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How Well Do You Know Your Dust?

July 23, 2008
When advanced testing is needed on combustible dusts, full-scale explosion test vessels can be used. (Fike Corp.)

The collection and testing of dust samples is a long-established practice used by many powder and bulk processors to make informed dust collection decisions. Dust testing protocols have not changed markedly in recent years. The importance of dust testing, however, has changed, and the implications are significant. While it has always been good practice for facilities to know their dust, it is rapidly becoming a necessity in today’s regulatory climate.

This article will review two separate types of testing: explosibility testing, which is used to determine whether a dust is combustible; and bench testing, which pinpoints numerous physical properties of dust. Both categories of tests are needed to determine the best dust collection system for an application and to decide whether explosion venting equipment must be part of that system.

Explosibility Testing

A June 8, 2008, segment on the CBS News series 60 Minutes reported that “devastating dust explosions at American factories are more common now than ever. Since 1980, there have been at least 350 such explosions in the U.S., killing 133 people and injuring hundreds more.” A measure that will require the Occupational Safety and Health Administration (OSHA) to impose new safety rules for combustible dust has been approved by the House of Representatives and awaits a Senate vote.

In the meantime, OSHA has identified and contacted 30,000 companies that are considered to be at risk of explosion from combustible dusts. OSHA is addressing the issue aggressively through its National Emphasis Program on the safe handling of combustible dusts and by enforcing the National Fire Protection Association (NFPA) 68 Standard on Explosion Protection by Deflagration Venting, a 2007 standard that sets explosion venting requirements for dust collection applications that involve combustible dusts. Since the main purpose of explosion venting is to save lives by preventing dust collectors from exploding, the importance of getting this equipment right cannot be overstated.

Test chamber used for standard explosibility testing (Fike Corp.)

What does all this have to do with dust testing? To determine whether a facility’s dust is combustible, it must undergo explosibility testing in accordance with ASTM test methods. NFPA 68 stipulates that if a dust sample is available, it must be tested. The standard further states that it is the responsibility of the end-user (i.e., the plant or safety engineer) to commission the required testing and report on results. This stipulation is a noteworthy change from past practice: What used to be a “guideline” is now a “standard.” It is regarded as mandatory.

Explosibility testing is offered by several companies that specialize in explosion protection services. Facilities can opt to go directly to such a company or commission testing through their dust collection supplier.

Collecting Dust Samples. Proper collection of a dust sample is important to ensure that it represents the state in which the filter collects it. Before collecting a sample, the facility should review the dust’s material safety data sheet to ensure that precautions are taken against hazards associated with the dust. If the facility has an existing dust collector, a dirty bag or cartridge is an ideal sample. If not, the facility should collect a sample that represents the dust that would be captured by a collector. For example, dust swept from the floor is not representative of plant dust because it may contain impurities and larger particles with enough mass to fall to the floor. Dust from a hopper is not recommended either because it will not represent the true particle-size distribution of the dust captured by filters. The facility should ask the test lab for further guidelines on proper sample collection.

Explosibility testing typically requires a 1-lb dust sample. Samples should always be placed in airtight containers to preserve their moisture content. Full-scale testing requires much larger dust samples. Again, the facility should consult the test laboratory for details.

Testing Procedures. Using a dust sample, a lab performs a screening test to determine whether the dust is inert or explosive. If the dust is inert, testing ceases. If it is combustible, the lab then conducts further testing on dust cloud explosibility parameters to pinpoint the deflagration index (Kst) of a dust cloud, or rate of pressure rise, and the maximum pressure in a contained explosion (Pmax). Costs range from around $600 for screening only to about $1400 for the standard battery of ASTM explosibility tests, if they are required.

Even if a facility believes that its dust is inert, the dust must still be tested under NFPA 68. For example, paper dust may be inherently inert. But if it is processed with a coating or glue, the combustibility characteristics of the dust can change drastically. Explosibility testing is the only certain way to determine whether such a change has taken place, and it is therefore the only way to guarantee compliance. When they submit dust samples for testing, companies may be surprised by the results.

If a dust sample is found to be even slightly combustible, the company will be required to use explosion-venting equipment on its dust collector. Explosibility testing enables dust collection suppliers to determine whether a facility can use a standard explosion vent or whether the vent size, ducting, and related components must be specially calculated and modified to ensure compliance. In either case, the facility should request documentation that the equipment has been manufactured in accordance with the latest NFPA standards.

Bench Testing

Test rig using multiple dust collectors to perform specialized full-scale dust testing

Whether a facility plans to install a new dust collection system or update existing equipment, dust collection is a complex process that is affected by dozens of variables. Dust sample bench testing is an excellent tool for knowing dust better, enabling plant personnel to make sound and accurate equipment choices.

Bench testing is beneficial in many ways. By identifying dust characteristics properly, it enables personnel to determine the types of collectors and filtration media they require. In addition, it helps the facility determine the size of the equipment it needs and the air-to-cloth ratio required for optimal energy savings and operational efficiency. This information can help to minimize maintenance problems, meet more-
stringent emissions requirements, and extend filter life.

Independent test laboratories with dust collection experience can perform bench testing at costs ranging from $300 to $1000. Some equipment manufacturers have in-house test labs, offering free testing as a value-added service to their customers. Companies researching dust collection equipment should inquire whether they are obligated to buy a dust collector if the manufacturer conducts tests for them.

Like explosibility testing, bench testing requires a 1-lb dust sample. When it requests a sample, the test lab should also ask to see detailed application data. Such data may include information on the process generating the dust, operating requirements, airflow and pressure-drop conditions, temperature and humidity, space constraints, and more. Without application data, no context will exist for the test
program, resulting in less-meaningful results.

Particle-size analysis is used to reveal dust’s particle-size distribution down to the submicron range.

Common bench testing involves several different types of diagnostic procedures:

1. Particle-size analysis reveals dust’s particle-size distribution down to the submicron range. This information determines the filtration efficiency required to meet emissions standards. A dual-laser particle
analyzer can pinpoint both the count (the number of particles of a given size) and the volume, or mass spread, of the dust. Knowing both particle count and volume is important because many types of dusts become mixed together. Sieve analysis is a related test that measures large particles (>100 µm).

2. A video microscope provides visual analysis of dust shape and characteristics. Together with particle-size analysis, this tool is vital for informing proper equipment selection, often helping to determine what type of collector should be used. For example, a microscope may be needed to see oil in the dust. Oil can cause serious problems with dry-dust collectors and may require the use of filter media with an oleophobic coating. In more drastic cases, an alternate collection system might be needed.

3. Pycnometer testing determines the true specific gravity of the dust, as opposed to its bulk density. Specific gravity is the weight of a given material as a solid block. This test can help to determine whether cyclonic-type dust collectors will work efficiently.

A video microscope provides visual analysis of dust shape and characteristics.

4. Moisture analysis measures a dust’s moisture percentage by weight. This information can help to prevent or troubleshoot moisture problems that can affect filter performance. A humidity chamber is used to see how quickly a dust will absorb moisture. This test helps to identify hygroscopic (moisture-absorbent) dust. Hygroscopic dusts require widely pleated filter cartridges or bag-type filters, since sticky dusts cause tightly pleated filters to plug up.

5. Abrasion testing measures the relative abrasiveness of dust. This test helps to determine the optimal design of dust-handling components, including valves, inlets, and ductwork. For example, when capturing a highly abrasive dust, the collector must be designed with low inlet velocity. Otherwise, the dust will reentrain on the filter elements, abrading the filters and causing premature wear.

6. Terminal-velocity testing pin-points the air velocity required to lift the dust. This test helps to determine the correct dust collector size and bag or cartridge filter size. Horizontal-velocity testing reveals the optimal velocity needed to move the dust horizontally, aiding in proper ductwork system design. Sliding angle/angle of repose testing determines the angle at which dust forms freely, aiding in hopper and dust discharge design. This test further identifies whether the dust will tend to stick or agglomerate.

Specialized Testing

Every dust is different. The only way for plant personnel to know their dust is to commission laboratory testing.

The explosibility and bench tests described here will be sufficient in the great majority of cases. However, in the case of highly unusual or trouble-prone applications, additional testing might be required. Explosion test labs can perform a variety of advanced tests using both small- and large-scale explosion test vessels, recommending further testing, if necessary, to analyze combustible-dust problems.

Similarly, after bench testing is completed, a plant may need further information to troubleshoot a serious collector problem or to predict the behavior of unusual or difficult dust. In these situations, full-scale dust testing using one or more dust collectors may be considered. Full-scale testing, available from a handful of labs, may be used for real-time or accelerated testing that simulates actual operating conditions.

After a facility has finished collecting and testing dust samples using the guidelines outlined in this article, it will know its dust well. Knowing dust is the key to enabling a facility to install the best possible dust collection equipment for ensuring emissions control, plant cleanliness, worker safety and comfort, and compliance.

Tony Supine is the technical director of Farr Air Pollution Control (APC), a leading worldwide manufacturer of dust collection equipment located in Jonesboro, AR. Supine received a BS in mechanical engineering from Arkansas State University. He can be reached at [email protected]. For more information on Farr APC, call 800-479-6801 or visit www.farrapc.com.