 10 to 8,000 ft/min. Static pressure range is up to 10 in.wg. Temperature range is from 0 to 255 F.

  Applicable for field and laboratory use. Integrity of probe must be maintained; the delicate wire can easily be damaged by mechanical shock, heavy dust loadings, or corrosive materials. Requires periodic calibration.
Aneroid type gauge Does not use liquid. Used as field
instrument in conjunction with Pitot tubes or other probes. Magnehelic gauge is the most commonly used in field installations.
  Easy to read. Provides a greater response than manometers. Portable. Requires less maintenance and mounting; allows use in any position without losing accuracy. Subject to mechanical shock and failure. Requires periodic calibration.

If the water flow rate is not adequate, the following steps should be conducted:
  • Check pump for-
- Excessive vibration, noise, and heat

- Seal or packing gland leakage

- Discharge pressure

- Adequate flow of mechanical seal flushing fluid

- Lubricant supply to bearing housings
The troubleshooting chart on the following page shows some common pump problems. For additional information, refer to the pump operations manual.
  • Inspect piping for leakage at valves, tees, elbows, and drains.
  • Inspect the water line filter element for excessive particle buildup.
  • Inspect nozzles and replace those with physical damage.
2. Measure Compressed Airflow (where applicable) - The flow rate of air is more difficult to measure since the direct readout instruments are often calibrated for a certain pressure. The flow rate for different pressures can be calculated using manufacturers' charts, actual pressure, and the flow rate at a calibrated pressure. If a flowmeter is not installed in the air line, a circuit as shown for testing water flow rate should be installed.

If the compressed airflow and pressures are not adequate, the following items should be checked:
  • Check air compressor for-
- Correct adjustment of upper and lower pressure set points

- Excessive noise and vibration

- Leakage in the line

- Proper lubrication

Refer to the operations manual for additional troubleshooting of the air compressor.
  • Inspect piping for leakage at valves, tees, elbows, and drains.
  • Drain the compressed air line filters.
  • Replace depleted compressed air dryer element.
Troubleshooting Chart - Pump Problems
 
Symptom Cause Solution



No liquid delivered. Lack of priming. Fill pump and suction pipe completely with liquid.
  Suction lift too high. If no obstruction at inlet, check for pipe friction losses,. If static lift is too high, liquid to be pumped must be raised or pump lowered.
  Discharge head too high. Check pipe friction losses. Check that valves are wide open.
  Impeller plugged. Dismantle pump and clean impeller.
Not enough liquid delivered. Air leaks in suction piping. Test flanges for leakage. Suction line can be tested by plugging inlet and putting line under pressure.
  Impeller partially plugged. Dismantle pump and clean impeller.
  Defective impeller. Inspect impeller and shaft. Replace if damaged or vane sections badly eroded.
  Defective packing or seal. Replace packing or mechanical seal.
  Suction pipe not immersed enough. Lower inlet pipe.
Not enough pressure. Speed too low. Check whether motor is receiving full voltage.
  Air leaks in suction piping. Test flanges for leakage. Suction line can be tested by plugging inlet and putting line under pressure.
  Mechanical defects. Inspect impeller and shaft. Replace if damaged or vane sections badly eroded. Replace packing or mechanical seal.
  Obstruction in liquid passages. Dismantle pump, inspect passages, and remove obstruction.
  Air or gases in liquid. (Watch for bubbles.) Possibility of overrated pump. Periodically exhaust accumulated air.
Pump operates for short
time, then stops.
Incomplete priming. Free pump, piping, and valves of all air. Correct any high points in suction line.
  Air leaks in suction piping. Test flanges for leakage. Suction line can be tested by plugging inlet and putting line under pressure.
  Air leaks in stuffing box. Increase seal liquid pressure to above atmosphere.
Pump takes too much power Mechanical defects. Inspect impeller and shaft. Replace if damaged or vane sections badly eroded. Replace packing or mechanical seal.
  Suction pipe not immersed enough. Lower inlet pipe.
  Shaft bent or damaged. Check deflection of rotor by turning on bearing journals.
  Failure of pump parts. Check bearings and impeller for damage.

3. Check unique components such as metering units, foaming units, and electrostatic charge generators for proper operation.

4. During freezing weather conditions, check for proper operation of heat tracing elements or tapes and insulation to avoid freezing in pipes.



Evaluating Dust Control Systems

After a dust control system is installed, it is essential to evaluate the system's effectiveness in reducing dust concentrations and associated employee exposure to dust. A sampling plan must be developed before any evaluation can be made.

Types of Samples

Two primary types of samples are normally obtained:
  • Process/source samples to measure airborne dust concentrations due directly to the source emissions
  • Ambient/background samples to measure the contribution of other sources to dust levels.
Sampling Locations

The selection of sampling locations is important because of the large differences in dust levels that may exist around a dust source. Many times, a change in the sampling location can have a greater effect on dust concentrations measured than the effect of engineering controls being evaluated.

Process or source samples should be located close to the source to reduce the interference of other sources in the vicinity. They may also be located near the worker whose exposure is most directly affected by the dust source.

Ambient/background samples should be located far enough so as not to be affected by dust emissions from the source, yet close enough to be representative of dust levels generated by all other sources in the vicinity.

Sampling Instrumentation

Two sets of sampling instruments are commonly used-
  • Instantaneous dust monitors, such as RAM-1, to obtain real-time dust concentrations
  • Gravimetric samplers to provide time-weighted average dust concentration data and mineral composition analysis
Instant dust monitors provide immediate dust concentration information. They may not be compared with the 8-hour time-weighted gravimetric samplers used for compliance monitoring; however, they are useful for rapid identification of major dust sources as well as rapid evaluation of dust control measures.

Gravimetric dust samplers provide a time-weighted average of dust concentration for the entire sampling period. They do not provide the information required to pinpoint sources of dust or determine how the concentrations vary as a function of operating conditions. Gravimetric dust sampler results should be used with caution.

Sampling Approaches

The three most common sampling approaches available are-
  1. Obtaining short-term samples with system "on" and "off".
  2. Obtaining samples before and after installation of a dust control system.
  3. Obtaining samples to compare the effectiveness of two different types of dust control systems.
Approaches 1 and 2 provide data to determine the effectiveness of a newly installed dust control system. In approach 1, the samples are obtained after the system is installed; in approach 2, the samples are obtained before and after the system is installed. Approach 3, commonly known as the A-B-A approach, is useful when it is essential to compare the effectiveness of two dust control systems during the same test period. It involves a period of testing with system A, followed immediately by an equivalent period of testing with system B, followed immediately by verification of the performance of system A. The return to system A provides an indication that changes observed with system B were not due to other changes in the process.

Data Collection

A data collection form should be developed to record all relevant information during field testing.

Strip chart recorders or more recent electronic data logging units can be attached to the instantaneous dust monitors to provide records and sound indications of dust levels. Small, portable tape recorders are often used, especially with instantaneous dust monitors, to record the data, field observation, and any other significant activity connected with the testing. When tape recorders are used, the data should be transcribed every day onto the appropriate data sheets. This will facilitate review of the data on a daily basis and prevent and future problems or omissions. It will also provide a preliminary feel for the system's effectiveness and indicate whether the testing program is going as planned.

Data Analysis

ABA Comparison of Dust Control Method
ABA Comparison of
Dust Control Method

To evaluate a system's effectiveness in reducing dust emissions, two methods are used to analyze the dust concentration data:
  • The graphical method
  • The mathematical method
The graphical method, illustrated on the following page, is a qualitative method. It provides a simple, yet effective, method to compare the dust concentration data with system "off" and system "on" and determines the magnitude of the system's effectiveness in reducing dust concentrations.

Mathematical methods are used to quantify the efficiency of system performance. Here, the system "off" and system "on" dust concentration data are compared and the system's efficiency is determined using the following equation:

η = Coff - Con x 100%

Coff
where:
 
η = system's efficiency, in percent
Coff = system "off" dust concentrations, mg/m3
Con = system "on" dust concentrations, mg/m3

If sampling is repeated for a number of times at the same location, then the data should be treated statistically for more detailed analysis. Information on statistical approaches can be obtained through several excellent reference books.