Combustible Dust and Its Hazard (Explosion Protection Design)

Important Considerations

The design for explosion protection must incorporate at least one of these methods:

      I.        Reduction in the oxidant concentration

    II.        Dilution with non-combustible dust

   III.        -Deflagration Control by ventilation, pressure containment, suppression or devices for flame suppression and containment. 

            Any part of the structure that is designed to relieve pressure during an explosion should be located and constructed so that it poses no accident risk while in use

The most common explosion control mechanisms are:

·         Confinement or containment,

·         Inerting

·         Isolation, suppression or deflagration release

Processing Equipment that generates combustible dust must be connected to a dust collection system. This system usually consists of a structure that conveys the dust through pipelines from the point where it is generated to the final collection point known as a dust collector. Dust can be transported through the system via mechanical energy (conveyor), or pneumatic (air). The NFPA 654, includes as collection and material handling equipment, systems for material transfer, pipelines, power machinery equipment, separators, conveyors, mixers, dryers and more.

Dust collectors must be equipped with safety devices that prevent or minimize the risk of

an explosion during normal operation, as established by NFPA 654

Filter systems for dust collectors cannot be replaced by alternative type systems unless

a complete fire risk assessment is performed and it has been properly documented and

approved by management.

It is recommended that whenever possible, locate bucket elevators outside the building.

For systems that handle metal particulate is necessary to classify the material as:

·         water, compatible

·         non- compatible with water, and

·         water-reactive,

to determine the fire protection system appropriate to the material properties.

Walls and  internal partitions construction in areas where combustible dust is present, 

usually require a minimum fire resistance protection of at least 1hr.

If the material transfer equipment is used for various products you must verify the

compatibility between products prior to using the equipment.

It is of upmost importance to always keep the horizontal surfaces and work area clean of

material accumulation.

In the event that some areas are difficult to reach or cannot be easily cleaned, the

recommendation is to completely seal them off.

The initial and refresher training to employees should include the area hazards, process

description, procedures, equipment, fire and explosion protection systems,

maintenance requirements, cleaning and emergency response plans.

The equipment maintenance includes frequent inspections to detect and remove product accumulation in critical areas. 

APPENDIX A – OSHA STANDARDS

What standards or guidelines are available?

OSHA (Occupational Safety and Health Administration)

Currently there is under approval process an OSHA standard that would cover industrial operations where combustible dusts are present. Although there is still no comprehensive standard for all operations that handle combustible dusts, several OSHA standards indicate certain key provisions for the control of risks associated with the handling of combustible dust in industrial operations. The standards discussed in this section are mandatory according to the type of industry applicable to your particular operation.

1910.22 - General Requirements (see Housekeeping)

1910.22 (a) (1) All places of employment, passageways, storage areas and service should be kept clean, in order and hygienic.

1910.22 (a) (2) the floor of each work area must be kept clean and kept dry as much as possible.

Since these are general requirements, they apply to all general industry operations covered by the Code of Federal Regulations (CFR) 29.1910

1910.38 - Emergency Action Plans

1910.39 - Fire Prevention Plans

1910.157 - Portable Fire Extinguishers

1910.165 - Employee alarm systems

These standards require employers to establish emergency action plans, including fire’ indicate the minimum necessary procedures to respond to emergencies, the equipment requirements for fire prevention and training requirements for personnel

1910.94 – Ventilation

Related to abrasive blasting systems,  the standard indicates that if flammable or explosive mixtures of dust are present, the work area must be constructed to provide secure mechanisms to release pressure in case of explosion. It incorporates by reference the NFPA 68 the requirements for the area construction and for security mechanisms.

1910.119 - Process Safety Management (PSM)

The purpose of this standard is to prevent or reduce the catastrophic consequences that arise from the handling of highly hazardous materials processes. The applicability of the standard depends on the type of material and the amount of material used in the process. Refer to Appendix A for a list of materials covered under the standard.

1910.132 - Personal Protective Equipment

The facility must have implemented the program including a risk analysis of each task for the selection of the equipment. Clothing and accessories for use in areas where combustible dust is present must include fire-resistant clothing and electrically rated footwear.

1910.176 - Material Handling and Storage

This standard generally covers the basic requirements for proper handling of materials. Section 1910.176 c) specifies that storage areas be kept free of accumulation of material that may cause danger of falling, tripping, fire, explosion or pests.

1910.178 - Industrial Forklifts

Table N-1 of the standard defines what kind of truck is suitable for use in handling and transportation of combustible dusts for Class II, Groups E, F, G. The type of forklift is identified by letters indicating which fuel or energy is used to operate the motor, and the equipment safety devices.

 For areas Class II, Groups E and F where combustible dust is present, the only trucks approved are those designated as EE or EX. For the Group G the approved designation will be only GS, LPS, DS, DY, ES, EE, or EX. All these designations provide mechanisms to prevent that electrical or combustion systems of the equipment enter unwanted ignition sources in areas where dust are present.

1910.263 - Bakery Equipment

Section 1910.263 (k) identifies the electrical safety requirements for equipment used to pulverize sugar and / or spices and section 1910.263 (l) establishes requirements for equipment / mechanisms to release pressure in case of explosion.

1910.265 - Sawmills Operations

The standard applies to sawmill operations such as, logging, handling, cutting, sanding, handling and storage of timber or related products. Establishes requirements to prevent fire or explosion of dust collection equipment, electrical protection for certain equipment and also minimum cleaning frequency requirements of the work area

1910.272 - Grain Handling Facilities

This was the first standard written specifically to control risks associated with grain handling, classified as combustible dust. It is aimed primarily at grain handling operations for the food industry and specifies the procedures, equipment 

requirements and safe work practices for that type of operation. Typically, the silos are considered confined spaces and therefore must also refer to Standard 1910.146 - Confined Spaces for safety considerations when working in silos or similar spaces.

1910.307 - Hazardous Locations

This standard defines  the requirements for electrical equipment and wiring to be used in electrically classified areas based on the properties of flammable liquids, solids or gases, combustible dusts and / or flammable fibers present in that area.  It references the NFPA 70 standard.

1910.1200 - Hazard Communication

This standard requires besides the identification of health hazards, the physical hazards present in the work area including but not limited combustible dusts. Each employer is responsible for obtaining the information about the safe handling of these materials and providing such information to  their employees through signs, labels, notices. The employer must provide training related to the handling and proper use of the material as well as ways to protect employees from the hazards associated with particular material.

Combustible Dust and Its Hazard (Risk Factors)

Risk Factors 

What elements should I consider to prevent an explosion?

Two of the most contributing factors to a combustible dust explosion are

(1)  the accumulation of dust in the work areas and surfaces and

(2)   the presence of ignition sources.

To control these factors we must pay particular attention to housekeeping programs and to the control or prevention of sparks, flame or heat sources that can provide energy to start the fire that might produce the explosion.

It is important to identify whether any of the materials that your industry handles is classified as a combustible dust. That information could be found in the Material Data Sheets (MSDS) or technical papers provided by the manufacturer. NFPA 61 provides a list of organic materials that generate combustible dust and are typically present or produced in agricultural operations. Some common examples are sugar, starch, flour, grains, etc.

Employees must also know the proper handling of any combustible dust present in the facility, so you must include the information of these materials in your Hazard Communication Program

There should be an inspection, maintenance and regular cleaning program that controls dust accumulation to prevent it from reaching dangerous concentrations.  Both equipments and working surfaces must be inspected to avoid dust accumulations.

Equipment and procedures used for cleaning purposes should not introduce risks elements such as dispersing dust while cleaning or non controlled ignition sources arising from equipment or vehicles.

The electrical rating of the work area determines the necessary controls for electrical equipment, machinery and lighting where dust is present. The ignition source controls should consider all energy sources, be it electrical, thermal or static. Table 4.5.2 of NFPA 499 indicates those chemical materials that have been identified as combustible dusts and assigns classification according to the electrical requirements of the groups E, F, G of Class II.

How do you identify a combustible dust problem?

To identify whether your industry is at risk is necessary to analyze the work areas that present problems in terms of cleanliness (maintenance, ventilation systems, horizontal surfaces), of ignition sources (electrical, mechanical, thermal) and of administrative controls (training, procedures).

Here you will find a series of questions that will help you identify these risk areas. The questions are designed to help you identify, in general, if your facility may be at risk of fire or explosion due to the presence of combustible dust. At the end of each section of questions are suggested standards and / or reference standards that can help reduce the risk you've identified.

House-keeping	Yes	No
Is there any solid particulate in the work area arising from processes, or raw materials?
What kind of dust? Metallic
Wood
Resin or plastic
Organic (food)
Carbon Derived
Chemical Materials
Is there dust accumulation in the work areas? Such as conduits / pipes
Ventilation systems
Hoods
Beams
Work surfaces
False Ceilings
Equipment or machinery
Others
If dust accumulated, is the thickness more than 1/16"?.
More of 1/8 "?
Does the dust cloud or dust accumulation interferes with visibility in the area?
Are there leaks in systems containing or transporting dust?
Are the cleaning methods, dry methods (brush, brushes, compressed air) that can disperse dust in the area?

If you answered YES to any of these questions, you may be at risk of fire or explosion due to accumulation of dust in work areas. You must set up a cleaning program that includes frequent inspections in the workplace. The program should indicate a regular schedule for cleaning floors and horizontal surfaces and should include immediate corrective actions in case of excessive accumulation of material. Refer to standards 1910.22, 1910,176 OSHA, the NFPA 654 and any particular standard that applies to your industry.

Ignition Control Measures	Yes	No
Do you know if your workspace has any electrical classification?
Is there a program for the control of static electricity?
Is the machinery that generates dust connected and grounded to minimize accumulation of static electric charge?
Are dust transport and handling equipment properly grounded and bonded?
Are the devices / electrical cleaning equipment approved for the hazard classification of Class II locations?
Do you use forklifts or industrial trucks that are approved for use in Class II areas where combustible dust is present?
The electrical equipment meets the requirements of NFPA 70 and NEC for Class II locations?
Is there a hot work permit program for areas where combustible dusts are present?
The areas where smoking is prohibited are marked with signs indicating "No Smoking"?
Do equipments that produce heat or flames in the work areas (i.e.  boilers, furnaces, dryers), contain safety devices to control these sources of ignition?
Are there methods of detection and / or fire extinction that may be caused by, sparks, temperature, or flame?
Are there methods for the detection of smoke / particles?
If there are devices to remove foreign materials such as magnets, are protected from causing ignition of combustible dust?
Are the construction materials of dust collection equipment and where dust is stored  fire resistant?
Does the construction of these areas/ equipment, limit the spread of damage in case of fire /explosion?
Are there controls to reduce the amount of oxygen in areas where dust is stored?
Are there devices installed to prevent explosion such as ventilation, pressure containment and / or suppression?

Measures to control auto-ignition-temperature	Yes	No
Are there ways to control the temperature in areas where combustible dust is generated?
Are there ways to control the temperature where combustible dust accumulates?
Are there moisture controls for combustible dust?

If you answered NO to any of the questions of these two sections, you can be at risk of fire or explosion due to the presence of ignition sources in work areas where combustible dust is present. You must establish a monitoring program of ignition sources, which considers the reduction or elimination of static charge, and ignition sources from equipment that produce flames, heat or sparks. You must also verify that vehicles, equipment or machinery that comes into classified areas is properly approved to operate in such area. Electrical facilities and fixtures of that area must meet the requirements of NFPA 499, National Electrical Code and NFPA 70. Check the OSHA standards 1910.176, 1910.307, and any particular standard that applies to your industry.

Protective measures
Are dust collectors are located outside the buildings?
The construction of dust collectors or storage site is combustible dust explosion-proof?
Does the vent / grille / duct exhaust blast is directed into a safe location away from employees?
Are dust collectors are designed so that the blast is released into places it does not affect other areas?
Are there isolation devices to prevent the spread of the conflagration between devices connected by duct system?
Are the ventilation equipment connected by ductwork are designed to prevent an explosion from spreading to other areas?
Will dust collecting systems have spark detection systems and explosion suppression systems / explosions?
Does the fuel dust powder is diluted with fuel, to reduce their flammability?

If you answered NO to any of these questions can be at risk of fire or explosion due to inadequate ventilation in work areas where combustible dusts are present. You must establish a ventilation system program that includes verifying the design and capacity of hoods, fans and dust collectors. Also verify, the materials of construction of these devices are adequate to resist fire / explosion.  Material handling and transportation equipment must have all safety devices to contain or safely release the effects of an explosion.

Employee Safety	Yes	No
Do you investigate accidents?
Are small explosions/ fires investigated and documented?
Are the persons in the combustible dust explosion risk area authorized to be there?
Is there an action plan in case of emergencies?
Are the emergency exit routes adequately maintained?
Are employees trained in the emergency plan?
Is there a hazard communication program at the facility?
Are employees trained in the hazard communication program (including combustible dust risks.)
Are employees trained before beginning work in areas where there is danger of a combustible dust explosion?
Are employees trained when they are reassigned to another area, if there are changes to processes or changes to the Hazard Communication program?
Does the Hazard Communication Program include Material Safety Data Sheets (MSDS) on combustible dusts?
Are the safety data sheets of materials that could be classified as combustible dust under normal operation conditions available to employees?.
Do employees use all necessary protective equipment for the task including fire retardant clothing?

If you answered NO to any of these questions employees may be at risk of injury from fire or explosion caused by combustible dust.

You must ensure that these programs are implemented:

1) Hazard Communication

2) Emergency Action Plan and Fire Prevention and Protection Plan.

3) Personal Protective Equipment

4) Accident Investigation

You must also verify that all employees are properly trained in the details of these programs, and have written procedures associated with them.