The manufacturing facility is one of two owned by a company that produces
wooden reels that house cable, cord, rope, and other flexible products. The main building
is approximately 300’ by 400’ having a square footage of 120,000. Located just outside
the city limits of Statesville in Iredell County, the building is of non-combustible construction with a metal roof covered with tar and gravel over flat metal decking and
steel trusswork.
The silo was used for the collection, retention, and distribution of wood waste
from the manufacturing process. The wood waste consists of shreds, shavings, and
sawdust collected from a ductwork system in the plant via a cyclone and ducting drawn
to the top of the silo. Another cyclone on top of the silo deposits the product into the silo.
The top configuration of the silo consisted of the integral dome shaped roof, (part
of the original manufactured structure), and the cyclone (a fan and collector structure for
the movement and direction of the wood product). The cyclone and housing were
supported by a framework of steel I-beams. A steel catwalk around the rim of the silo
was accessed by two separate steel-cage ladders. As the wood waste was collected and
stored, it was removed from the bottom by a means of an electric-powered double shaft
screw auger to a chute in the bottom of the silo to the facility boiler system. The North
Carolina State Department of Environment and Natural Resources issued the permit for
this installation.
Photo #1 gives a perspective of the silo and its orientation to the building. The
trailer shed to the right received wood waste via ducting and cyclone arrangement similar
to that of the silo, for short-term storage in an open-box trailer and then disposal off site.
The silo was an oxygen-limiting type designed to store food for livestock. The
design permitted the silage to be stored for long periods by preventing the oxygen in the
air to penetrate the structure. By their nature, oxygen-limiting silos are very strong
structures built to exclude air exchange with the ambient atmosphere outside the silo. The
shell of this unit lacked total integrity because it was compromised at the two access
panels. These panels were not part of the original configuration that featured airtight
coating over the bolts in the interior. The access panel bolts lacked this coating.
In an agricultural setting, this type of silo has been the cause of fatal explosions,
which have taken the lives of firefighters, (8/27/85, Marshallville OH, 3 firefighters
killed, 8/5/93, Morgan County GA, 2 firefighters killed.) The mechanism of this type of
explosion is a backdraft-like phenomenon caused when oxygen-containing air is
introduced into areas of built-up heat and gases in deep-seated, slow-burning areas of the
stored agricultural products. All that is needed for a violent combustion explosion is a
source of oxygen. The presence of wood dust could likely have contributed to and
aggravated the explosion.
In an agricultural setting, with the fuel source being finely chopped organic
material (silage), the commonly recommended tactic is to close off all silo openings,
assure there is no fire outside the silo structure, and close off the silo and allow the fire to
burn itself out. A factory-installed connection permits the introduction of fire-inerting
nitrogen to blanket the fire in the interior. This is the only recommended extinguishing
action of an oxygen-limiting silo in an agricultural setting.
Over the course of approximately ten years, the fire department responded to at
least thirteen fire alarms at this facility. The silo itself was the scene of a previous fire
and explosion on July 29, 1993. In that incident the explosion bowed the roof upward
into a mushroom configuration. No one was injured in that explosion and damage from
the incident was minor.
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