By Ed Ravert, United Air Specialists
Thousands of dust collectors are used in manufacturing processes each day. Some are more modern, equipped with the latest and greatest safety features and energy-efficient design; while others may be more than 20 years old and still working as good as new, but without the advancements of modern technology. Although seemingly cost-effective for a company to hold onto a dust collection system for so many years, this may also be a potentially hazardous decision when it involves combustible dusts.
OSHA has been placing more emphasis on combustible dust safety in recent years, and sent letters to approximately 30,000 companies across the U.S. that may be at risk for a combustible dust incident. The letters warned of the potential dangers at these facilities based on the type of processing dusts used. According to OSHA and the U.S. Department of Labor, there have been at least 350 combustible dust explosions in the U.S. since 1980, killing 130 workers and injuring almost 800.
|Dust collectors equipped with explosion venting, shown on the left side of the unit, is one form of protection when handling combustible dusts.|
Also noteworthy is the re-issue of the National Fire Protection Association’s NFPA 68 Standard on Explosion by Deflagration Venting. The completely revised Guideline is now a Standard that is enforceable by OSHA and highlights five main points:
1. NFPA 68 spells out mandatory requirements for collecting explosive dusts and is to be treated as mandatory code that is enforceable by local, state, and national authorities.
2. Companies that handle dusts are required to test and document explosibility of dust(s) within their facilities. Any dust above 0 Kst is considered explosive, and explosion venting or alternate protection is necessary.
3. Companies that handle dusts are required to conduct a Commission Hazard Analysis (also called a Risk Assessment) on their dust collection system(s).
4. Companies are required to maintain specific documentation, including test reports, equipment owner’s manuals and maintenance reports on their dust collection systems for inspection by an Authority Having Jurisdiction (AHJ) agent.
5. Companies are required to schedule inspections on explosion venting equipment at least annually and possibly more often.
Combustible Dust: What Is It?
According to OSHA, combustible dust is a solid material composed of distinct particles or pieces, regardless of shape or chemical composition, which due to its small particle size presents a flash-fire or explosion hazard as a result of its ability to propagate combustion when dispersed in air or the process-specific oxidizing medium.
Materials that may be combustible include metals (such as aluminum and magnesium), wood, coal, plastics, biosolids, sugar, paper, soap, and certain textiles. A combustible dust explosion hazard may exist in a variety of industries, including: food (e.g., candy, sugar, spice, starch, flour, feed), grain, tobacco, plastics, wood, paper, pulp, rubber, furniture, textiles, pesticides, pharmaceuticals, dyes, coal, metals, and fossil fuel power generation.
Options for Existing Equipment
Once you have determined that you are working with combustible dusts, it is necessary to take a look at the location and construction of your current dust collector. If your system is indoors, it needs to be relocated outside to an open, non-populated area to avoid potential damage to an adjacent property or structure. Next, two important areas need to be defined: the pressure capacity of the cabinet and the construction of the cabinet itself.
|If outdoor space is limited, dust collectors can be placed on top of a new or existing facility. The explosion vents can be installed on top of the collector and protected by weather covers as shown here.|
Most dust collection systems installed within the last 40 years were designed for pressures of ±15 to 20 in. water gauge Static Pressure (w.g. SP). A new state-of-the-art explosion suppression system may be required to withstand up to +105 in. w.g. SP. So, it is recommended that you reference the equipment owner’s manual and contact the original dust collector manufacturer to obtain the parameters of the cabinet design pressure in inches water gauge static pressure, or lb/sq in gauge (PSIG). Once you have this data, you can make an educated decision on whether or not it is even feasible to install an explosion protection system onto your existing collection equipment.
The assembly of the dust collector is also important, and typically involves two types of manufacturing processes—fully welded construction and bolted construction.
It is much more difficult to add explosion protection to a fully welded system, because the very act of burning or cutting a hole to add an explosion vent, for example, can cause an explosion. This is due to the fact that there is dust inside the collector that needs to be removed and then washed off prior to cutting into the cabinet. Once the explosion vent hole is cut and finished, a frame then needs be bolted or welded to the housing so the explosion protection system can be mounted. If attempting this type of solution, it is strongly recommended that a fire watch crew be on hand while this work is done.
On the other hand, dust collectors with bolted construction are much more manageable because sections can be taken apart. A side wall can be unbolted, properly cleaned, and then cut on or welded. This makes it easier to add an explosion vent or suppression fittings, and then bolt the panel back into place. Another option is to purchase a complete replacement panel with an explosion vent already installed, and bolt the new panel into place. Like all work performed on explosion protection, you should work directly with the equipment manufacturer or a professional in the dust collection protection industry.
The truth is that not all existing dust collectors can be retrofitted with explosion protection. However, it is possible under the right set of construction conditions, and it may provide your company a cost-effective solution that complies with recent combustible dust safety standards. Regardless of your decision to retrofit an existing dust collector system or to purchase new equipment, it is important that your company stays up-to-date on explosion protection.
Ed Ravert is a senior application engineer at United Air Specialists (Cincinnati). He has more than 44 years in the dust collection industry and has been an industrial ventilation instructor for more than 30 years. For more information, visit www.uasinc.com