Three key points from the document: 1. Two maintenance mechanics were fatally injured in a combustible dust flash fire caused by failure to follow lockout/tagout procedures when restarting a bucket elevator during maintenance. The elevator restarted, lofting iron dust into the air and igniting it. 2. Bucket elevators present risks of combustible dust fires and explosions if proper safety procedures are not followed during inspection, cleaning and maintenance when dust can be dispersed. Lockout/tagout procedures must address explosive atmosphere hazards. 3. Existing OSHA regulations do not have a specific combustible dust standard, relying instead on NFPA standards which do not provide clear guidance on identifying combustible dust fire

1. Bucket Elevator Safe Work Procedures
Teresa Long
University of Houston-Clear Lake
An off-track condition exacerbates the problem. Intense frictional heat generated by the belt rubbing on the inside of the elevator casing can ignite a dust cloud. Risks of explosive atmospheres
within bucket elevators should be communicated to employees.
Introduction
Bucket elevator conveyors are used for vertically conveying bulk
materials. Dust is generated and dispersed as buckets are loaded with product, move through the elevator, and then unload the
product at the top. Problems arise when the dust generated by vertical conveyance of the product is combustible.
Combustible dust fire and explosion hazards must be recognized
in addition to unexpected energization or startup of process equipment that could cause harm to employees in close proximity. LOTO procedures that address combustible dust hazards (explosive
atmospheres) must be included in OSHA Regulations and followed prior to maintenance operations on bucket elevators. Engineering controls, such as a black box to record mechanical data
(similar to flight data recorder) for use in accident investigations,
should be implemented for bucket elevators located inside a building (Kauffman).
Bucket elevators are the most common location of primary explosions in the grain industry but combustible dust fire and explosion
hazards are not as prevalent in other sectors. Other than an ignition source, elements of the explosion pentagon are inherent to
bucket elevators.
Manufacturing facilities generating combustible dust are not following safe work procedures as outlined by Conveyor Equipment
Manufacturers Association (CEMA) during inspection, cleaning,
or maintenance operations.
“Conveyors shall not be operated unless all covers and/or guards
for the conveyor and drive unit are in place. If the conveyor is
to be opened for inspection cleaning, maintenance or observation, the electric power to the motor driving the conveyor must
be LOCKED OUT in such a manner that the conveyor cannot be
restarted by anyone; however remote from the area, until conveyor cover or guards and drive guards have been properly replaced (CEMA).”
Mechanics Standing on Elevated Platform Source: CSB
When the bucket elevator was restarted combustible iron dust was
lofted into the air, forming a dust cloud. The dust cloud ignited
and a fireball engulfed the workers, causing their burn injuries.
One mechanic died from his injuries two days later. The other mechanic suffered nearly four months before succumbing.
Trapped Key Interlock Switches could be installed to ensure bucket elevators cannot start if access doors are open. This would minimize the occurrence of combustible dust fires and explosions as
well as injuries incurred from moving parts.
Alternative process equipment, such as screw conveyers, have fewer inherent risks for a dust fire / explosion and can be installed in
place of bucket elevators in some manufacturing processes where
vertical bulk material transport is necessary.
References
Control of hazardous energy (Lockout / Tagout) procedures designed to safeguard employees during startup are ignored.
Kauffman, C. William PhD, Professor (Retired), Department of
Aerospace Engineering, University of Michigan, President, Explosion Research and Investigation Corporation, GEEIT Member.
Telephone interview. 03 Apr. 2012.
Discussion
Astad, John C. Director, Combustible Dust Policy Institute. Personal interview. 18 Feb. 2012.
On January 31, 2011 a combustible dust flash fire claimed two lives
due to failure to follow Lockout / Tagout (LOTO) procedures during maintenance operations on a bucket elevator at a Tennessee
manufacturing facility that produces atomized iron powder for the
automotive industry.
Elevator #12 is located downstream of an annealing furnace and
conveys fine iron powder to storage bins and had been shut down
until maintenance personnel could inspect it. The elevator was reported to be malfunctioning due to a misaligned belt. A dust collector associated with the elevator was also reported to have been
out of service for two days prior to the incident. The elevator had
experienced off-track conditions three times in the six months prior to the incident. The inspection panel at the base of the elevator
was typically removed without implementing LOTO procedures.
Witness statements indicate that the access panel near the head
was also open.
Two maintenance mechanics on the night shift were standing
alone on an elevated platform adjacent to an open access panel at
the bucket elevator head, checking the belt alignment. Neither believed the belt was off-track; LOTO procedures were not followed
prior to starting maintenance operations. Since the elevator had
been shut down due to a malfunction, product remained in the
buckets. They requested via two-way radio that the operator in the
control room restart the elevator. The elevator could neither be
seen nor heard from the control room.
Zeeuwen, Pieter. Principal Process Safety Specialist. Pieter
Zeeuwen Consulting. Email interview. 20 Apr.2012.
Remnants of Flame Retardant Clothing Worn by Mechanics
Source: Confidential
Bucket elevator access / inspections covers were left off the bucket elevator during the restart. Employees were aware of dangers
from placing hands inside the elevator, but the risk of a dust explosion from a potentially explosive atmosphere existing within
the elevator wasn’t effectively communicated.
Conclusions
Existing OSHA regulations for the control of hazardous energy
(ignition sources) during bucket elevator LOTO procedures do
not include a specific, formal combustible dust (explosive atmosphere) standard. OSHA generally issues citations via the General Duty Clause and relies on National Fire Protection Association
(NFPA) Combustible Dust Standards. NFPA 654 does not provide specific safety guidance for identifying the hazards of a combustible dust fire or explosion.
Leaving an access cover off provides the opportunity for a dust
cloud or fireball to exit the opening during bucket elevator restart.
Anderson, Eric P.E.. Consulting Engineer. Email interview. 12
Aug.2012.
Safety, Installation & Service Instructions For Bucket Elevators.
n.d. Screw Conveyor Corporation. Web. 06 Mar. 2012 http://
www.screwconveyor.com/bucketsafety.pdf.
State of Tennessee. Department of Labor and Workforce Development, Division of Occupational Safety and Health. Hoeganaes Corporation Case File (#315334060). Nashville, Tennessee: n.p.,Print. 15 Feb. 2012.
United States Department of Labor, OSHA. Establishment
Search. n.d. Web. 17 Mar. 2012 http://www.osha.gov/pls/imis/
establishment.inspection_detail?id=315334060
United States. Chemical Safety Board. Hoeganaes Corporation Fatal Flash Fires. n.p.,Web. 05 Jan. 2012. http://www.csb.gov/investigations/detail.aspx?SID=100&Type=2&pg=1&F_All=y
"Warning and Safety Reminders for Screw, Drag, and Bucket Elevator Conveyors", CEMA Document: SC 2004-01. n.d. Conveyor Equipment Manufacturers Association. n.d. Web. 02 Feb. 2012
http://ww.cemanet.org.