Identifying the relevant NFPA standards on combustible dust
In trying to sort through the list of combustible dust standards, a good starting point for every plant engineer is NFPA 654, the Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids. Simply stated, NFPA 654 is an all-encompassing standard on how to design a safe dust collection system. It is regarded as the guiding dust document and the most general on the topic, and it will lead you to other relevant documents.
Depending on the nature and severity of the hazard, NFPA 654 will guide you to the appropriate standard(s) for explosion venting and/or explosion prevention, as follows:
NFPA 68 – Standard on Explosion Protection by Deflagration Venting: This document focuses on explosion venting – i.e., on devices and systems that vent combustion gases and pressures resulting from a deflagration within an enclosure, for the purpose of minimizing structural and mechanical damage. The current edition, published in 2007, contains much more stringent requirements than past editions, essentially elevating it from a guideline to a standard.
NFPA 69 – Standard on Explosion Prevention Systems: This standard covers explosion protection of dust collectors when venting is not possible. It covers the following methods for prevention of deflagration explosions: control of oxidant concentration, control of combustible concentration, explosion suppression, deflagration pressure containment, and spark extinguishing systems.
The general document (NFPA 654) also directs the reader to appropriate standards for specific manufacturing industries. The NFPA recognizes that different industries and processes have varying requirements, and it relaxes or tightens some aspects of its dust standards accordingly. Wood dusts, for example, tend to contain high moisture content that make for a potentially less explosive environment, resulting in a less stringent overall dust standard for that industry. Conversely, metal dusts can be highly explosive and subject to more vigilant regulation.
The industry-specific standards most commonly employed are:
NFPA 61 – Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities. This standard covers facilities engaged in dry agricultural bulk materials including grains, oilseeds, agricultural seeds, legumes, sugar, flour, spices, feeds, and other related materials; facilities that manufacture and handle starch; seed preparation and meal-handling systems of oilseed processing plants not covered by NFPA 36, Standard for Solvent Extraction Plants. Examples of facilities covered by NFPA 61 include but are not limited to bakeries, grain elevators, feed mills, flour mills, corn and rice milling, dry milk products, mix plants, soybean and other oilseed preparation operations, cereal processing, snack food processing, tortilla plants, chocolate processing, pet food processing, cake mix processing, sugar refining and processing, and seed plants.
NFPA 484 – Standard for Combustible Metals. This standard covers all metals and alloys in a form that is capable of combustion or explosion, and it outlines procedures that shall be used to determine whether a metal is combustible or noncombustible form. It also applies to processing or finishing operations that produce combustible metal powder or dust such as machining, sawing, grinding, buffing and polishing. Parts that contain multiple metals or alloys are subject to the requirements of the metal whose combustion characteristics they most closely match. The standard also defines exclusions such as the transportation of metals or the primary production of aluminum, magnesium, and lithium.
NFPA 664 – Standard for the Prevention of Fire and Explosions in Wood Processing and Woodworking Facilities. This standard establishes the minimum fire and explosion prevention requirements for facilities that process wood or manufacture wood products using wood or cellulosic fibers, creating wood chips, particles, or dust. Examples include wood flour plants, industrial woodworking plants, furniture plants, plywood plants, composite board plants, lumber mills, and production-type woodworking shops and carpentry shops that meet minimum requirements for plant size or dust collection flow rates.
Using Performance-Based Codes: In 1995, the NFPA created a Performance-Based (PB) Support Team to assist NFPA Technical Committees with the transition to performance-based documents. Since that time, the NFPA has been incorporating performance-based options into its updated standards: The NFPA 654 general dust document first adopted this concept in 2006, with the other more specific combustible dust standards following suit since that time. Using the newer performance-based codes, solutions no longer have to follow NFPA standards to the letter if the variance is backed by full-scale, real-world destructive test data.
Performance-based provisions state specific life safety objectives and define approved methods to demonstrate that your design meets these objectives. They give equipment manufacturers and plant engineers greater flexibility by allowing methods to quantify equivalencies to existing prescriptive-based codes or standards, as long as the proposed solution demonstrates compliance.
A performance-based design procedure includes the following steps: (1) establish safety goals; (2) evaluate all aspects of the facility with regard to safety; (3) identify potential hazards; (4) define appropriate hazard scenarios; (5) establish performance objectives and criteria; (6) select calculation methods (e.g. computer models); (7) develop a proposed solution; (8) assess the solution; and (9) obtain approval.