I plant aren’t aware of the plant’s dust explosion hazards · 2000. 6. 1. · Dust explosion hazards are seldom given the same careful analysis as a plant’s fire hazards. This

Powder and Bulk Engineering, June 2000 31

Three steps to protecting your process from dust explosions

Clive I. Nixon and David Grandaw Fenwal Safety Systems

While explosions account for less than 4 percent of processing plant interruptions in the US each year, they account for nearly 40 percent of all losses, in- cluding personnel injuries and deaths, equipment losses, and downtime. This article details how you can prevent such a costly disaster at your plant by taking a planned approach to explosion protection. Sections outline the three steps in this approach and list sources for further reading on explosion protec- tion and applicable National Fire Protection Associ- ation codes and standards.

n many cases, personnel in a dry bulk solids processing plant aren’t aware of the plant’s dust explosion hazards I until an explosion occurs. Dust explosion hazards are

seldom given the same careful analysis as a plant’s fire hazards. This is despite the fact that a relatively small amount of dust can generate a devastating pressure wave when ignited.

Dust from common dry bulk solids like sugar, starch, and plastics can produce a dust explosion as hazardous as any created by propane, butane, or gasoline. Unlike a fire, a dust explosion can propagate rapidly throughout your process in fractions of a second. The explosion can destroy the process unless an integral explosion protection system checks its progress.

Insurance statistics show that the average dollar loss per plant explosion is $3.4 million. But this doesn’t have to be

the story in your plant. You can prevent this kind of disas- ter at a reasonable cost by taking a planned approach to ex- plosion protection.

This approach includes three steps:

1. Analyze the combustion and explosibility character istics of materials in your process.

2. Understand the applicable codes and standards.

3 . Select the right explosion protection system.

Analyze the combustion and explosibility

Like a fire, an explosion is a combination of basic ele- ments: fuel, oxygen, and an ignition source. The fuel for the explosion can come from a dust cloud generated by a dry bulk material or from a process by-product such as a flam- mable gas or volatile chemical vapor. The oxygen is readily available in almost any plant process. The ignition source can be any of several items: a flame, a welding arc, sponta- neous combustion, a fictional spark, or an electrostatic dis- charge. f i t these three elements into an enclosed vessel - a dust collector, spray dryer, cyclone, or other process ves- sel- and you’ve created a dust explosion hazard.

1 characteristics of materials in your process

Before you can analyze your process material’s explosibil- ity characteristics, you must first consider whether the ma- terial is explosible. The answer can be obvious if your process produces a flammable gas or volatile vapor by- product. For explosibility information on specific gases and vapors, you can refer to a chemical handbook or direc-

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tory, such as Sax S Dangerous Properties of Industrial Ma- terials, N. Irving Sax and Richard J. Lewis Jr. (New York Van Nostrand Reinhold Co.).

But if your material produces a dust cloud, the answer may not be so obvious. You can find explosibility data for some common dusts in National Fire Protection Association (NFPA) 68, Guide for Venting of Deflagrations. You can also have your material tested in what’s called a go-no go test at an independent explosion research and testing lab. In the test, your material is suspended in a dust cloud at various concentrations and is subjected to a strong (10- kilojoule) ignition source. If this test shows your material is explosible, additional tests can determine more about the material’s hazards.

For such a detailed analysis of your material’s explosibil- ity hazards, you can send samples of the material to an in- dependent lab that specializes in analyzing these hazards or to the lab of an explosion protection system supplier that provides this service. Each lab uses sophisticated spherical pressure test vessels to test samples of your bulk material (or your process by-product) for a variety of ex- plosibility characteristics. For instance, the lab will test for a dust’s pressure-generation characteristics by deter- mining the dust’s K,, value, which is the internationally recognized index for classifying dust explosibility. The lab can measure the burning velocity of gas and vapor mixtures and dozens of parameters that affect how and under what conditions a combustible material will ex- plode, including the material’s lower flammability limit, minimum ignition energy, and autoignition temperature. Analyzing these parameters will not only provide a basis for designing an explosion protection system for your process, but will help you determine the safest operating conditions for the process.

Understand the applicable codes and 2 standards Many plant operations and safety personnel work under the misconception that process equipment doesn’t require explosion protection. But in fact, codes and standards do govern explosion protection for this equipment.

The primary source for explosion protection codes and standards is the “A, which develops and publishes sev- eral. Your plant’s insurance carrier will typically have copies of them or have recommended procedures based on them. The “ P A codes and standards usually will be man- dated by authorities having jurisdiction in your area. These authorities vary from area to area, but an example is the local f i e marshal. The f i e marshal may enforce local fire and explosion protection regulations and may dictate the use of specific explosion protection methods for that area. Other authorities that may have jurisdiction at the federal, state, or local level include OSHA, building officials, elec- trical inspectors, and insurance companies.

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The NFPA explosion protection codes and standards can be broadly categorized by industry, by process equipment, and by explosion protection system.

Industry codes and standards. Since World War 11, NFPA technical committees have been issuing explosion protec- tion codes and standards covering several at-risk indus- tries. Listed in the sidebar, “NFPA explosion protection codes and standards,” page 34, the codes and standards focus on preventing, controlling, and extinguishing fire and explosions in these industries.

A typical standard, NFPA 61, Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Products Facilities, prescribes the requirements for a range of items, such as process construction; explosion prevention, relief, and venting; safety controls; and hot work (such as welding and blowtorch cutting) on the types of equipment typically found in these facilities.

Process equipment codes and standards. The NFPA pro- vides explosion protection codes and standards, also listed in the sidebar, for facilities with certain types of process equipment that are subject to explosion hazards.

The most comprehensive of these and the most applicable to the dry bulk solids processing and handling industries is NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids. The stan- dard’s major requirements are:

Segregation: Where combustible dusts are present, you must ensure that penetrations through floors, walls, and ceilings (for such items as conveying lines, chutes, pipes, and support legs) are dust-tight.

Separation: If dust accumulations greater than % inch are possible or if hazardous dust clouds are present, you must perform an engineering evaluation to determine the safe separation distance (which is at least 30 feet) between the process generating the combustible dust and other opera- tions in the same room.

Means of egress: To provide safe egress for people in your plant, your building should conform to require- ments in NFPA 101, Life Safety Code.

Deflagration venting: In a room or building with a com- bustible dust hazard external to the process, you must vent the room or building to a safe location, such as an open area outside the plant, to direct a deflagration out of the enclosed area.

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-. *Ignition sources: Reduce the potential for ignition by

doing one or more of the following: comply with the Na- tional Electric Code for removing potential ignition

and maintain process equipment; prevent conveyor belt

slippage occurs; use conductive conveyor belts and con-

sources; remove tramp metal from the process; lubricate

slippage or provide automatic conveyor shutdown when

ductive belt carriers; use roller or ball bearings rather than bushings; eliminate manual dust dumping into flammable

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atmospheres; obtain hot work certificates for all opera- tions that can be potential ignition sources; locate direct- rn

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Explosionprotection: You must use one of these forms of explosion protection for process equipment: inerting, fuel dilution, deflagration venting, containment, or sup- pression. Equipment handling combustible dusts, such as bucket elevators, mixers and blenders, particle size re- duction equipment, and dryers, must be protected.

Powder and Bulk Engineering, June 2000

Explosion isolation: You must provide explosion isola- tion in locations where deflagration propagation can occur, including the return-air side of material-air separa- tors that return air to work areas. Isolation devices include chokes, rotary valves, fast-acting valves, flame-front di- verters, and flame-front extinguishing systems.

*Duct systems: You must use dust-tight, conductive, and bonded ducts in duct systems.

36 Powder and Bulk Engineering, June 2000

In a test explosion at a combusfion mseurch center, a vent pnel on a dust collecior ruptures to &/y vent h e explosion. Flashback down the collecbr‘s inlet dud (at boftomj illusimtes why it‘s importcrnt to incorporate an exphion isolafion system beiween connededpmcess equipment.

fired process equipment (such as a kiln) or direct-fired comfort heating equipment (such as a furnace) outdoors or in a separate building; and maintain hot surfaces outside your process at either 80 percent of the ignition tempera- ture of your combustible dust hazard or 165°C.

=Inspection testing and maintenance: Implement a pro- gram to ensure that your fire and explosion protection systems operate as designed, and ensure that a change in your process equipment doesn’t increase the explosion hazard.

Explosion protection system codes and standards. Two major standards, also listed in the sidebar, address explo- sion protection systems: NFPA 68, Guide for Venting of Deflagrations, and NFPA 69, Standardfor Explosion Pre- vention Systems. You or an explosion protection engineer (who can be either an independent consultant or can be on the staff of an explosion protection system supplier) can use both standards to help design the proper explosion pro- tection system for your process.

NFPA 68 covers deflagration fundamentals, deflagration venting in low- and high-strength enclosures, and defla- gration venting of gas mixtures from high-strength enclc- sures and from pipes and ducts. The standard also describes types of deflagration vents and vent closures and provides vent inspection and maintenance guidelines.

NFPA 69 covers oxidant and combustible concentration reduction, deflagration suppression, deflagration pressure containment, spark detection and extinguishing systems, and isolation methods.

3 Select the right explosion protection system Once you’ve analyzed the explosion protection hazards in your process and consulted the applicable NFPA stan- dards, work with an explosion protection engineer to se- lect an explosion protection system (or combination of systems). This engineer can also oversee the explosion protection system’s design and installation.

Three of the most common types of explosion protection systems are vent, isolation, and suppression. A vent system includes a vent panel mounted on the process equipment; the panel ruptures quickly and reliably at a predetermined pressure, allowing an explosion’s pressure shock wave and flame to vent to a safe area. An isolation system is mounted on the process equipment or related equipment and uses one or more detector, one or more mechanical or chemical isolation device, and a control device to prevent a deflagration from spreading to other equipment. The system can also include passive devices such as suitably constructed rotary gate valves or cylindrical screw con- veyors. The components in a suppression system, includ- ing one or more detector, one or more extinguisher, and a control device, are mounted on the process equipment or related equipment to discharge an explosion suppressant that will contain an explosion within milliseconds, before it can create destructive pressures. [Editor’s note: Find more detailed information about these systems in articles listed in the later section “For further reading.”]

For an example of the steps to follow in choosing an explo- sion protection system for your process equipment, see Figure 1. The figure shows a decision matrix for choosing

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“How to protect your drying process from explosions,” an explosion protection system for a dust collection sys- tem, but the same steps apply to choosing explosion pro- Henry Garzia and Dan Guaricci, April 1995, page 53. tection for other process equipment.

“How to protect your dust collector from dust explo- sions,” John A. Constance, October 1994, page 67.

Keeping the system in good shape Once you’ve selected and installed an explosion protec-

“Using explosion vents in dust collectors,” Michael A. Maxwell, January 1992, page 19.

tion system, work with the explosion protection engineer to set up an inspection and maintenance program. This will keep your system functioning properly and ensure

“Investigation: How dust exploded during railcar load- ing,” Dr. Vahid Ebadat, January 1999, page 57.

that it continues to comply with NFPA standards. PBE

For further reading These Powder and Bulk Engineering articles provide more information about explosion protection systems and related topics:

CliVe 1. Nixon is marketing managerfor industrial eXpl0- sion Protection at Fe~wal safe0 Systems, 700 AW~erson Road, Marlborough, MA 01 752; 508/481-5800, ext. 2501, fax 508/485-3115 (clive.nixon @fenwalsafety.com). He holds an HNC in electric and electronic engineering and an ONC inphysics, both from Slough College of Technol- ogy in Slough, Berkshire, England, as well as a BS in tech- nical communication from Northeastern University in

“HOW to safely handle explosible dust,” Dr. K. Chatrathi, Part 1: January 199 1, Page 22; Part 11: February 1991, page 12.

Boston. David Grandaw is regional sales manager for the company and has served on the National Fire Protection Association 30B technical committee since 1990. He holds

“Testing to assess your powder’s fire and explosion haz- ards,” Dr. Vahid Ebadat, January 1994, page 19.

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“Thermal hazards: How to identify and minimize them in your drying process,” Dr. Vahid Ebadat and James C. Mulligan, April 1997, page 61.

an associate degree infire protection engineering technol- ogy from Northeast Wisconsin Technical Institute in Marinette .