Process Safety

NFPA 652 compliance starts with a dust hazard analysis

Combustible dust safety standard requires understanding the codes.
By Vahid Ebadat, Ph.D. August 12, 2019
Courtesy: Stonehouse Process Safety Inc.

Formation of explosive dust clouds are possible during powder/dust handling, transfer, processing, dust collection and packaging operations. If there is an energetic ignition source present, a dust deflagration or explosion can result.

The National Fire Protection Association (NFPA) publishes several codes and standards to assist with efforts to assess and control dust fires and explosions in operations.

The relevant NFPA publications include:

  • NFPA 61: Standard for the prevention of fires and dust explosions in agricultural and food products facilities
  • NFPA 68: Guide for venting of deflagrations
  • NFPA 69: Standard on explosion prevention systems
  • NFPA 77: Recommended practice on static electricity
  • NFPA 484: Standard for combustible metals, metal powders and metal dusts
  • NFPA 499: Recommended practice for the classification of combustible dusts and of hazardous (classified) locations for electrical installations in chemical process areas
  • NFPA 654: Standard for the prevention of fire and dust explosions from the manufacturing, processing and handling of combustible particulate solids
  • NFPA 655: Standard for prevention of sulfur fires and explosions
  • NFPA 664: Standard for the prevention of fires and explosions in wood processing and woodworking facilities.

These publications also have created some confusion regarding consistency and specific applicability of some of their requirements.

NFPA 652 (2019 Edition) has sought to consolidate the best general practices for all combustible solids, regardless of industry and powder/dust type and to direct the user to appropriate NFPA industry- or commodity-specific standards.

A practical approach to identifying and controlling dust fire and explosion hazards in line with the requirements of NFPA 652 is required.

Dust cloud flash fires and explosions

Dust cloud flash fires (deflagrations) occur if the following conditions are present:

  • A combustible powder, with enough dust content (small particle size) to support flame spread, forming a cloud with a concentration above its minimum explosive concentration (MEC)
  • Sufficient oxidant, which typically atmospheric oxygen provides
  • An ignition source.

The first two conditions usually are present at least at some point during any powder/dust handling, transfer, processing, dust collection and packaging operations. In the presence of a simultaneous energetic ignition source, this can result in a dust cloud deflagration. If a deflagration is confined in a closed process vessel or room/building, a pressure can build up sufficient to rupture the confining enclosure; the event becomes a dust explosion.

Industry-specific standards

According to NFPA 652, the industry- or commodity-specific standards include NFPA 61, NFPA 484, NFPA 654, NFPA 655 and NFPA 664.

In terms of applicability of the standards, the general principle is summarized in Table 1.

Table 1: Applicability of NFPA 652 versus industry- or commodity-specific NFPA standards. Courtesy: Stonehouse Process Safety Inc.

Table 1: Applicability of NFPA 652 versus industry- or commodity-specific NFPA standards. Courtesy: Stonehouse Process Safety Inc.

If the combustible particulate solid is a mixture, the approximate proportions of each general particulate solid category shall be determined and documented according to available information and shall be used to determine representative samples. Table 2 summarizes NFPA 652 criteria for classifying combustible particulate solid mixtures.

NFPA 652, 2019 edition requirements

NFPA 652 requires the owner/operator of a facility where potentially combustible dusts might be present to be responsible for determining combustibility and explosiveness of materials (go/no go) based on the following:

  • Laboratory analysis of representative samples (the most reliable approach)
  • Historical facility data or published data, if representative of current materials and process conditions
  • Assumption that a material can explode, forgoing the laboratory analysis.

However, absence of previous incidents cannot be used as a basis for assuming non-combustibility. NFPA 652 also permits the use of the worst-case characteristics of the various materials being handled as a basis for design, but this can lead to expensive and unnecessary precautions.

Table 2: Classification of combustible particulate solid mixtures. Courtesy: Stonehouse Process Safety Inc.

Table 2: Classification of combustible particulate solid mixtures. Courtesy: Stonehouse Process Safety Inc.

Conduct a dust hazard analysis

The owner/operator of a facility where combustible or explosive dusts are present shall be responsible to ensure a dust hazard analysis (DHA) is completed. The absence of previous incidents cannot be used as the basis for not performing a DHA.

DHA is a systematic evaluation of potential dust fire, deflagration and explosion hazards in a process or facility where combustible/explosive powder is handled or processed and includes a list of measures to manage the associated risks.

Table 3 summarizes the conditions for fire and explosion hazards in process systems, buildings or building compartments.

Table 3: Conditions for fire and explosion hazards. Courtesy: Stonehouse Process Safety Inc.

Table 3: Conditions for fire and explosion hazards. Courtesy: Stonehouse Process Safety Inc.

Where a fire and/or deflagration hazard exists, identification and evaluation of specific hazard scenarios must include:

  • Identification of safe operating ranges
  • Identification of the safeguards in place to manage fire, deflagration and explosion events
  • Recommendation of additional safeguards where warranted, including a plan for implementation.

The DHA must be conducted by someone with proven expertise in hazards associated with handling and processing of combustible particulate solids.

For new constructions, a DHA shall be completed as part of the project. For existing processes and facilities, DHAs must be completed by September 7, 2020. The owner/operator must demonstrate reasonable progress each year in completing DHAs prior to the deadline.

The DHA must be reviewed and updated at lease every five years.

Fire, explosion hazards

Facility owners/operators have the responsibility for managing the identified fire, flash fire and explosion hazards within their process, building or building compartment. Meeting life safety, mission continuity and mitigation of fire spread and explosions can be achieved by either of the following means:

  • A prescriptive approach in accordance with Chapters 5, 7, 8 and 9 of NFPA 652, 2019 Edition, in conjunction with any prescriptive provisions of applicable commodity-specific NFPA standards.
  • A performance-based approach in accordance with Chapter 6 of NFPA 652, 2019 Edition. As stated in Chapter 9 of NFPA 652, 2019 Edition, the use of performance-based alternative designs for a process or part of a process, specific material or piece of equipment in lieu of the prescriptive requirements is permitted.

Establish written safety management systems

NFPA 652 requires owners/operators to retroactively establish written management systems for operating their facility and equipment to prevent or mitigate fires, deflagrations and explosions from combustible particulate solids. These written management system requirements must apply to new and existing facilities and processes.

Tips for managing dust hazards

Effective management of dust flash fires and explosions in facilities requires:

  • Having pertinent data for understanding the combustibility/explosiveness, ignition sensitivity, electrostatic, self-heating and explosion characteristics of dusts
  • Having a good understanding of all operations and processes
  • Conducting a dust hazard analysis (DHA), including the following:
    • Identification and evaluation of the process or facility areas where fire, flash fire and explosion hazards exist
    • Identification of safe operating ranges
    • Identification of the safeguards that are in place to manage fire, deflagration and explosion events
    • Specification of additional safeguards where warranted, including a plan for implementation
    • Ensuring safety through properly defined fire and explosion prevention, protection and isolation measures to meet the life safety, mission continuity and mitigation of fire spread and explosions
    • Establishing effective written safety management systems
  • Regularly reviewing and maintaining management programs, process safety data and information, training, process-control systems, processing equipment and the facility to ensure the continued safety of people, the community and the business.

Vahid Ebadat, Ph.D.
Author Bio: Vahid Ebadat Ph.D., M.Inst.P, MIET, C.Phys. is CEO of Stonehouse Process Safety Inc. He has worked extensively as a process safety consultant for the chemical, pharmaceutical, food, paper/wood and other process industries globally for more than 30 years.