NFPA 654 - New Requirements for Explosion Protection and Isolation

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 since 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 changes or additions include requirements for air-material separators and vacuum systems, safety management practices such as hazard analysis, management of change, training, emergency procedures, and contractor and sub-contractor safety. This article will focus on some of the more important changes with regard to fire and explosion protection of air-material separators and vacuum systems.

Explosion Protection

Defining the Hazard
This revision contains a much needed definition of what constitutes an explosion hazard inside process equipment. The hazard is defined to exist if combustible dust is present in sufficient quantity to cause enclosure rupture if suspended and ignited, and a means of suspending the dust is present. This definition will help the individuals responsible for conducting the process hazard analysis required by the standard under Section 4.2 to assess the specific equipment inside the plant where combustible dust handling, processing, or collection may be involved. If the design of the equipment can prevent the development of an explosible dust cloud inside it, then explosion protection, for example, may not be required. Based on dust collection techniques, dust-loading parameters, and airflow, an analysis can be performed to determine if an explosion hazard exists inside a specific piece of equipment.

Ductwork
The revised standard imposes some new requirements for ductwork that handles particulate solids and where flexible hose with wire re-enforcing is used or where the flexible hose or boots are made of non-metallic materials. For wire-reinforced plastic hose, the resistance between the conductive components must be less than 106 ohms. This can be achieved by bonding the wires at each end of the hose to the respective sections of pipe to which the hose is attached. In the case of flexible hoses or boots, the following requirements are imposed:

1. Hybrid mixtures must not be present.
2. Conductive dusts must not be conveyed.
3. Minimum Ignition energy (MIE) of the dust must be greater than 3 mJ.
4. The nonconductive components of the hose or boot must not allow isolation of conductive components from ground.
5. The breakdown voltage across non-conductive sheets or membranes must not exceed 4 kV when used in high powder charging applications.

    The last requirement is predicated on the powder demonstrating high chargeability. The services of an expert may be needed to assist the operator in making this determination. Powder “volume resistivity” and “chargeability” testing will probably be required to make the determination.

Air-Material Separators (AMS)
The standard recognizes enclosureless air-material separators (AMS) that are frequently found in wood processing plants and already recognized by the standard (1) that governs the control of wood dust hazards. These units are used to remove dust from the transport air and blow it into large filter socks that are not enclosed. They are permitted to be located inside the plant without explosion protection if the following conditions are met:

1. The filter medium is not shaken or pressure-pulsed to dislodge dust during operation.
2. The AMS is not used to vent or serve metal grinders, hot work processes, or machinery that can produce sparks.
3. The AMS is not used to vent or serve sanders, abrasive planers, or similar sanding process equipment.
4. Each collector system has a maximum air flow handling capacity of 3000 CFM (1.4 cu m/sec).
5. The fan motor is suitable for Class II, Division 2, or Class III, as appropriate.
6. The collected dust is removed daily or more frequently if necessary to ensure efficient operation and to limit the collected dust to less than 22 lb.
7. The collector is located at least 20 ft from any means of egress or area routinely occupied by personnel.
8. Multiple collectors in the same room are separated from each other by at least 20 ft.
9. The minimum ignition energy (MIE) of the collected materials is greater than 500 mJ.
10. The fan construction is spark resistant and meets the criteria for spark resistant construction as outlined in Section 7.12.2.5 (blowers shall be of Type A or B construction where dust/air stream concentration is higher than 10 percent of MEC or Type C if protected with spark detection and extinguishment located downstream of the fan).
11. The filter medium is not located within 35 ft of any open flame or hot surface capable of igniting a dust cloud of the material it contains.

Wet Scrubbers
Some requirements are also added for wet scrubbers (wet collection) for dust collection that include:
1. Interlocks must be provided to shut down the system if the flow rate of the scrubbing medium is less than the designed minimum flow rate;
2. The scrubbing medium must not be a flammable or combustible liquid.
3. The separator must be designed to prevent the formation of a combustible dust cloud within the air-material separator (AMS).

Explosion Isolation
New language has been added with regard to explosion isolation. The previous version of the standard required explosion isolation where an explosion hazard exists between pieces of equipment connected by ductwork and also upstream of air-material separators (dust collectors, for example) to the work areas. The concerns are twofold. In the former case, an explosion initiated in one piece of equipment could propagate to the connected equipment, causing “pressure-piling” and resulting in a deflagration that could produce significantly higher pressure and turbulence in the connected vessel. Thus, the connected equipment could fail even if it was protected with a suitable explosion protection system such as a deflagration vent. In the latter case, the potential fireball developed inside the equipment could propagate back upstream in the ductwork, potentially injuring personnel working near the aspiration ducts or hoods; for example, at bagging and hand-dumping operations.
    The requirement in the previous version of the NFPA 654 has been revised. The word, "ductwork" has been eliminated and the requirement now reads, "where an explosion hazard exists, isolation devices shall be provided to prevent deflagration propagation between connected equipment in accordance with NFPA 69”. The revision also modifies requirements with regard to the need for explosion isolation between pieces of connected process equipment, where an explosion hazard is deemed to exist in each. The new requirement exempts the need for isolation where all of the following requirements are met:

1. Metal dust or hybrid mixture is not being conveyed.
2. The connecting ductwork is smaller than 4 in. nominal diam.
3. The maximum concentration of dust conveyed through the duct is less than 25 percent of the minimum explosive concentration (MEC) of the dust.
4. The conveying velocity is sufficient to prevent accumulations of combustible dust in the duct.
5. All connected equipment is properly designed for explosion protection by means other than deflagration pressure containment.

    The exemption, based on the size of the ductwork, is likely based on information in Annex A 11.2 of the NFPA 69 standard regarding isolation which states, "Piping less than 4 in. diameter is far less likely to provide a conduit for flame spread than larger diameters".
    With regard to cases where the fireball developed inside the equipment could propagate back upstream through the ductwork into the work area, the standard has been modified to include all equipment, instead of just air-material separators where an explosion hazard exists. The revision also adds a section that permits the use of systems designed for pre-deflagration detection and control of ignition sources, for reducing the probability of occurrence of a deflagration in (1) ductwork supplying air-material separators, (2) recycled air from air-material separators and (3) ductwork between process equipment. However, installation of these systems does not preclude the isolation requirement.
    The NFPA recently published a guide (2) to combustible dusts and provides some insight with regard to the need for isolation where personnel are working upstream near aspiration ducts. An estimate of how far upstream from an air-material separator that a fireball, from a deflagration inside the protected vessel, may travel can be made by using the following equation:
 
Lupstream = Vdirty{[(Pmax + 1)/(Pinitial + 1)] -1}/(Aduct + Av)

where:

Lupstream = upstream duct length for a simple flame expansion (m),
Vdirty = initial dirty-side volume of the air-material separator, excluding the volume displacement of the filter elements (cu m),
Pmax = maximum pressure of an unvented deflagration initially at atmospheric pressure (barg),
Pinitial = enclosure pressure at the moment of ignition (barg),
Aduct = inlet duct cross sectional area (sq 2), and
Av = vent area (sq m)

    If an upstream work area pick-up location is nearer than this distance to the air-material separator, explosion isolation should be considered, even when all other risk reduction measures are in place.

Recycling Exhaust Air from Duct Collectors
The requirements in the revised version have been expanded and modified. Recycling of air-material separator exhaust to buildings or rooms is now permitted only where all of the following requirements are met:

1. Combustible or flammable gases or vapors or combustible solids are not present either in the intake or the recycled air in concentrations above applicable industrial hygiene exposure limits or 1 percent of the LFL (gases or vapors) or MEC (solids), whichever is lower.
2. The oxygen concentration of the recycled air stream is between 19.5 and 23.5 percent by volume.
3. Provisions are incorporated to prevent transmission of flame and pressure effects from a deflagration, or smoke and flame from a fire, in air material separator back to the facility unless a process hazard analysis indicates that those effects do not pose a threat to the facility or the occupants.
4. The system includes a method for detecting air-material separator malfunctions that would reduce collection efficiency and allow increases in the amount of combustible particulate solids returned to the building.
5. The building or room to which the recycled air is returned meets the fugitive dust control and housekeeping requirements of the standard.
6. Recycled air ducts are inspected and cleaned at least annually.

    Item 1 adds a requirement for gases and vapors and replaces the former requirement of 99.99% filtering of solid particulate greater than 10 microns and is consistent with practical industrial hygiene measuring techniques and control measures.

Vacuum Systems
More definitive requirements for both fixed and portable vacuum cleaners have been added to the revised standard.

Portable Vacuum Systems
The standard no longer requires use of a portable vacuum cleaner "listed" for use in Class II hazardous locations unless the area is classified as a Class II area. The standard imposes several design requirements for portable units which include:

1. Materials of construction must be noncombustible, except for filter media and support frames.
2. Portable containers used to collect material are separated from the unit by a valve.
3. Hoses to be conductive or static dissipative.
4. All conductive components, including wands and attachments, to be bonded and grounded.
5. Dust laden air does not pass through the fan or blower.
6. Electrical motors not be in the dust laden air-stream unless listed for Class II, Div 1 atmospheres.
7. Where liquids are picked up, paper elements are not to be used.

    In addition, vacuum cleaners used for metal dusts need to meet NFPA 484 requirements (listed specifically for metal dusts) and if flammable vapors or gases are also present the vacuum cleaners must be listed for both Class I and Class II hazardous locations.

Fixed (Centralized) Systems
Similar to portable vacuum cleaners, there are also requirements that the hoses, couplings, and tools must be made of conductive or static-dissipative materials that are bonded and grounded. Other requirements added to this revision include:

1. The system must be designed to ensure minimum transport velocities at all times.
2. The system must be operated only with the hoses and tools that have been designated in the design documentation for the specific hose connection station.
3. Vacuum controls must be provided to prevent overfilling the air-material separator (AMS), which could disable the system.
4. The maximum number of hose connection stations that can be simultaneously used must be included in the system documentation.

       The new requirements for vacuum systems should result in increased safety. Frequently during hazard assessments conducted in industrial settings, vacuum system hoses are found to be nonconductive in nature and, in some cases, where they are re-enforced with wire, the wires are not attached to the hard pipe and vacuum tool. This can lead to the development of Propagating Brush Discharges inside the hose which can ignite a combustible dust cloud inside and also create an electrostatic shock hazard outside the hose to employees responsible for vacuuming activities.

Summary
The 2013 revision of the NFPA 654 standard includes several new requirements for air material separators including portable vacuum cleaning systems. These requirements address measures to improve control of ignition sources as a basis of safety, provide a definition of an explosion hazard inside process equipment, expand upon when and where explosion isolation is necessary, and recognize the use of enclosureless dust collection systems. The prescriptive requirements outlined in this article should serve to assist personnel involved in the preparation of process hazard assessments, as required by the standard.
    
    Steven J. Luzik, PE, CFEI, is a senior process safety specialist at Chilworth Technology Inc., with more than 30 years of experience in the area of fire and explosion hazards, including gas/vapor explosions, dust explosions, and fire and explosion protection strategies. Luzik is a Certified Fire and Explosion Investigator (CFEI) with the National Association of Fire Investigators (NAFI). He is a member of the American Society for testing and Materials (ASTM) E-27 Committee on Hazardous Properties of Chemicals, the National Association of Fire Investigators (NAFI), and the National fire Protection Association (NFPA). He has authored numerous publications in the areas of fire and explosion prevention, protection, and investigation. For more information on Chilworth, visit www.chilworth.com.

1 NFPA 664, "Standard for the prevention of Fires and Explosions in Wood processing and Woodworking Facilities", (2012), The National Fire Protection Association, Quincy, MA.
2 “NPFA Guide to Combustible Dusts”, Walter L. Frank, P.E., and Samuel A. Rodgers, P.E., The National Fire Protection Association, Quincy, MA

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