Incident Investigation
Case Study 2
Caustic Dilution Tank Eruption
Dennis C. Hendershot
Rohm and Haas Company, retired
SACHE Workshop
September 2005
Bristol, PA
Incident 2 – Caustic Dilution Tank
Eruption
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Note – This incident description is derived
from a real incident (actually contains
aspects of several), but the detailed
information in this exercise is not
historically accurate. Aspects of the
incident, including procedure descriptions
and personnel “testimony”, have been
created for purposes of this exercise. It
should not be considered to be a
description of an actual incident.
General description of operation
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Batch polymer plant
Dilute caustic solution preparation from water
and solid (flake) sodium hydroxide
Small (a couple hundred gallons) agitated tank
with loose fitting top and hatch to charge solids
Tank cooled with cooling tower water through a
coil. No flow meter on cooling water, no sight
glass or other way to confirm flow.
On third floor of plant
Facility layout
Water
F
Meter
Other
ingredients
in small
containers
Solid Caustic
Addition Hatch
Third Floor
Dilute
Caustic
Tank
Second Floor
Note: There are several other similar tanks on the
third floor used for blending various ingredients
from small containers.
Safety
Shower
Written and EHS Reviewed
procedure summary
Charge specified amount of water to the dilution
tank through water meter
 Wearing specified Personal Protective Equipment
(PPE), weigh the appropriate amount of solid
NaOH in a 5 gallon pail
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– PPE includes long rubber gloves, boots, jacket, pants,
goggles, face shield, and hard hat
– Solid NaOH and weigh scale are located on 3rd floor
near the dilution tank
– The product of this operation is a 10% aqueous
NaOH solution.
Procedure summary, continued
Put cooling water on the charging tank coil.
Take the bucket of solid NaOH to the dilution
tank and charge it slowly through the charging
hatch.
 Watch the tank temperature during charging
(local thermometer on top of tank right next to
the charging hatch). If it exceeds 60 Deg C, stop
adding NaOH until the temperature goes below
40 Deg C.
 When the NaOH is all added, close the hatch
and mix for 30 minutes before using in the
process.
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What happened
During charging, the batch erupted through the
open charging hatch, spraying on the operator,
who was not wearing his face shield and rubber
suit.
 The operator had difficulty finding the safety
shower, slipped on the slippery floor, and fell in a
puddle of hot diluted NaOH ejected from the
tank. He was able to get up and get to the
safety shower, but suffered chemical burns and
had to be hospitalized.
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Some information from the
operator
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“I opened the valve on the cooling water
to the dilution tank at the vessel, and also
the cooling water return valve.”
– A check of the system after the incident
showed that the cooling water valves at the
dilution tank were open.
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“The reactor operator called me on the
radio and told me that he needed the
dilute caustic solution immediately.”
More information from the operator
“My rubber suit was in the locker room on
the ground floor in another building, and I
was in a hurry to get the batch made
because it was needed in the process.”
 “I’ve done this a million times with no
problem.”
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– Actually, the operation has been done about
400 times over a several year period by any
one of the building shift operators, without
incident.
More information from the operator
“The caustic was really sticky. I was trying to
charge it slowly, but a big pile of it stuck
together and fell in the tank all at once.”
 “The hatch was open on this tank, and on
several nearby tanks. There was a bunch of
other stuff in pails and small drums in the area
being used for other blends by other people.”
 “I remembered where the safety shower was,
but the floor was slippery and I fell down on the
way to the shower.”
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Some other information
The plant has done a chemical interaction matrix for the
materials handled on the 3rd floor. These materials
include acids (acetic, aqueous HCl, dilute sulfuric acid),
bases (ammonia) and various solid additives which are
relatively non-reactive. Incompatible materials are stored
in separated areas.
 The 3rd floor has 4 similar looking mix tanks, lined up in
a row against the wall. They are identified by number
with labels (~ 6 inch letters) – Tanks 301, 302, 303, 304.
The incident occurred in Tank 304, which is only used for
the caustic dilution operation.
 Parts of the plant cooling water system had been shut
down for maintenance the night before the incident.
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Your job
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Construct a root cause incident investigation tree
for this incident
Identify potential root causes for the incident
Determine what additional information or
evidence you need to identify the most likely
true root causes
Identify any laboratory experiments or
calculations you recommend to help identify the
root causes
Develop recommendations for the plant
– Consider process changes and opportunities for
inherently safer design.