Making Sense of Combustible-Dust Hazard Analysis

Several standards on combustible dust contain provisions for conducting process hazard analyses. This article summarizes the requirements and presents a simple approach to meet them.

Process hazard analysis (PHA) uses structured brainstorming techniques to identify and evaluate hazards associated with chemical processes, understand their causes and consequences, and develop safeguards to manage the risks (1). Conceived in the 1960s (2), PHA has been conducted in many industries for decades, and the concept of hazard evaluation has been refined and adapted for a variety of applications. PHA is now finding its way into combustible-dust hazard management.

Several National Fire Protection Association (NFPA) standards on combustible dust contain provisions for conducting PHAs. The newest, NFPA 652: Standard on the Fundamentals of Combustible Dust (3), became effective on Sept. 7, 2015. It requires that dust hazards analyses (DHAs) be completed on existing facilities and significant modifications. The legacy standard, NFPA 654: Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids (4), contains requirements for PHAs that include dust hazard assessments. Different hazard analysis requirements apply to facilities that fall under an industry- or commodity-specific NFPA standard (e.g., metals, agricultural and food, wood processing and woodworking, sulfur, dust).

The differing requirements and recommendations can make it difficult to know where to start and which PHA technique to use. This article presents a flowchart to guide you through the dust hazard assessments and provides guidance on how to incorporate them into a PHA that meets the hazard analysis requirements found in the various standards.

The NFPA standards

The standards may be classified as:

• general requirements that apply to all combustible-dust handlers
• specific dust standards (which the NFPA refers to as industry- or commodity-specific).

NFPA 652, the newest standard, falls into the general category and is intended to ensure that fundamental requirements are met and to guide compliance with other, existing standards. NFPA 652 is intended to apply alongside the relevant specific standards. Thus, when it comes to combustible-dust hazard management, facilities will need to comply with two standards — NFPA 652, and the specific standard that applies to their operations and/or industry sectors.

Table 1 lists the NFPA standards that govern the management of combustible-dust hazards. Additional standards (not included in the table) apply to some specific operations, equipment (e.g., ovens) or mitigation systems (e.g., relief vents).

NFPA 652. The new standard’s requirements for a dust hazards analysis include:

1. Complete a DHA of existing processes within three years of the standard’s publication (i.e., by Sept. 7, 2018).
2. Complete a DHA of significant modifications if their cost exceeds 25% of the original cost.
3. Evaluate the fire, deflagration, and explosion hazards of each part of the process and each building and its compartments. During this evaluation, identify safe operating ranges for equipment and processes, identify existing safeguards designed to prevent operation outside of the safe range, and, if necessary, recommend additional safeguards and develop a plan for their implementation.
4. Provide recommendations for managing combustible-dust hazards.
5. Document the results of the DHA.

NFPA 654. The legacy standard on preventing combustible dust flash fires and explosions applies to combustible dusts that are not covered by dust-specific standards. Although often considered a general dust standard because of its broad applicability, it functions as a specific standard for operations not covered elsewhere, such as pharmaceuticals, plastics, petrochemicals, and textiles manufacturing and processing. For a process hazard analysis that includes a dust hazard assessment, NFPA 654 requires:

1. Design the fire and explosion safety provisions based on a PHA.
2. Specify and document the maximum allowable accumulation of dust. In the dust hazard assessment portion of the assessment, evaluate both the facility compartments and individual process equipment for flash fire and explosion hazards, calculate the allowable dust accumulation per unit area, measure the amount of accumulated dust, and determine whether the process is operating above or below the allowable accumulation.
3. Specify and document the minimum personal protective equipment for employees.
4. Document the results of the PHA
5. Review and update the PHA every five years.

NFPA 61, 484, 655, 664. Some of the dust-specific standards contain requirements or recommendations for conducting PHAs. For example, the...