POWER April 2014 - 84

PLANT SAFETY
4. Explosive opening. The explosion folded over the roof, killing the welder and severely
injuring the foreman. Source: U.S. Chemical Safety Board
The subsequent investigation found that
the contractor correctly completed the required
hot work permit paperwork. The
plant safety procedures required the " area
proprietor " to inform the worker of any
process changes or of any additional safety
information that might affect the work. The
final incident report noted, " On the day of
the incident however, neither the construction
field engineer nor the employee signing
off as the area proprietor had understanding
of the area, the process, or potential hazardous
conditions. "
The portion of the hot work permit that
line was not referenced in the lockout report
because it had no valves.
The welder began work on Tank 1 after
completing a hot work permit on Nov. 9, as
planned, although the welder did not confirm
the valves were locked out before beginning
work. A plant lab technician also manually
checked the area on top of the tank before
weld repairs began and found no flammable
vapors present, but failed to sample the interior
of the tank. The interior of the tank
wasn't tested because when empty of slurry,
it was considered as low hazard.
Just after 9 a.m., the welder attached his
safety harness to the tank and began using an
electric arc welder to weld a C-channel beam
to the roof of the tank. The arc welding increased
the metal temperature to in excess of
1,200F, well above the VF autoignition temperature
of 725F (the autoignition temperature
of diesel oil is about 410F). The foreman
was stationed on the catwalk adjacent to
the closed manway cover serving as the fire
watch. Two hours later, an alarm in the main
control room reported a fire inside Tank 1.
Shortly after the alarm, the overpressure
within the tank blew the manway cover off
the tank roof, landing over 100 feet away.
The explosion also tore open a 25-foot
length of the top-to-shell weld, leaving only
a 2-foot seam acting as a hinge. The roof of
the tank folded completely over the side of
the tank pivoting on the remaining weld. The
welder was killed immediately by blunt force
trauma. The foreman faired better, suffering
first-degree burns and other minor injuries.
The fire quickly consumed the flammable
vapor and self-extinguished (Figure 4).
The incident investigation determined
that an earlier plant hazard analysis model84
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POWER
| April 2014
ing did not consider that VF could accumulate
in sufficient amounts in a tank to reach
flammable levels, although the hazard analysis
did determine that loss of the compressor
would double the VF vapor produced.
The cracked looped seal pipe in Tank 2
had the unfortunate effect of increasing the
amount of flammable vapor present in Tank
1. In addition, the insulation was removed
from Tank 1 during the overhaul and not yet
been replaced at the time of the incident,
which also resulted in the production of
more flammable vapor due to condensation
(colder tank metal temperatures).
asked if flammable material would be within
35 feet of the work was left blank. The owner's
construction engineer was also found to
have " had no working knowledge of the . . .
process " and did not recognize the impact of
the failed compressor on the production of
flammable vapor. Also, the area proprietor
that signed the hot work permit was from the
service department and knew little about the
process, a safety shortcut that had been going
on for months. Internal controls require
the area proprietor to be " knowledgeable "
and to walk down the area where the work
was to be done.
It was later determined that the area proprietor
who signed the permit didn't even know
where the hot work was to be performed.
OSHA issued 10 citations to the plant
owner and eight to the construction company
doing the work for violations of OSHA
29CFR 1910.147 (Control of Hazardous Energy
(lockout/tagout standard) and 29 CFR
1910.252 (Welding, Cutting and Brazing).
5. Explosive mixture. A pharmaceutical manufacturing plant allowed polyethylene dust
to collect above a suspended ceiling that eventually led to an explosion, destroying the plant.
Workers were not informed that the dust residue was a safety concern. Source: U.S. Chemical
Safety Board

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POWER April 2014

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