POWER April 2014 - 83

PLANT SAFETY
3. Dangerous work environment. A still shot taken from
a CSB animation shows the location of the workers at the time the
fumes in the tank were ignited by the welding. Source: U.S. Chemical
Safety Board
tall, with a volume of 10,800 gallons.
The tank shell and top were quarter-inch-thick stainless steel. A
flash tank vented small amounts of VP as a gas to the atmosphere and
drained to the tank, and the area was equipped with flammable vapor
air monitoring to alert operators when an explosive gas mixture is
present. An alarm was set at 10% of the lower explosion limit (LEL)
and the high alarm was at the 2% LEL. The sample monitor was located
directly under the flash tank vent. At the time of the incident,
the area surrounding the three VF slurry tanks was classified as general
purpose electrical at ground level, on the top of the tanks, and on
the elevated catwalks connecting the tanks.
The plant's three VF slurry tanks were " locked out " by the plant
owner on Oct. 22, the day after the plant began a two-week outage
for repairs. Emergent work on the roof of Tank 1 was delayed due to
lack of materials and was rescheduled for Nov. 9, when the contract
welder was available, which was after the plant was restarted.
In the meantime, a loop seal pipe (separating the VF slurry from the
way around the spool with his second pass when the " pressure vessel "
exploded. A portion of the reheated root pass and the second pass weld
material struck the welder in the chest, causing serious injuries, and hit
the welder's helper in the face and eyes, causing what was probably a
permanent loss of vision.
These preventable injuries were caused by a series of mistakes by
the workers and plant supervision. The lessons learned from this tragedy
are self-evident:
■ Welding on an enclosed structure when you don't know what is
on the other side of the weld surface is extremely dangerous. Also,
any enclosed volume must be thoroughly vented before and during
a weld repair because any liquids present will evaporate and pressurize
the container. The strength of the molten weld to contain a
buildup of pressure inside the container is essentially zero.
■ Any welding on plant piping, particularly high-pressure steam piping,
regardless of how insignificant the repair appears, requires
up-to-date welding qualifications for the work to be performed, in
accordance with the ASME piping and pressure vessel codes. It's
unclear whether the quality of the welding was an issue. However,
playing fast and loose with well-established welding processes is
always a safety mistake.
■ Plant supervision and management failed to instill a safety culture
in these employees and failed to have safety inspectors present during
the work. Supervisors should have also recognized the safety
implications of a steam valve leak in an adjacent boiler.
Hot Work Ignites Flammable Fumes
In November 2010 at a process plant in upstate New York, a welder and
foreman were repairing equipment located on top of an atmospheric
storage tank. The hot work ignited flammable fumes contained within
the tank, and the subsequent explosion killed the welder. Although the
fumes present in the tank were from a unique source, the result was
no different than if the tank contained fuel oil or another of the many
flammable liquids found in a power plant. The lessons learned remain
the same (Figure 3).
In this particular incident, the tank contained flammable vinyl fluoride
(VF) gases produced from the liquid VF used in the manufacture
of photovoltaic panel films and other consumer goods. VF is a colorless,
heavier-than-air, highly flammable gas at ambient conditions.
The tank containing the VF vapor was 11 feet in diameter and 19 feet
April 2014 | POWER
Made in
Canada
vapors) located inside of Tank 2 split open. An engineer inspected the
pipe and determined that the damage was not significant and the tank
was returned to service. The engineer that performed the inspection
did not recognize that the split in the loop seal provided an open path
for flammable vapors to travel to adjacent tanks, a " low hazard " area.
The engineer did not recognize that the split overflow line could not be
secured as part of the lockout process.
On Nov. 6 the plant was restarted, and Tanks 2 and 3 were returned
to service while Tank 1 waited on parts. On Nov. 8 a plant process
compressor malfunctioned with the effect of doubling the amount of
waste vapor sent to the tanks. The following day, a lockout card was
completed for Tank 1 that confirmed that all five pipes connected to the
tank were secured and isolated from the online processes. The overflow
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POWER April 2014

Table of Contents for the Digital Edition of POWER April 2014

Contents
POWER April 2014 - Cover1
POWER April 2014 - Cover2
POWER April 2014 - Contents
POWER April 2014 - 2
POWER April 2014 - 3
POWER April 2014 - 4
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POWER April 2014 - Cover3
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