July 5, 2017
Commonly used to lift dusty product ranging from raw material to finished goods, bucket elevators are a workhorse on which many plants depend for efficient handling. Connecting different areas of a process facility, an unprotected bucket elevator can become a conduit that conveys the destructive forces of a dust explosion beyond its point of initial combustion.
For facilities that manufacture, process, blend, convey, repackage, generate, or handle combustible dust, bucket elevators are deployed to lift material between process locations. New National Fire Protection Association (NFPA) standards, and even the first deadline to conduct a dust hazard analysis, are now on the books, adding to the responsibilities of owners and operators of such equipment.
Explosions can result from an ignition of combustible dust when mixed with air. The empty buckets on the down side of an elevator act like a fan pushing dusty air around the device. In the event of a dust explosion, a rapid rise in pressure occurs in the typically lightweight elevator structure due to the combustion event.
The NFPA published NFPA 652 – Standard on the Fundamentals of Combustible Dust in 2016. This standard consolidates best engineering practice and provides reference to all of the existing combustible dust standards in a single, overarching document that applies to all facilities that handle potentially explosive dusts.
NPFA 652 includes a deadline for owners and operators of at-risk facilities to conduct a dust hazard analysis (DHA) within three years. Recent revisions to NFPA 654 and 61 that additionally apply to many facilities have extended that deadline to five years.
Arising from the DHA of NFP 652 are requirements for prevention and/or protection strategies for bucket elevators as defined by long-established standards, in particular NFPA 68 - the Standard on Explosion Protection by Deflagration Venting and NFPA 61 - the Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities. NFPA 68 is currently in its 2013 revision with a 2018 update underway, and NFPA 61 is in its newly published 2017 revision.
Explosion Venting of Bucket Elevators
During the early stages of a dust or gas explosion, explosion vents open rapidly at a predetermined burst pressure, allowing the combustion process to escape to the atmosphere and limiting the pressure generated inside a bucket elevator to calculated safe limits.
Bucket elevator protection uses multiple vents to meet NFPA 68 requirements that assess both dust reactivity (or Kst value) and the design strength of a bucket elevator.
In cases where a vented flame ball must be avoided, flame arrestors can be deployed – so called flameless venting. These devices are designed to absorb the pressure wave, flame, and at least some of the dust that would normally be ejected by a vented explosion. The boot of a bucket elevator, which is typically at or below grade level, would be suited to protection by flameless venting.
Explosion Suppression of Bucket Elevators
Explosion suppression equipment detects a dust explosion in the first milliseconds of the event and then signals extinguishing modules to release a flame quenching medium into the bucket elevator. This stops the explosion in its infancy and only low pressure is produced that is safe for the bucket elevator and connected equipment.
Explosion Isolation of Bucket Elevators
With their task being to connect different areas of a manufacturing or storage facility, bucket elevators are a conduit through which a dust explosion can spread between areas of a plant. This requires the careful placement of isolation barriers per the NFPA standards to stop such propagation. Mechanical barriers are obstructive to normal bucket elevator function and are round devices, whereas elevators often have rectangular connections. In this application scenario, bucket elevators are well suited for protection by chemical isolation, which is the same rapid extinguishing technology used for explosion suppression applied to connecting ductwork. Chemical isolation operates by rapidly injecting extinguishing medium, presenting a barrier to flame passing through.
There is always more than one way to achieve combustible dust safety with a bucket elevator. It is important to review each option for a particular application and arrive at a combination of technologies that are technically effective and cost effective in meeting the owner/operator responsibilities under NFPA Standards.
Geof Brazier, chairman, BS&B Pressure Safety Management LLC, has almost 40 years of experience in the field of combustible dust prevention and protection technology design in both the U.S. and Europe. He has been a member of the NFPA 68 and 69 Technical Committee on Explosion Protection Systems for the past 10 years. He was educated in the UK at the University of Bristol as a physicist.
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