MARCH 11, 2015
WASHINGTON, DC
The July 2010 explosion and fire at the former Horsehead
zinc refinery in Monaca, Pennsylvania, likely resulted from a buildup of
superheated liquid zinc inside a ceramic zinc distillation column, which then
“explosively decompressed” and ignited, according to a technical analysis released
today by the U.S. Chemical Safety Board (CSB).
Two Horsehead operators, James Taylor and Corey Keller, were
killed when the column violently ruptured inside the facility’s refinery
building, where multiple zinc distillation columns were operating. The
rupture released a large amount of zinc vapor, which at high temperatures
combusts spontaneously in the presence of air. The two men had been
performing unrelated maintenance work on another nearby column when the
explosion and fire occurred. A third operator was seriously injured and
could not return to work.
The incident was investigated by multiple agencies including
the CSB and the U.S. Occupational Safety and Health Administration, but its
underlying cause had remained unexplained. In the fall of 2014, CSB
contracted with an internationally known zinc distillation expert to conduct a
comprehensive review of the evidence file, including witness interviews,
company documents, site photographs, surveillance videos, laboratory test results,
and data from the facility’s distributed control system (DCS). The
57-page report of this analysis, prepared by Mr. William Hunter of the United
Kingdom, was released today by the CSB. Draft versions of the report were
reviewed by Horsehead and by the United Steelworkers local that represented
Horsehead workers in Monaca; their comments are included in the final report as
appendices.
In the years following the 2010 incident, the Horsehead
facility in Monaca was shut down and dismantled. The “New Jersey” zinc
process, a distillation-based method that was first developed in the 1920’s and
was used for decades in Monaca, is no longer practiced anywhere in the United
States, although a number of overseas companies, especially in China, continue
to use it.
“Although this particular zinc technology has ceased being
used in the U.S., we felt it was important to finally determine why this
tragedy occurred,” said CSB Chairperson Dr. Rafael Moure-Eraso. “Our hope
is that this will at last provide a measure of closure to family members, as
well as inform the safety efforts of overseas companies using similar
production methods.”
The Hunter report was based on expert professional opinion,
and did not involve any onsite examination of the evidence. CSB
investigators made several short deployments to the Horsehead site in 2010
following the incident, interviewing a number of witnesses and documenting
conditions at the site.
The explosion involved an indoor distillation column several
stories tall. The column consisted of a vertical stack of 48 silicon
carbide trays, topped by a reflux tower, and assembled by bricklayers using a
specialized mortar. The bottom half of the column was surrounded by a
masonry combustion chamber fueled by natural gas and carbon monoxide waste
gas. Horsehead typically operated columns of this type for up to 500
days, at which time the columns were dismantled and rebuilt using new trays.
The explosion on July 22, 2010, occurred just 12 days after
the construction and startup of “Column B.” Column B was used to separate zinc
– which flowed as a liquid from the bottom of the column – from lower-boiling
impurities such as cadmium, which exited as a vapor from the overhead
line. The column, which operated at more than 1600 °F, normally has only small
amounts of liquid metals in the various trays, but flooding of the column
creates a very hazardous condition, the analysis noted. Such flooding
likely occurred on July 22, 2010.
“Under extreme pressure the tray wall(s) eventually failed,
releasing a large volume of zinc vapor and superheated zinc that would flash to
vapor, and this pressure pushed out the combustion chamber blast panels,” Mr.
Hunter’s report concluded. “The zinc spray and vapor now had access to
large amounts of workplace air and this created a massive zinc flame across the
workplace.”
After examining all the data, the report determined that the
explosion likely occurred because of a partial obstruction of the column sump,
a drain-like masonry structure at the base of the column that had not been
replaced when the column was rebuilt in June 2010. The previous column
that used this sump had to be shut down prematurely due to sump drainage
problems, the analysis found. These problems were never adequately
corrected, and various problems with the sump were observed during the July
2010 startup of the new Column B. Over at least an hour preceding the
explosion, DCS data indicate a gradual warming at the base of Column B, as
liquid zinc likely built up and flooded the lower trays, while vapor flow to
the overhead condenser ceased.
Ten minutes before the explosion, an alarm sounded in the
control room due to a high rate of temperature change in the column waste
gases, as zinc likely began leaking out of the column into the combustion chamber,
but by then it was probably too late to avert an explosion, according to the
analysis. Control room operators responded to the alarm by cutting the
flow of fuel gas to Column B but did not reduce the flow of zinc into the
column. The unsafe condition of Column B was not understood, and
operators inside the building were not warned of the imminent danger.
The technical analysis determined that there was likely an
underlying design flaw in the Column B sump involving a structure known as an
“underflow” – similar to the liquid U-trap under a domestic sink. The
small clearance in the underflow – just 65 millimeters or the height on one
brick – had been implicated in other zinc column explosions around the world,
and likely allowed dross and other solids to partially obstruct the sump and
cause a gradual accumulation of liquid zinc in the column. Liquid zinc in
the column causes a dangerous pressure build-up at the bottom and impairs the
normal evaporation of vapor, which would otherwise cool the liquid zinc.
Instead the liquid zinc becomes superheated by the heat from the combustion
chamber, with the pressure eventually rupturing the column and allowing the
“explosive decompression.”
The report noted that the Column B sump had previously been
used with a different type of column that had a much lower rate of liquid
run-off through the sump, so the problem with the sump was only exacerbated
when Column B was constructed to separate zinc from cadmium, increasing the
liquid flow rate into the sump by a factor of four to five.
The report concluded that Horsehead may have missed several
opportunities to avoid the accident, overlooking symptoms of a blocked column
sump that were evident days before the accident. “Missing these critical
points indicates that, in large measure, hazardous conditions at Monaca had
been ‘normalized’ and that process management had become desensitized to what
was going on. This raises the question whether sufficient technical
support was provided to the plant on a regular basis,” according to Mr. Hunter.
The report noted that New Jersey-type zinc distillation
columns have been involved in numerous serious incidents around the
world. In 1993 and 1994, two column explosions at a former French zinc
factory killed a total of 11 workers. An international committee of
experts who investigated the incidents in France identified up to 10 other
major incidents at other sites attributable to sump drainage problems.
The Monaca facility had suffered five documented column explosions prior to 2010,
but none with fatalities, according to the CSB-commissioned report.
The CSB is an independent federal agency charged with
investigating serious chemical accidents. The agency's board members are
appointed by the president and confirmed by the Senate. CSB investigations look
into all aspects of chemical accidents, including physical causes such as
equipment failure as well as inadequacies in regulations, industry standards,
and safety management systems.
The Board does not issue citations or fines but does make
safety recommendations to facilities, industry organizations, labor groups, and
regulatory agencies such as OSHA and EPA
//--------------------------------//
PITTSBURGH (AP)
A 2010 explosion that killed two workers
at a former Horsehead Corp. zinc oxide plant in western Pennsylvania was caused
by recurring problems with a sump system that the company failed to address,
the U.S. Chemical Safety Board reported.
“Human factors played a dominant role” in the explosion at
the plant near Monaca, about 30 miles northwest of Pittsburgh, according to the
report by William H. Hunter, a British metallurgist hired to study the blast.
In reference to the system meant to drain the volatile
chemical from a distillation column, Hunter said, “In short, because of a
history of column blockage and explosions at the Monaca facility, hazardous
conditions had been ‘normalised.'”
James Taylor, 53, of Aliquippa, and Corey Keller, 41, of
Newell, West Virginia, died of smoke inhalation from the flash fire and
explosion July 22, 2010. The plant has since closed.
The chemical board doesn’t have the power to fine or cite
businesses, but investigates accidents so lessons can be passed on to
manufacturers, unions and government regulators designed to prevent similar
incidents.
“We felt it was important to finally determine why this tragedy
occurred,” said board Chairman Rafael Moure-Eraso. “Our hope is that this will
at last provide a measure of closure to family members, as well as inform the
safety efforts of overseas companies using similar production methods.”
The company filed a four-page objection letter to the
report, which was issued Wednesday, saying the scientist didn’t visit the site
or personally inspect the debris.
“Generally speaking, I guess the report really reflects an
opinion based on the file review rather than a review of the evidence involved
or a site visit,” Ali Alavi, Horsehead’s senior vice president of corporate
affairs, said Thursday.
“We completely reject the notion that any kind of hazardous
operating condition was ‘normalized'” at the plant, Alavi said.
Earlier investigations by the chemical board and the federal
Occupational Safety and Health Administration were “inconclusive as to the root
cause,” Alavi said.
OSHA nonetheless collected $42,000 in workplace safety fines
from Horsehead after the blast, after first seeking $45,000 in penalties.
Board spokesman Daniel Horowitz defended the report
Thursday, saying in an email that Hunter had full access to all witness
statements, photographs, computer data and other records Chemical Safety Board
investigators gathered after the explosion and that Horsehead never provided
any alternative theory or disputed any facts in the board analysis.
“So we do not think (Horsehead’s) critique was fair or
accurate,” the email said.
The plant used an indoor distillation column several stories
tall to boil off impurities from liquid zinc. When the sump drain at the bottom
of the column backed up, a “hazardous condition” resulted from more liquid zinc
in the column than normal. The column walls failed, and the flammable “zinc spray
and vapor now had access to large amounts of workplace air and this created a
massive zinc flame across the workplace,” the report found.
The particular technology, invented in the 1920s and used
for decades thereafter, is no longer used in the United States, although it is
used in China and other overseas plants.
Alavi said the plant shut down at the end of 2013 and the
rest of the operation closed in April.
Shell Chemical LP in November exercised its option to buy
the property, which it hopes to convert to a petrochemical plant though that
hasn’t been finalized, and the sale has yet to close.