Page 38 - Occupational Health & Safety, May 2019

COMBUSTIBLE DUST
Towards Compliance: NFPA 652 Standard on the FundEamentals of Combustible Dust
BY VAHID EBADAT
34 Occupational Health & Safety | MAY 2019
www.ohsonline.com
xplosible dust clouds can be expected at some Requirements of NFPA 652
point during any powder/dust handling, The intended life safety, mission continuity, and transfer, processing, dust collection, or pack- mitigation of fire spread and explosions objectives of aging operation. Of course, if there is an igni- NFPA 652 are as follows:
tion source present, a dust deflagration or explosion ■ Reasonably protect occupants not in the im-
may occur. It is clearly essential that effective precau- tions are taken to control the risk and protect employ- ees, contractors, the public, and the facility against the effects of fires and explosions.
Over the past few decades, the National Fire Pro- tection Association (NFPA) has published a number of codes and standards to help industry with its ef- forts to assess and control dust fires and explosions. Although very useful, these publications also have created some confusion regarding consistency and applicability of some of their requirements.
The NFPA 652 Standard on the Fundamentals of Combustible Dust has sought to consolidate the best general practices for all combustible solids, irrespec- tive of industry and powder/dust type and to direct the user to appropriate NFPA industry- or commodity- specific standards. It also establishes relationship and hierarchy between these standards, while ensuring that fundamental requirements are addressed consis- tently across industries, processes, and dust types.
This article will discuss the requirements of the 2019 edition of NFPA 652 and provide some practical measures for controlling dust flash fire and explosion hazards associated with the handling, processing, and storing of combustible powders.
Conditions for Dust Cloud Flash Fires
and Explosions
Dust cloud flash fires (deflagrations) must be antici- pated when the following conditions are simultane- ously present:
1. A combustible powder forming a cloud with a concentration above its Minimum Explosible Con- centration (MEC). [There needs to be enough dust content (small particle size) to support flame spread.]
2. Sufficient oxidant, which the oxygen in air will usually provide.
3. An ignition source with sufficient energy.
It should be noted that conditions 1 and 2 are usu- ally present at least at some point during any powder/ dust handling, transfer, processing, dust collection, or packaging operation. The simultaneous existence of an energetic ignition source may then result in a dust cloud flash fire. If the ignition occurs in a closed pro- cess vessel or room/building, pressure can build that will be sufficient to rupture the confining enclosure and cause a dust explosion event.
mediate proximity of the ignition from the effects of fire for the time needed to evacuate, relocate, or take refuge
■ Reasonably prevent serious injury from flash fires and explosions
■ Reasonably protect adjacent properties and the public from the effects of fire, flash fire, or explosion
■ Limit damage to levels that ensure the ongo- ing mission, production, or operating capability of the facility to a degree acceptable to the owner/operator
■ Prevent or mitigate fires and explosions that can cause failure of adjacent buildings, compartments, enclosures, properties, storage, the facility’s structural elements, or emergency life safety systems
To achieve these objectives, NFPA 652 requires that the owner/operator of a facility where potentially combustible dusts might be present shall be respon- sible for the following:
■ Determining combustibility and explosibil- ity (Go/No-Go) of materials. It should be noted that the Go/No-Go—as the name implies—would only provide information on whether or not a dust is com- bustible (would burn) or explosible (deflagrate) under the right conditions. No information would be ascer- tained regarding the ease of ignition or the severity of the explosion. NFPA 652 permits the determina- tion of combustibility or explosibility (Go/No Go) to be based on laboratory analysis of representative samples from the process or historical facility data or published data, so long as the data is representative of current materials and process conditions. One is also permitted to assume that a material is explosible, forgoing the laboratory analysis. However, absence of previous incidents shall not be used as basis for deem- ing a particulate non-combustible or non-explosible.
■ Conducting a Dust Hazard Analysis (DHA). DHA is a systematic review to identify and evaluate potential dust fire, flash fire, and explosion hazards in a process or facility where combustible/explosible powder is handled or processed. The DHA must be conducted by someone with proven expertise in haz- ards associated with handling and processing com- bustible particulate solids.
■ For existing processes, the owner/operator shall schedule the DHA to be completed by Septem- ber 2020. For new construction or modifications more than 25 percent of the original cost, the DHA