July 2014 - Dust is a nuisance in any metalworking facility: But when dust is combustible, it poses a serious threat to life and property. That’s why it’s so important to understand National Fire Protection Association guidelines and to subject your dust to the necessary testing.
The NFPA 484: Standard for Combustible Metals focuses on “the production, processing, finishing, handling, recycling, storage and use of all metals and alloys that are in a form that is capable of combustion or explosion, as well as to operations where metal or metal alloys are subjected to processing or finishing operations that produce combustible powder or dust.” The current standard was published in 2012, with the next update scheduled for 2015.
Using NFPA 484, we outline the steps you should take to prevent combustible dust explosions in your shop.
The importance of dust testing
There is no quick and easy way to know if your dust is explosive because every operation is somewhat different. Most metals that are ground create dust containing active carbons that are potentially explosive. Metals that are cut thermally by plasma or laser may or may not be combustible, depending on the material.
Thin, flat metals are typically pickled and oiled, which creates a heavy surface coating. Metal products greater than 3⁄4-inch thick are typically dry finished. Rust inhibitors, coolants, paints and/or other fugitive materials can impact what ends up in the dust collector. All these different elements may combine into an explosive mixture that varies from application to application.
It is your responsibility to identify what exactly is in the dust(s) generated during metalworking processes. Testing the dust on a case-by-case basis is the only way to know for sure.
Hazard analysis and risk assessment
A hazard analysis and risk assessment is the only way to get a true reading on potential risks in your facility. Such an analysis helps to determine the required level of fire and explosion protection, according to NFPA. The analysis can be conducted internally or by an independent consultant, but either way, authorities with jurisdiction will ultimately review and approve the findings.
The first step in a hazard analysis is testing the dust and many commercial laboratories offer a low-cost test to establish whether a dust sample is combustible. If the test is positive, then additional testing will be needed to determine the explosive index (Kst) and the maximum pressure rise (Pmax) of the dust.
A risk assessment identifies all aspects of a given operation and all risks involved. It leads end-users to find the best available technology to manage and mitigate their risks, to protect employees and property.
Most dust collectors are manufactured with sealed motors and enamel or powder coat paint so that they can be used outdoors, a safer location that also saves on floor space. Collectors can also be located inside for small, mobile and flexible manufacturing cells or for growing companies that move their equipment frequently. Another option is to install collectors on mezzanines or rooftops. If a dust collector must be installed inside where an explosive dust is present, a sodium bicarbonate or other suppression system will be needed, or the exhaust air will have to be vented safely outside through a wall.
There is a wide range of explosion prevention and suppression equipment to choose from, too. As a rule of thumb, we recommend passive explosion prevention systems with a certified inlet isolation damper, explosion vent and safety monitoring filter to protect the inlets entering the collector and the return air. Passive systems are preferred when feasible because they are less costly and do not require monthly or quarterly inspections by an outside party.
Active suppression systems can vary by material: Sodium bicarbonate is the most common but there are also gas-based systems like CO2. The type of equipment selected should be based on its compatibility with the material being manufactured and the dust created by the manufacturing process.
Finally, invite a dust collection specialist in for a site visit before selecting equipment. A knowledgeable supplier can help determine the best explosion prevention components for your application, the type of ducting needed, the right filtration media, where to locate the collector, and whether air can be safely recirculated downstream of the collector to save on heating and cooling costs. MM