Layer Ignition Test (LIT)
Important Differences between NFPA 499 and OSHA Requirements
When dealing with electrical area classification for combustible dusts, there are several different approaches suggested by scientific and regulatory bodies such as the NFPA, OSHA, and NMAB. The one you choose could have a significant impact on your plant’s safety, and could be costly.
1. The historical practice has been to use the recommendations from the National Materials Advisory Board (NMAB) report 353-3-80, “Classification of Combustible Dusts in Accordance with the National Electrical Code”, which defines dusts having Ignition Sensitivity (IS) ratios greater than or equal to 0.2 or Explosion Severity (ES) ratios greater than or equal to 0.5 to be appreciable explosion hazards requiring electrical equipment suitable for Class II locations.
2. These ratios are relative measures relying on extensive testing (such as the Pmax, KSt, MIE, MEC, and MIT). The ratios compare these explosibility characteristics of the dust under consideration against Pittsburgh Pulverized Coal. OSHA has adopted the NMAB interpretation of a Class II dust in their combustible dust national emphases program.
3. Recent changes to the recommended practice NFPA 499 stipulate that only a Go/No-Go explosibility screening test per ASTM E1226 is needed to determine if a sample requires special attention for electrical classification followed by a Layer Ignition Test (LIT) and a Minimum Ignition Temperature Test (MIT) for dust clouds to establish appropriate electrical “T”-codes, thus replacing the former ES and IS ratio determination. This paradigm shift seeks to make a hazardous location determination more accessible (i.e., cost effective); however, it ignores key information that is essential for safe dust management.
4. The explosibility data which constitutes the ES and IS ratios can be used in the design of deflagration relief vents, deflagration suppression and isolation systems, housekeeping level requirements, set pneumatic transport limits, help establish electrostatic hazard avoidance like grounding and bonding protocols and provide guidance on safe operation temperatures. In fact, these data will be required for safe operation of a given process.
5. The Go/No-Go test alone does not provide the data required for effective explosion mitigation engineering. While these tests increase the costs of sample analysis, they provide the data that could have prevented an explosion.
When considering dust, different regulatory bodies have different approaches. If you have questions about what approach is appropriate for your material, please contact Dr. Ashok Dastidar, Fauske & Associates, LLC at 630-887-5249 or via email at Dastidar@fauske.com. www.fauske.com