BACKGROUND OF HOUSEKEEPING
Maintaining good housekeeping in facilities handling or processing combustible dust is crucial. Over the past 30 years, investigations by the Chemical Safety Board and other agencies into serious dust explosions have identified several common causal factors, with inadequate housekeeping programs being a significant one. These programs are essential to keep combustible dust accumulations at safe levels, preventing flash fires or explosions.
Plant management must be committed to controlling accumulations of combustible dust in their facilities to protect employees and property against secondary flash fires and explosions. The design and implementation of a rigorous housekeeping program will ensure that combustible dust accumulations are effectively managed.
A well-designed housekeeping program will also include cleanup frequencies designed to ensure that dust levels do not exceed hazardous accumulation levels and will include timetables for the cleanup of unscheduled spills.
Risks of Secondary Dust Explosions
Secondary dust explosions occur when dust accumulations on elevated surfaces or floors become airborne and ignite due to a primary event. These explosions are particularly dangerous as the initial blast can cause more dust to become suspended and ignite, potentially leading to multiple, powerful secondary explosions.
Regulatory Requirements
- NFPA Standards: Effective housekeeping programs are mandated by NFPA standards to manage the risk of secondary explosions, which can cause greater loss of life, injuries, and facility damage than primary explosions.
- OSHA Housekeeping Standard (29 CFR Part 1910.22): Requires that workplaces, passageways, storerooms, and service rooms be kept clean and orderly. Floors must be maintained in a clean and dry condition. This standard is somewhat subjective and open to interpretation by compliance safety and health officers.
- NFPA 652 and 654: These standards require the removal of dust from floors, structural members, and other surfaces concurrently with operations. Each facility must develop and implement a written housekeeping program outlining the frequency and methods for reducing fugitive dust accumulations. NFPA 654 is particularly detailed in providing guidelines for establishing a comprehensive housekeeping program.
Methods to determine dust accumulation limits
To assess the risk of flash fires or explosions due to dust accumulation, four primary methods can be utilized. These methods ensure that housekeeping programs are effectively structured to prevent hazardous dust accumulation levels, safeguarding the facility from potential fire and explosion risks. The methods are:
The Layer Depth Criterion Method
This method sets a maximum allowable dust layer thickness based on the bulk density of the dust. It calculates the layer depth criterion (LD) by using a reference bulk density value and adjusting it for the specific dust in question. This criterion helps determine when dust accumulation poses a hazard. The layer depth, LD, is calculated by multiplying the previous benchmark accumulation level (NFPA 654, section 6.1.3) of 1/32 in. by 75 lb/ft3, (assuming a dust with a bulk density of 75 lb/ft3 as a reference), and dividing this number by the bulk density of the dust of interest (ρ), to establish dust a new maximum allowable accumulation thickness or layer depth criterion.
Equation 1: LD (in.) = [(1/32) * 75]/ρdust
Where: LD = layer depth (in.) and BD (ρdust )= Bulk density (lb/ft3)
Mass Method A
This approach evaluates whether a flash fire or explosion hazard exists by determining the total dust mass accumulated externally to the equipment. Using specific equations, it establishes threshold dust masses that signify hazard levels. It is a more straightforward method that doesn’t require detailed physical property measurements, making it a conservative yet practical option. 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 Equations 2 and 3. The threshold dust mass establishing a building or room as a dust explosion hazard area, M basic-exp is determined by the following equation:
Equation 2: M basic-exp = 0.004*Afloor*H
where:
Mbasic-exp =threshold dust mass (kg) based on building damage criterion
Afloor=lesser of enclosure floor area (m2) or 2000 m2
H=lesser of enclosure ceiling height (m) or 12 m
The threshold dust mass establishing a building or room as a dust flash-fire hazard area, Mbasic-fire, is determined by the following equation:
Equation 3: Mbasic-fire = 0.02*Afloor
Where:
M basic fire is threshold mass (kg) based on personnel fire exposure criteria.
A floor is lesser of enclosure floor area (m2) or 2000 m2
These equations do not require measurement of any physical or combustibility properties for an application and tend to lead to a more conservative outcome.
Mass Method B
A more complex method involves separate equations to calculate the threshold dust mass establishing the hazard. It requires inputs on various parameters such as building strength, worst-case dust concentrations, dust entrainment factors, and the probability of flame impingement. This method is detailed in NFPA 654 and provides a thorough hazard assessment but can be challenging due to the need for precise data. NFPA 654 (section 6.1.5.1) provides more information with regard to this method.
Risk Evaluation Method
This method involves a documented risk evaluation, accepted by the authority having jurisdiction (AHJ), to determine the presence of a dust explosion or flash fire hazard. It focuses on material properties and facility design features. Specialist expertise and specific test data are usually required, making it suitable for scenarios with unique dust characteristics, such as low bulk density or low ignition sensitivity.
Typically, the use of this method will require specialist expertise with extensive knowledge of combustible dust and the associated fire and explosion hazards. In addition, appropriate test data will be required with regard to the particular dust being generated. The method can offer significant advantages over the other three methods described here, for example, where:
- the bulk density of dust is low.
- hygroscopic dust is being handled/generated.
- the ignition sensitivity properties of dust are low.
- the nature of the solid particulate makes the formation of fine dust clouds difficult.
Summary Methods to Determine Dust Accumulation Limits Estimation
The goal of any effective housekeeping program should be to maintain dust accumulation levels below those at which flash fire or explosion hazards will exist in the plant if the accumulated dust were to become airborne. The development and effective implementation of such a plan, including the inspection component, should ensure that the goals are achieved.
The Layer Depth Criterion or Mass Method A can be used to determine the threshold limits of dust accumulations to manage the risk of dust flash fire and explosion. If these methods prove to be impractical or problematic to implement, then management should consider a risk-based evaluation.
Implementation of an effective housekeeping program can significantly reduce the risk of having a major industrial dust flash fire or explosion incident.
RELATIONSHIP BETWEEN HOUSEKEEPING AND HAZARDOUS AREA CLASSIFICATION
The housekeeping plan should include the following:
- To prevent accumulated dust on walls, floors, and horizontal surfaces of equipment, ducts, pipes, hoods, ledges, beams, and concealed areas like above suspended ceilings and the interior of electrical enclosures from exceeding established levels, a frequent cleanup schedule is crucial.
- Implementing a planned inspection process is necessary to evaluate dust accumulation rates and determine the required housekeeping frequencies to keep dust levels below the set limits.
- Specific requirements, establishing time to clean up local spills or short-term accumulations (see Tables 1 and 2)
- The housekeeping plan must address specific cleanup requirements, including the time frame for addressing local spills or short-term accumulations, as outlined in Tables 1 and 2. It should also incorporate the electrical classification of the facility, referencing NFPA 70 (National Electrical Code) Article 500.4 (B) and NFPA 499 for guidelines on hazardous area classification in areas where combustible dust is present. NFPA 499 provides the specifications for hazardous area boundaries, defining Class II, Division 1, and Division 2 areas.
- Class II, Division 1 areas are those where combustible dust is in the air under normal conditions in sufficient quantities to create explosive or ignitable mixtures, or where mechanical failure or abnormal equipment operation could produce such mixtures and an ignition source. Dust layers over 1/8 inch thick under normal conditions classify the area as Division 1. Division 2 areas, on the other hand, are where combustible dust is not typically in the air, and dust accumulations are usually insufficient to interfere with normal operations. However, dust may be suspended due to infrequent malfunctions. Good housekeeping practices can distinguish between Division 1 and Division 2 classifications and unclassified areas based on dust thicknesses, ensuring safety and compliance with established standards.
- Based on the thicknesses of dust, good housekeeping can determine the difference between a classification of Division 1 and a classification of Division 2 and between a classification of Division 2 and unclassified (see Table 1).
Table 1: Unscheduled Housekeeping
| Accumulation on the Worst Single Square Meter of Surface | Longest Time to Complete Unscheduled Local Cleaning of Floor-Accessible Areas | Longest Time to Complete Unscheduled Local Cleaning of Areas |
|---|---|---|
| >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 |
Table 2: Cleaning frequencies based on the depth of the dust layer
| Depth of Dust Accumulation (in.) | Frequency | Housekeeping Requirements | Area Electrical Classification |
|---|---|---|---|
| < 1/32 | Continuous / frequent | Clean up as necessary to maintain an average accumulation below 1/64 in. | Unclassified; however, electrical enclosures should be dust-tight |
| 1/32 to 1/8 | Infrequent | Clean up during same shift | Unclassified; however, electrical enclosures should be dust-tight |
| 1/32 to 1/8 | Continuous / frequent | Clean as necessary to maintain an average accumulation below 1/16 in. | Class II, Division 2 |
| > 1/8 | Infrequent | Immediately shut down and clean | Class II, Division 2 |
HOUSEKEEPING PROCEDURE
- Establishing a visual baseline in the process area where dust can be seen and measured over specific periods, such as a shift or a day, is critical. This helps in identifying the proper cleaning frequency required to ensure dust accumulation remains below the benchmark thickness of 1/32 inch, as indicated in Tables 1 and 2.
- Ensuring that dust-handling systems, including exhaust ducts, dust collectors, vessels, and processing equipment, are designed to prevent fugitive dust from leaking into the work area is essential for maintaining a safe environment.
- Regular cleaning of horizontal surfaces, floors, decks, walls, bulkheads, and various equipment components such as ducts, pipes, hoods, ledges, beams, stair rails, and concealed surfaces above suspended ceilings should be performed at a frequency sufficient to prevent dust accumulations of 1/32 inch or greater. Housekeeping methods should be designed to avoid generating dust clouds that can resettle at higher elevations.
- The use of vacuum cleaners, whether mobile units or central vacuum systems, is the preferred method for housekeeping. Central systems typically offer larger storage capacities and provide connection points at various locations without needing to move mobile units in cramped quarters, although both solutions are acceptable for effective housekeeping.
- Portable electric vacuum cleaners must be listed for use in Class II atmospheres, with hoses and tools that are static dissipative or conductive and properly grounded. Mobile vacuum cleaners should meet specific requirements to ensure safety and efficiency in maintaining cleanliness and preventing hazardous dust accumulations.
- The operation of portable vacuum cleaning devices should be subject to a dust hazard analysis to ensure that the risk to personnel and facility operations from deflagrations is minimized. This analysis will help identify potential hazards and establish appropriate safety measures for the use of such devices in areas where dust accumulation could pose a risk.
- Hoses and vacuum tools should be appropriate for use in environments with dust hazards and must be static dissipative or conductive. Ensuring that these tools meet safety standards is crucial for preventing static discharge and reducing the risk of ignition in dust-laden atmospheres.
- Portable vacuum cleaners should not be used on processes generating hot embers or sparks. The presence of these ignition sources could lead to dangerous situations, and alternative cleaning methods should be employed in such environments to maintain safety.
- Portable vacuum cleaners that meet the following minimum requirements should be used to collect combustible particulate solids in unclassified (nonhazardous) areas,
- Hoses used in vacuum cleaning should be conductive or static dissipative. Additionally, all conductive components, including wands and attachments, must be properly bonded and grounded to prevent static discharge and ensure safety in dust-prone environments.
- The fan or blower of the vacuum cleaner should be situated on the clean side of the primary filtration media or wet separation chamber to avoid contamination and potential hazards. Electrical motors should also be positioned on the clean side unless they are listed for Class II, Division 1 locations, ensuring they are safe for use in environments with combustible dust.
- When vacuum cleaners are used to pick up liquids or wet materials, paper filter elements should not be used, as they are unsuitable for such conditions. Instead, appropriate filtration media should be utilized to maintain the effectiveness and safety of the cleaning equipment.
- The facility should consider the following when using a vacuum system
- Never suck up an ignition source or glowing ember into the vacuum to prevent fire hazards. It is crucial to review the instruction manual for complete operating instructions to ensure the safe and effective use of the vacuum cleaner.
- Use only approved accessories from the vacuum manufacturer to maintain compatibility and safety. Always empty the vacuum before and after each use to prevent the buildup of dust and debris, which could pose a fire risk.
- Regularly inspect the filter pleats to ensure there is no excessive buildup of material, which can reduce the vacuum’s efficiency. Additionally, inspect all power cables for damage and replace them immediately if there are excessive signs of wear to avoid electrical hazards.
- The central vacuum cleaning systems should meet the following requirements (NFPA 652, Section 8.3.3.)
- The vacuum system should be designed to ensure minimum conveying velocities are maintained at all times, whether the system is used by a single operator or multiple simultaneous operators. This design consideration helps maintain the efficiency and safety of the vacuum system.
- The hose length and diameter should be appropriately sized for the specific application and operation. Proper sizing ensures optimal performance and prevents issues related to inadequate airflow or excessive resistance.
- When collecting ignition-sensitive materials, vacuum tools should be constructed of metal or static dissipative materials and provide proper grounding to the hose. Additionally, the vacuum cleaning hose should be static dissipative or conductive and properly grounded to prevent static discharge and ensure safe operation.
- The facility can use scoops, brooms, and brushes for housekeeping, but it is essential to take care to prevent the formation of dust clouds. This can be achieved by using gentle sweeping techniques and selecting appropriate cleaning tools.
- For instance, using soft brooms or brushes with natural fiber bristles can help minimize the dispersion of dust into the air. These tools are less likely to generate dust clouds compared to harsher cleaning implements.
- Ensuring that housekeeping methods are designed to control dust effectively is critical for maintaining a safe and clean environment within the facility.
- Avoid vigorous sweeping or blowdowns with steam or compressed air when dealing with combustible dust to prevent the formation of hazardous dust clouds.
- If such methods are necessary, ensure the following requirements are met to maintain safety and prevent dust ignition or dispersion.
- Implementing proper safety measures and following established guidelines can help mitigate the risks associated with using steam or compressed air for cleaning in environments with combustible dust.
- The use of compressed air for cleaning (“blow down”) is not allowed. An exception can be made for the cleaning of inaccessible areas where there are no other alternatives, but this should be done under the restrictions outlined in NFPA 652, Section 8.4.2.6.2 and NFPA 654, Section 8.2.2.4.
- Before using compressed air for cleaning, ensure that vacuuming, sweeping, or water wash-down methods are first employed to clean surfaces that can be safely accessed. These methods help to minimize the amount of dust that could be dispersed into the air. By prioritizing these initial cleaning techniques, the risk of creating hazardous dust clouds is significantly reduced. This step is crucial in environments where combustible dust is present, as it helps maintain a safer work area. Using vacuuming, sweeping, or water wash-down methods as the primary means of cleaning ensures that the use of compressed air is limited to areas where these safer methods are not feasible, thereby enhancing overall safety.
- It is crucial that dust accumulations in the area after these cleaning methods do not exceed the threshold of housekeeping dust accumulation.
- Compressed air hoses should be equipped with pressure relief nozzles limiting the discharge pressure to 30 psi (207 kPa), in accordance with OSHA requirements in 29 CFR 1910.242(b), “Hand and Portable Powered Tools and Equipment, General.” Additionally, all electrical equipment, including lighting, potentially exposed to airborne dust during cleaning should be suitable for use in Class II, Division 2 hazardous locations per NFPA 70, such as NEMA 12 or equivalent.
- Ensure all ignition sources and hot surfaces capable of igniting a dust cloud or dust layer are shut down or removed from the area. After the blowdown is complete, any residual dust on lower surfaces should be cleaned before reintroducing potential ignition sources to maintain a safe environment.
THE USE OF PERSONAL PROTECTIVE EQUIPMENT
- Personnel who are exposed to combustible dust, whether during cleaning or handling, should be provided with and wear flame-resistant garments. This protective clothing is essential for minimizing the risk of injury in the event of a dust ignition.
- Flame-resistant garments are designed to protect workers from the hazards associated with combustible dust by reducing the likelihood of burn injuries. Ensuring that all personnel in dust-prone areas are equipped with this protective gear is a critical safety measure.
- Providing appropriate flame-resistant clothing is part of a comprehensive safety program aimed at protecting workers from potential fire and explosion hazards in environments where combustible dust is present.
- The selection of the flame-resistant garments should take the following into consideration,
- When selecting flame-resistant garments for personnel exposed to combustible dust, consider the thermal protective characteristics of the fabric over a range of thermal exposures. Evaluate the physical characteristics of the fabric to ensure it meets safety standards and provides adequate protection.
- Garment construction and components should be examined for quality and durability. Proper garment fit and design are crucial for both safety and comfort. Additionally, consider the conditions under which the garment will be worn to ensure it is suitable for the work environment.
- Assess the garment’s durability and wear life to ensure it provides long-term protection. Follow the recommended laundering procedures to maintain the integrity of the flame-resistant properties. Finally, take into account the conditions and features affecting wearer comfort to ensure that the garments are practical for everyday use
- The facility should provide personnel with the manufacturer’s instructions on the care, use, and maintenance of flame-resistant garments, including any warnings. Informing personnel about the limitations, use, care, and maintenance of these garments is essential, including guidelines on the use of undergarments or overgarments.
- Appropriate storage practices for flame-resistant garments include storing them inside out of direct sunlight in a dry, ventilated area, preferably on hangers. Cleaned flame-resistant garments should be stored separately from uncleaned ones. Manufacturers should advise on any known deleterious effects of storage conditions.
- Personnel involved in handling dusty or granular materials should wear eye protection. In environments where personnel are exposed to dust clouds with more than “nuisance” toxicity hazards, they should wear tight-fitting masks or masks with positive pressure inside to ensure adequate respiratory protection.
EMPLOYEE TRAINING
- Ensure that all operators, mechanics, functional, and technical personnel receive awareness training on the explosion risk from combustible dust hazards, including housekeeping procedures. For a specific operational location, train personnel on preventive and protective measures to control combustible dust hazards. New employees, groups of employees, or those returning after more than six months must undergo basic training specific to their location, focusing on preventive measures and housekeeping protocols.
- Train employees on emergency procedures for incidents involving combustible dust within their location. Emphasize the critical safety components and necessary employee actions to maintain them properly. Ensure employees understand the proper use of fire extinguishers and receive training in their application. Highlight the hazards associated with smoldering piles of powder and stress that gas-driven fire extinguishers should not be used as they can create dust/air suspensions, presenting a deflagration hazard.
- Ensure employees are trained on housekeeping procedures and the use of appropriate housekeeping equipment. Provide refresher training once a year, focusing on changes that have occurred during the year and reviewing emergency procedures rather than repeating the onboarding training.
MANAGEMENT SYSTEMS
- Ensure that appropriate Management Systems are in place and used effectively in combustible particulate processing areas to minimize risk. Changes to operations involving combustible dust, including process, product, or systems changes, should meet the site Management of Change program requirements. Management of Change associated with combustible dust is critical.
- Any changes involving material composition, method of manufacturing of the raw material, vendor changes, process design changes, or equipment design changes handling combustible dust should initiate a Management of Change process. This ensures that all potential risks are assessed and mitigated properly.
- Due to the high-risk level associated with dust handling equipment, any work on (including maintenance and sanitation) or modifications to this equipment require the use of an approved Permit to Work. All procedures as part of the permit system should be followed, including reviewing work area rules to mitigate risks of combustible dust, reviewing the work area after task completion, and sign-off of the permit prior to turning over the system to production.
MANAGEMENT REVIEWS
- This housekeeping program should be renewed or reviewed at least every three years to ensure its continued effectiveness. However, updates and verifications should be completed under specific circumstances.
- An update and verification of the program should occur if any explosible dust cloud-related incident, such as a fire, explosion, or near-miss, has occurred. Additionally, updates should be made when a need is identified through the Management of Change Procedure or following any significant change to the facility or its operations.
- Regular review and timely updates of the program are crucial for maintaining safety and addressing any new risks or changes in the operational environment
SERVICES OFFERED BY PRIME PROCESS SAFETY CENTER
Finally, Prime Process Safety Center offers the following Combustible Dust Consulting Services; Combustible Dust Hazard Analysis (DHA), Ignition Sources Assessment, Electrostatic Hazard Assessment, Hazardous Area Classification, Fire and Explosion Hazard Analysis, Explosion Prevention and Protection Consulting Services, Fire and Building Code Services, Incident Investigation, Expert Witness and Litigation. Moreover, Prime Process Safety Center offers the following combustible dust testing services Go/No Go Explosibility Screening, Burn Rate / Fire Train Test, Dust Explosion Severity (Kst/Pmax/dP/dt), Minimum Explosible Concentration (MEC)/Lower Explosible Limit (LEL), Limiting Oxygen Concentration (LOC) Test, Minimum Ignition Energy (MIE), Minimum Autoignition Temperature-Cloud (MAIT – Cloud), Layer Ignition Temperature of Dust (LIT), Volume Resistivity, Surface Resistivity, Charge Decay (Relaxation) Time, Breakdown Voltage, Flexible Intermediate Bulk Containers (FIBC), Electrostatic Chargeability Testing, Basket Self-Heating, Grewer Oven Test, Air Over Layer/Powder Layer Test, Bulk Powder Test, Aerated Powder Test.