The long-awaited 2013 revision of NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, has arrived. This revision was issued by the Standards Council and became effective on June 18, 2012.
The goal of the NFPA 654 standard is to provide safety measures to prevent and mitigate fires and explosions in facilities that handle combustible particulate solids. This standard applies to all phases of the manufacturing, processing, blending, conveying, repackaging and handling of combustible particulate solids or hybrid mixtures — regardless of concentration or particle size — where the materials present a fire or explosion hazard. The owners or operators of affected facilities are responsible for implementing the requirements.
The 2013 edition has incorporated a number of changes. These include administrative controls such as safety-management practices, added training requirements for contractors and subcontractors, and incident investigation and reporting requirements. Important sections regarding housekeeping programs and a hierarchy of cleanup operations also are included in this revision. This is an important area because incident history and statistics clearly indicate that secondary dust explosions — caused by inadequate housekeeping and excessive dust accumulations — have caused much of the damage and casualties experienced in major industrial dust explosions.
This article will focus on the hazard assessment that is required to be conducted to determine if a flash fire or explosion hazard exists inside the plant.
Changes in the Facility and Design Requirements
In the NFPA 654 standard, the sixth chapter, “Facility and Systems Design Section 6.1.1 Hazard Assessment,” details four methods for determining if flash fire or explosion hazards exist within a particular area of the plant. Each method is considered to provide an equivalent level of safety.
For the purposes of each method, accumulated mass and bulk density are to be determined on a dry basis (less than or equal to 5 percent moisture). Dust accumulations are deemed to be nonseparated unless segregation, separation or detachment is used to limit the hazard area, and all dust accumulated on structures above the lowest footprint are evaluated as if accumulated on the lowest footprint.
Dust-accumulation amounts should reflect conditions that exist just prior to routinely scheduled cleaning and should not include short-term accumulations that are cleaned in accordance with Chapter 8 of the standard. Housekeeping procedures to allow for cleanup of local spills and short-term accumulations must be in place to receive credit.
Methods to Determine the Flash Fire or Explosion Hazard
The four methods of determining the flash fire or explosion hazard are: the layer depth criterion method, mass method (version A), mass method (version B) and risk evaluation method.
Layer Depth Criterion Method. This method establishes a baseline combustible-dust accumulation based on the bulk density of the dust. This criterion divides the previous benchmark accumulation level (2006 Edition) of 1/32", which was based on a dust with a bulk density of 75 lb/ft3, by the bulk density of the dust of interest to establish a new threshold accumulation thickness. The total area of nonseparated dust accumulation cannot exceed 5 percent of the footprint area, and the total volume in the footprint area cannot exceed the layer depth criterion multiplied by 5 percent of this area.
For areas greater than 20,000 ft2, the maximum accumulations and total volumes are based on a footprint area of 1,000 ft2. For instance, in a room with an area of 1,000 ft2, where dust having a bulk density of 37.5 lb/ft3 has accumulated, the layer depth criterion is 1/16", and the accumulation of dust in the room is limited to not greater than 1/16" over 50 ft2. Additionally, the calculation to determine the total volume of dust that this room may not exceed is shown:
Total volume of dust =
1/16" ÷ 12 in/ft x 50 ft2
Total volume of dust = 0.26 ft3, or 9.8 lb
A few examples will demonstrate calculations based on the layer depth criterion method.
Example 1.In the first example, assume a single-floor accumulation area in a small portion of a 100' by 40' room. The givens are:
• The dust has a bulk density of
37.5 lb/ft3.
• Layer depth criterion is 1/16".
• Room footprint area is 4,000 ft2.
• Actual accumulation area is 25 ft2.
• Average layer depth in the accumulation area is 1/8".
Given the above criteria, the calculation to determine the total volume of dust that this room may not exceed is:
Ratio = (50 ft2 x 1/8") ÷
(0.05 x 4,000 ft2 x 1/16")
Ratio = 6.25 ft2-in ÷ 12.5 ft2-in
Ratio = 0.5
Because the ratio is less than or equal to 1 — in this example, the ratio is 0.5 — a dust deflagration hazard does not exist in the room. When the actual accumulation area is less than 5 percent of the room footprint, the layer thickness can be greater than the layer depth criterion without resulting in a dust deflagration hazard.
Example 2. In the second example, assume a machining operation that results in piles of dust accumulations. The machining operation is located in a 10' by 10' area in a 100' by 100' room. Between periodic housekeeping, the machining operation results in three 12-by-12-by-16" piles of dust with a bulk density of 40 lb/ft3 and a uniform dust layer thickness throughout the room of less than 1/64". The givens are:
• Room footprint area is 10,000 ft2.
• Actual accumulation area is 100 ft2.
The first step is to determine the adjusted layer depth criterion:
Layer depth criterion =
(1/32" x 75 lb/ft3) ÷ 40 lb/ft3
Layer depth criterion = 0.059"
The second step is to determine the allowable volume of dust:
Allowable volume of dust =
0.05 x 10,000 ft2 x 0.059" x [1' ÷ 12"]
Allowable volume of dust = 2.46 ft3
The third step is to determine the actual volume of dust. In this example, the layer of dust is less than 1/64" and does not need to be factored into the calculation.
The fourth step is to calculate the volume of dust per pile and the total volume of dust in the three piles.
Volume of dust per pile =
(12" x 12" x 16") x (1 ft3 ÷ 1,728 in3)
Volume of dust per pile = 1.33 ft3
Total volume of dust = 3 x 1.33 ft3
Total volume of dust = 4 ft3
The total volume of dust exceeds the allowable volume of dust; therefore, a dust deflagration hazard exists.
Mass Method A. A dust flash fire or dust explosion hazard area is judged to exist when the total accumulated dust external to the equipment exceeds the quantities determined from Equation 1 and 2 below.
The threshold dust mass establishing a building or room as a dust explosion hazard area, Mbasic-exp, is determined by the
Equation 1:
Mbasic-exp = 0.004 x Afloor x H kilograms
The threshold dust mass establishing a building or room as a dust flash-fire hazard area, Mbasic-fire, is determined by the Equation 2:
Mbasic-fire = 0.02 x Afloor kilograms
where
Mbasic-exp is the threshold mass (kg) based on building damage criterion.
Mbasic-fire is the threshold mass (kg) based on personnel fire exposure criterion.
Afloor is the lesser of enclosure floor area (m2) or 2,000 m2.
H is the lesser of enclosure ceiling height (m) or 12 m.
These equations do not require measurement of any physical or combustibility properties for application and tend to lead to a more conservative outcome.
Mass Method B. This method incorporates a number of parameters to determine if a flash fire or dust explosion hazard exists. A dust flash fire or dust explosion hazard area exists when the total accumulated dust external to process equipment exceeds the quantities determined from Equations 3 and 4.
The dust explosion hazard area equation — shown as Equation 3 below — has been adapted from NFPA 68, which adjusts the amount of venting needed when the combustible mixture only fills part of the room, and also considers explosion dynamics based on the properties of the dust of interest. Thus, it is possible to more accurately determine the potential for explosion inside the facility.
Equation 3:
Mexp = [Pes ÷ DLF] x [Cw ÷ Pmax] x Afloor x [H ÷hD] kilograms
where
Mexp is the threshold dust mass (kg) based on building damage criterion.
Pes is the enclosure strength evaluated to vent or fail (bar g) per NFPA 68.1
DLF is the dynamic load factor, the ratio of maximum dynamic deflection to static deflection per NFPA 68.
Cw is the worst-case dust concentration (kg/m3) at which the maximum rate of pressure rise results in tests conducted per ASTM E1226.
Pmax is the maximum pressure (bar g) developed in ASTM E 12262 tests with the accumulated dust sample.
Afloor is the enclosure floor area (m2).
H is the enclosure ceiling height (m).
hDis the entrainment factor, which is 0.25.
Equation 4 below for calculating the area for a flash-fire hazard estimates the fraction of the volume that could be filled by an expanded fireball from burning dust. The room or building volume up to a person’s height is taken as the total volume of the expanded fireball, regardless of the actual room or building height. The threshold for the flash fire hazard is based on allowing the accumulated dust mass to reach the worst case concentration in an unburned volume, which, when expanded in a fireball, is only a fraction of the volume described by the product of person height and floor area. The risk tolerance criterion is the probability of an occupant being in the same location as the deflagration flame. This choice implies that some residual risk remains.
Equation 4:
Mfire = p x Cw x [Pinitial ÷ (Pinitial + Pmax)] x Afloor x [D ÷ hD] kilograms
where
Mfire is the threshold mass (kg) based on personnel fire exposure criteria.
p is the probability of flame impingement on a person, not to exceed 0.05 (5 percent probability).
Cw is the worst-case dust concentration (kg/m3) at which the maximum rate of pressure rise results in tests conducted per ASTM E1226.
Pinitial is 1 bar absolute.
Pmax is the maximum pressure (bar g) developed in ASTM E 1226 tests with the accumulated dust sample.
Afloor is the enclosure floor area (m2).
D is nominal height of a person (2 m).
H is enclosure ceiling height (m).
hDis the entrainment factor, which is 0.25.
Risk Evaluation Method. This method affords the evaluator some flexibility when assessing the hazard, and it supplements the process hazard analysis outlined in Chapter 4 of the standard. The risk assessment, to be performed by a competent professional, determines that the proposed strategy will achieve the goals that one of the other prescriptive methods is designed to achieve. Also, this method can be used where other methods may be viewed as impractical. The method is based on scientific facts and test data generated with regard to the combustible dust(s) and provides equivalent safety when compared to the other methods.
The approach can be a very important alternative to the first three methods for industries that generate significant amounts of combustible dusts due to the inherent nature of their manufacturing operations, or where dust control or cleanup may present significant challenges.
The risk-based approach considers the physical, chemical, explosibility and ignitibility properties of the dust as part of the overall risk assessment. Typical questions the assessor asks include:
• Is the combustible dust (or dusts) very difficult to ignite?
• Does the combustible dust have very weak explosibility properties?
• Does the nature of the dust particle (size and shape) make generation of a combustible dust cloud improbable?
• Are incendive ignition source(s) likely?
Where a flash fire or explosion hazard exists, based on the outcome of applications of these methods, the operator must provide the employees with personal protective equipment (PPE), including flame-resistant garments, in accordance with the workplace hazard assessment required by NFPA 2113.3
In addition, areas demonstrating an explosion hazard must be detached, segregated or otherwise separated from other occupancies. This requirement may include the need to provide deflagration venting in the room or building as well as a hazardous area electrical classification for electrical equipment in the area.
References
1. NFPA 68 “Standard for Explosion Protection by Deflagration Venting” (2007 Edition); The National Fire Protection Association, 1 Batterymarch Park, PO Box 9101, Quincy, MA 02169-7471.
2. ASTM E 1226 “Standard Test Method for Explosibility of Dust Clouds”; ASTM International,100 Bar Harbor Drive, P.O. Box C700, West Conshohocken, PA 19428-2959.
3. NFPA 2113, “Standard on the Selection, Care, Use, and Maintenance of Flame Resistant Garments for Protection Against Flash Fire,” (2012) Edition, National Fire Protection Association.
Report Abusive Comment