A large explosion on March 21, 2011, at a Carbide
Industries plant that killed two workers and injured two others
resulted from a failure by the company to investigate similar
but smaller explosive incidents over many years while deferring
crucial maintenance of the large electric
arc furnace that blew up.
These findings are according to the final investigation report, released
on Thursday by US Chemical Safety Board (CSB).
The report, which contains recommendations to prevent such
explosions in similar furnaces, is scheduled to be discussed at
a CSB public meeting on Thursday night in Louisville, Kentucky.
Louisville is the site of the calcium carbide plant.
The deaths and injuries likely resulted when water leaked into
the electric arc furnace causing an over-pressure event,
ejecting furnace contents heated to approximately 3800 degrees
Fahrenheit, the CSB said.
Along with molten calcium carbide, the furnace spewed powdered
debris and hot gases, which blew through the double-pane
reinforced glass window of the furnace control room that was
located just 12 feet from open vents atop the furnace. The two
workers inside died within 24 hours from severe burn
"This accident is literally a case study into the tragic,
predictable consequences of running equipment to failure even
when repeated safety incidents over many years warn of
impending failure," said CSB chairperson Rafael
"When control room windows blew out during previous furnace
incidents, the company merely reinforced them, rather than
taking the safe course and moving the control room farther from
the furnace and investigating why the smaller furnace
overpressure events were happening in the first place. It is
what we call a normalization of deviance, in which
abnormal events become acceptable in everyday operations.
The facility, located by the Ohio River in the
Rubbertown section of western Louisville, supplies
calcium carbide primarily to the iron and steel industry and to
acetylene producers. It employs about 160 workers in
operations, maintenance, and
The investigation report proposed two scenarios for the
development of cooling water leaks that likely resulted in the
overpressure and explosion. In one scenario, fouling -- or
the accumulation of solids inside the hollow chamber where
water flows -- resulted in localized overheating,
eventually causing sections of the cover to sag and crack.
Another possible cause of the leaks could have been the sudden
eruption of hot liquid from the furnace, which operators called
a boil-up. Hot liquids contact the underside of the
furnace cover, eroding its ceramic lining, and eventually
melting holes through which water leaks.
Post-incident examination revealed recurring water leaks in
multiple zones of the furnace cover. Rather than replacing the
furnace cover, the company directed workers to attempt repairs,
according to the CSB.
The investigation found that the company would inject a mixture
of oats and commercially available boiler solder
into the cooling water, in an effort to plug the leaks and keep
the aging cover in operation.
Water leaks into the furnace interfere with the steady
introduction of lime and coke raw materials, through an effect
known as bridging or arching, the
report noted. In a carbide-producing electric arc furnace, this
can result in an undesirable and hazardous side reaction
between calcium carbide and lime, which produces gas much more
rapidly that the normal reaction to produce calcium carbide
Industry literature described the phenomenon as early as 1965,
and an independent CSB analysis confirmed that operating
conditions at Carbide on the day of the incident could have
resulted in this effect, causing hot materials to be expelled
from the furnace.
One of our key findings was that Carbide Industries
issued 26 work orders to repair water leaks on the furnace
cover in the five months prior to the March 2011 incident,"
said CSB lead investigator Johnnie Banks.
"It was distressing to find that the company nonetheless
continued operating the furnace despite the hazard from ongoing
water leaks. We also found that the company could have
prevented this incident had it voluntarily applied elements of
a process safety management program, such as hazard analysis,
incident investigation, and mechanical integrity.
Investigator Banks noted that Carbide was not required to
follow the OSHA Process Safety Management standard, since the
company did not use threshold amounts of covered hazardous
The report notes that Carbide continued operating the furnace
even though it planned to replace the furnace cover in May
2011. The accident occurred in March of that year.
The investigation found that National Fire Protection
Association (NFPA) industry codes governing the safe operation
of potentially hazardous Class A furnaces, such as the one at
Carbide, do not have specific requirements for appropriate
safety devices, interlocks, and safe distances between the
furnaces and occupied work areas.
The draft report recommended that the NFPA develop a national
standard requiring companies to provide adequate safety
instrumentation and controls to prevent explosions and
overpressure events, mechanical integrity and inspection
programs, and a documented siting analysis to ensure that
control rooms and other occupied areas are adequately
Carbide Industries was urged to modify the design and
procedures for the electric arc furnace and related structures,
including the control room, to comply with the standard the
NFPA was recommended to develop, and to implement a mechanical
integrity program for the electric arc furnace and cover,
including preventive maintenance based on periodic
inspections, and timely replacement of the furnace cover.
At a minimum, the CSB said, the mechanical integrity program
should include leak detection and repair and monitoring
refractory lining wear.
In my view, a national standard adopted by industry and
incorporated into state and federal requirements would go a
long way to prevent the kind of tragedy that befell the workers
at Carbide Industries," said Moure-Eraso.
"While that is important, its clear that Carbide
displayed a chronic lack of commitment to figuring out what was
going wrong, ignoring all the warning signs, even as its
workers were exposed to a potential massive explosion just a
few feet away from their control room.
The CSB is an independent US federal agency charged with
investigating industrial chemical accidents. The agency's board
members are appointed by the president and confirmed by the
The board does not issue citations or fines but does make
safety recommendations to plants, industry organizations, labor
groups, and regulatory agencies such as OSHA and EPA.
The board's website can be visited