Safe Confined Space Entry for Cement Silos and Hoppers
By: Daniel Marshall, Contributor
Whether it’s humidity, moisture content, size/texture of the raw material or increased volume, many factors can cause bulk materials to adhere to the sides of silos and hoppers, resulting in lost capacity or clogging. Accumulation contributes to ratholing or bridging that eventually stops production—causing expensive downtime and requiring extra labor to clear the obstruction—often involving confined space entry.
The consequences of untrained or inexperienced staff entering a silo or hopper can be disastrous, including physical injury, burial or asphyxiation. Disrupted material adhered to the sides of the vessel can suddenly break loose and fall on a worker. If the discharge door is in the open position, cargo can suddenly evacuate, causing unsecured workers to get caught in the flow. Vessels containing combustible dust—without proper testing, ventilation and safety measures—could result in a deadly explosion.
This article highlights some of the current general rules and best practices of confined space entry of a silo or hopper.
What is Confined Space Entry?
OSHA defines “confined space” as an area not designed for continuous employee occupancy and large enough for an employee to enter and perform assigned work, but with limited or restricted means for entry or exit. “Permit-required confined space” means a confined space that has one or more of the following characteristics:
- The vessel contains or has the potential of containing a hazardous atmosphere, such as exposure to explosive dust, flammable gas, vapor or mist, in excess of 10% percent of its lower flammable limit (LFL). Atmospheric oxygen concentration below 19.5% or above 23.5%.
- There is the potential for material to engulf, entrap or asphyxiate an entrant by inwardly converging walls or by a door which slopes downward and tapers to a smaller cross-section.
- Contains any other recognized, serious safety or health hazards.
Entering a Confined Space
Working in confined spaces requires securing a permit for the job and takes longer than the same work in unconfined spaces. With requirements that include special personnel training, safety harness and rigging, extensive preparation and added personnel for a “buddy system,” permit-required confined space entry entails cumbersome and costly—but necessary—safety procedures.
Unique to every operation, improved systems that incorporate easier access for entering and exiting the enclosure and natural ventilation of the internal work areas are cost-effective over time, from a safety and labor standpoint. When maintenance and repair work can be done without requiring permits or specially trained personnel, the risk and labor expense associated with such tasks is reduced. Consequently, designing systems to minimize permit-required confined spaces can provide a significant return on investment.
The best time to reduce the amount of confined-space entry for component maintenance and replacement is during the specification and design stages of a project. Once the system is constructed and in operation, it is far more difficult to redesign equipment to eliminate confined space entry.
Many manufacturers offer systems and products that can reduce the need for confined space entry. Examples would include:
- Chutes with abrasion-resistant liners using modular designs for quick replacement
- Chutes that hinge open and lay down for liner replacement
- Skirtboards with external liners
- Belt cleaners that can be withdrawn from the side of the conveyor for service without confined space entry
- Installation of flow aids, such as air cannons and vibrators, to reduce buildup
- Modular air cleaners for specific locations rather than centralized dust collection
Regulations and Standards
Rules regarding confined space entry vary greatly depending on the country, even down to the state, province or prefecture level. As always, regional and local codes should be identified and followed, but general rules can be drawn from regulations established in major industrial markets, such as Australia/New Zealand, Canada and the U.S.
Australian Standard (AS) and New Zealand Standard (NZS) 2865: Confined Spaces Persons conducting a business or undertaking (PCBU) or other responsible party issues a confined space entry permit or letter of authorization. The permit acts as a safety checklist.
AS Regulation 67 & 77: Confined Space Permits The permit must be completed in writing and:
- Specifies the confined space
- Records the names of persons permitted to enter the confined space and the work period
- Enacts risk-control measures based on the risk assessment
- Contains space for an acknowledgement that work in the confined space has been completed and all persons have left the space
Ministry of Labour: Confined Spaces The evaluation of the space to determine if the regulatory requirements apply may include consideration of previous data (including air sampling); knowledge of the process and space; data on space configuration (size, design, areas for pocketing, double wall, etc.); and knowledge and data on the generation and accumulation of contaminants.
Canadian Centre for Occupational Health and Safety (OHS) The Entry Permit system is an example of an administrative control used in confined spaces. The results of the tests for these hazards are to be recorded on the Entry Permit, along with the equipment or method(s) that were used in performing the tests.
Having summarized the requirements for an OSHA confined space entry permit in the “What is Confined Space Entry?” section of this article, the provisions below define the environment where a permit is not needed.
U.S. OSHA: 1910.146(c)(7), 1910.146(c)(7)(i), 1910.146(c)(7)(ii) A space classified by the employer as a permit-required confined space may be reclassified as a non-permit confined space under the following procedures:
- If the permit space poses no actual or potential atmospheric hazards, and if all hazards within the space are eliminated without entry into the space, the permit space may be reclassified as a non-permit confined space for as long as the non-atmospheric hazards remain eliminated.
- If testing and inspection during entry demonstrate that the hazards within the permit space have been eliminated, the permit space may be reclassified as a non-permit confined space for as long as the hazards remain eliminated.
Commonalities between governmental regulations provide employers with a measured approach to safety. These procedures include:
Prior to starting the job
- Review the permit and the job specific work procedures.
- Gather and inspect all necessary PPE.
- Test and/or calibrate any safety gear, test instrumentation or communication tools.
- Perform a short risk-assessment.
- Conduct proper tests for toxins, vapor, dust levels, oxygen levels and material-specific hazards.
- Perform as much cleaning and maintenance as possible outside of the vessel.
- Post completed confined space entry permit outside of the vessel.
- Isolate contaminants and moving parts to prevent accidental introduction of materials.
- Proper lock-out/tag-out/block-out/test-out procedures must be completed and documented prior to entry.
- Perform maintenance/cleaning using non-toxic substances, such as water, and avoid using heat/fire in the confined space. Never use oxygen to purge a confined space, as this can create a fire and explosion hazard.
- Provide ventilation if possible.
- Select personal protective/safety equipment, such as safety helmet, gloves, hearing protectors, safety harness, and lifeline and breathing apparatus.
- Assign a trained observer to monitor the procedure and internal conditions.
- Practice fast evacuation of the confined space.
Closing Thoughts: Covering Your Access
For a clean, safe and productive system, access to equipment for maintenance and repair is essential. No trade-off between safety, accessibility and cost is necessary when designing access to a silo, chute or other confined area. Safe access that is carefully located and adequately sized will increase dependability and reduce downtime—and the associated labor expense—required for maintenance, thus minimizing hazards such as dust and confined space entry.
Over time, well-designed access improves safety and saves money. Consider equipment designs that eliminate the need for confined space entry, including improved access doors, vibrators, air cannons or silo cleaning services. Vessels that are properly retrofit with the right cleaning and material discharge equipment experience longer life and less downtime, while creating a safer workplace.
Daniel Marshall is a Product Engineer with Martin Engineering. © 2022 Martin Engineering Company. All rights reserved. Martin Engineering products are protected by U.S. Patents, corresponding foreign patents and patents pending. Additional information can be obtained at www.martin-eng.com/trademarks.
 Dougherty, Dorothy, “Permit-Required Confined Spaces,” Occupational Safety and Health Administration, Department of Labor, Dec. 12, 2011. https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=9797&p_table=STANDARDS
 Staff, “Confined Spaces: Planning Entry and Working Safely in a Confined Space,” Worksafe New Zealand, Government of New Zealand. Jan. 29, 2016. http://www.worksafe.govt.nz/worksafe/information-guidance/all-guidance-items/confined-spaces-planning-entry-and-working-safely-in-a-confined-space/confined-spaces-planning-entry.pdf
 Staff, “Confined Spaces: Code of Practices,” Safe Work Australia, Government of Australia. Feb. 1, 2016. http://www.safeworkaustralia.gov.au/sites/SWA/about/Publications/Documents/634/Confined_Spaces.pdf
 Staff, “Confined Spaces: Confined Space Guidelines,” Ministry of Labour, Government of Canada. July 2011. https://www.labour.gov.on.ca/english/hs/pubs/confined/cs_4.php
 Staff, “Confined Space Introduction,” Canadian Centre for Occupational Health and Safety, Government of Canada. Feb. 17, 2017. https://www.ccohs.ca/oshanswers/hsprograms/confinedspace_intro.html
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