March 2013 OH&S

NFPA 654 2013 Edition Revised Requirements for Housekeeping

Although the strategies offered in the new revision can provide a benchmark for triggering housekeeping efforts, plant management should strive for zero dust accumulations.

The 2013 revision of the NFPA 654 "Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids" was issued by the Standards Council on May 29, 2012, with an effective date of 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 incorporates several significant changes, most notably in the areas of housekeeping and establishing whether or not a flash fire or explosion hazard exists within the facility. This addition is particularly important 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. Other important areas that have been revised or where new requirements have been added include safety management practices such as hazard analysis, management of change, training, emergency procedures, and contractor and subcontractor safety.

This article will focus on the changes to the 2013 edition of NFPA 654 with regard to housekeeping requirements, including cleaning strategies.

Housekeeping is an extremely important administrative control to manage combustible dust accumulations that can result in a flash fire or explosion hazard. Managing dust levels below values calculated from Section 6.1 of the standard will obviate the need for personal protection equipment, deflagration venting in the building, and possibly not having to install electrical equipment specifically designed for hazardous Class II areas (combustible dust).

There are four methods to determine whether a flash fire or explosion hazard exists within a building or room in the plant. These methods include a layer depth criterion method, two mass accumulation methods, and a risk evaluation method that allows for a documented risk evaluation, acceptable to the Authority Having Jurisdiction (AHJ), to be conducted to determine whether or not the flash fire or explosion hazard exists. The Layer Depth Criterion Method and Mass Method A tend to be more conservative in nature and do not require input of specific dust properties or building parameters, other than the bulk density of the dust being generated. Mass method B considers additional factors, including the combustibility properties of the dust and building design.

Background
The most notable method of determining whether a flash fire or explosion hazard exists is the Layer Depth Criterion Method. This method is based on a layer depth criterion calculated by dividing the previous benchmark dust accumulation level (2006 Edition) of 1/32 inch, which was based on a dust with a bulk density of 75 pounds per cubic food, by the bulk density of the dust of interest to establish a new threshold dust accumulation thickness.

The total area of nonseparated dust accumulation exceeding the layer depth criterion 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 larger than 20,000 square feet, the maximum accumulations and total volumes are based on a footprint area of 1,000 square feet. For example, in a room with an area of 1,000 square feet where dust having a bulk density of 37.5 pounds per cubic foot has accumulated, the layer depth criteria is 1/16 inch and the accumulation of dust in the room is limited to not greater than 1/16 inch over 50 square feet. Additionally, the total volume of dust in this room cannot exceed (1/16 inch)/12 inches/ft*50 ft2 = 0.26 ft3 or 9.77 pounds.

A calculation can be performed to determine the airborne dust cloud concentration in a room if the bulk density of the dust, accumulation depth, and dispersion height above the floor is known. The genesis of the 1/32 inch accumulation, as a benchmark thickness, was based on this calculation using a dust bulk density of 75 pounds/cubic foot (1,200 kg/m3) and an assumed concentration of 0.32 ounces/cubic foot (320 g/m3).

Equation 1 - C = ρ*h/H

where
C = dust concentration (g/m3)
ρ = bulk density of dust in g/m3
h = dust accumulation (m)
H = height of room (m)

 

 

If the equation is solved for "h," the dust accumulation in meters, the calculation shows that a dust layer averaging 1/32 inch (0.8 mm) thick and covering the floor of a building is sufficient to produce a uniform dust cloud of optimum concentration (320 g/m3), 10 feet (3 m) above the floor throughout the building. This calculation assumes that the deposit would be uniform throughout the floor area and that the layer would be completely dispersed throughout the volume of the room by some initial pressure wave presumably caused by a primary explosion event.

If only 50 percent of the accumulation were suspended, the concentration in the air space would still be within the explosible range of most dusts. (Typical Minimum Explosible Concentrations [MECs] of combustible dusts range from 0.02 to 0.12 ounces/cubic foot (20-120 g/m3).

The rationale for establishing a threshold limit accumulation value prior to cleaning was that consideration should be given to the proportion of the building volume that could be filled with a cloud of combustible dust. The percentage of the floor area covered can be used as a measure of the hazard. A 10 foot by 10 foot room with a 1/32 inch layer of dust would obviously be hazardous; however, the same 100 square foot area in a building with a floor area of 2,000 square feet would be considered to be, relatively speaking, less hazardous. This represents about 5 percent of the total floor area and was judged by the standard committee to be as much accumulated dust as should be allowed before cleanup. The Layer Depth Criterion Method reflects this point.

Generally speaking, dust deposits on the floor do not readily become suspended to fill the volume or a significant portion of a room due to a pressure disturbance caused by some primary explosion event. Deposits at higher elevations, however, easily can be disturbed and lofted into suspension, creating a combustible dust cloud. The energy necessary to cause these disturbances can come from a primary explosion event, rupture of a steam or compressed air line, mechanical failure of equipment, a forklift striking a structural member of the building, lighting strike on the building, etc. Therefore, the importance of good housekeeping cannot be overemphasized. Although the strategies offered in the new revision can provide a benchmark for triggering housekeeping efforts, the goal of plant management should be to strive for zero dust accumulations. Even small accumulations can become airborne and present localized flash fire hazards.

Housekeeping Plan Requirements
Chapter 8, Fugitive Dust Control and Housekeeping - Section 8.2 Housekeeping, outlines housekeeping requirements. These requirements are intended to be applied retroactively. Where the facility is intended to be operated with less than the dust accumulation levels determined by the requirements of Section 6.1.1 (the hazard assessment that determines whether a flash fire or explosion hazard exists in the plant), housekeeping frequencies must be established that prevent the threshold accumulation levels from developing, and a planned inspection process needs to be in place to maintain the accumulations below the threshold dust mass/accumulation.

The housekeeping plan also must include requirements establishing time to clean local spills or short-term accumulations. Table A.8.2.1.3.(a) in the Annex provides guidance for these time intervals based on the level of accumulation on the worst single square meter of surface.

NFPA 654 (2013) Annex A - Table A.8.2.1.3(a) Unscheduled Housekeeping

Accumulation on the Worst Single Square Meter of Surface Longest Time to Complete Unscheduled Local Cleaning of Floor-Accessible Surfaces Longest Time to Complete Unscheduled Local Cleaning of Remote Surfaces 
 > 1 to 2 times threshold dust mass/accumulation  8 hours  24 hours
 > 2 to 4 times threshold dust mass/accumulation  4 hours  12 hours
 > 4 times threshold dust mass/accumulation  1 hour  3 hours
 

If the facility is operated at dust levels that exceed the chosen criterion as outlined in Section 6.1, then a documented risk assessment acceptable to the AHJ needs to be conducted to determine the level of housekeeping consistent with any dust explosion and dust flash fire protection measures provided in accordance with Section 6.4 and 11.2.2 of the standard. These measures include room or building deflagration venting and use of PPE, including flame-resistant garments, in accordance with the workplace hazard assessment required by NFPA 21131.

The revised standard also states that vacuuming is the preferred method of cleaning and establishes a hierarchy of cleaning methods: vacuuming first, followed by sweeping or water wash, and finally, if necessary, blowing down with compressed air, but only under the following conditions:

1. Vacuuming, sweeping or water wash down is performed first.

2. Dust accumulations in the area after cleaning as per item 1 do not exceed the threshold dust accumulation.

3. Compressed air hoses are provided with pressure relief nozzles limiting discharge pressure to 30 psig.

4. All electrical equipment potentially exposed to airborne dust in the area meets requirements of NFPA 702, NEMA 12, or equivalent.

5. All ignition sources and hot surfaces capable of igniting a dust cloud or layer are shut down or removed from the area.

Elements of a Sound Housekeeping Plan
Important items that should be part of any sound housekeeping plan include:

1. A risk analysis that considers specific characteristics of the dust being cleaned (particle size, moisture content, Minimum Explosible Concentration, Minimum Ignition Energy [MIE]) and other safety risks introduced by the cleaning method used.

2. Personal safety procedures, including fall protection when working above ground level.

3. Use of PPE, including flame-resistant garments in accordance with NFPA 2113.

4. Cleaning sequence.

5. Specific cleaning methods to be used.

6. Equipment used in the cleanup, including lifts, vacuum systems, etc.

The NFPA position on portable vacuum cleaners has also been relaxed when compared to the 2007 revision. The revision now allows use of non-listed or non-approved [Note: Listed or approved refers to equipment that has been included in list published by an organization, that is acceptable to the AHJ, and that indicates this equipment meets appropriate designated standards or has been tested and found to be suitable for a specified purpose.] portable vacuum cleaners to collect combustible particulate solids in non-electrically classified areas if they are made of noncombustible material, except for filter media and support frames, and where portable containers used to collect material are separated from the unit by a valve.

Hoses must be conductive or static dissipative and all conductive components, including wands and attachments, are required to be electrically bonded and grounded. Where metal dusts are being collected, vacuum cleaners must meet NFPA 4843 requirements (listed specifically for metal dusts). In Class II electrically classified areas, portable vacuums are required to be listed for the purpose and location, and in instances where flammable vapors or gases are present, vacuum cleaners must have a dual listing for both Class I and Class II hazardous locations.

Development and implementation of a sound housekeeping plan that meets the requirements of this standard will significantly reduce the risk of flash fire and explosion in the industrial plant setting. Training the employees to recognize combustible dust hazards and in the proper methods for cleaning is also of paramount importance as part of the overall safety culture of the company.

 

References
1. NFPA 2113, "Standard on the Selection, Care, Use, and Maintenance of Flame Resistant Garments for Protection Against Flash Fire," (2012) Edition, The National Fire Protection Association, 1 Batterymarch Park, PO Box 9101, Quincy, MA 02169-7471.

2. NFPA 70, "National Electrical Code," (2011) Edition, The National Fire Protection Association, 1 Batterymarch Park, PO Box 9101, Quincy, MA 02169-7471.

3. NFPA 484, "Standard for Combustible Metals," (2012) Edition, The National Fire Protection Association, 1 Batterymarch Park, PO Box 9101, Quincy, MA 02169-7471.

 

 

 

 

 

This article originally appeared in the March 2013 issue of Occupational Health & Safety.

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