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14-06-2009, 09:03 AM
Post: #1
PYROPHORIC IRON FIRE
PYROPHORIC IRON FIRE
NARENDRA KUMAR
FIRE SAFETY OFFICER
IOCL, BARAUNI
Email - narendrak[at]iocl.co.in
What is Pyrophoric Iron Oxidation?
The word pyrophoric is derived from the Greek for
fire-bearing". According to Webster's dictionary,
"pyrophoric material" means "any material igniting
spontaneously or burning spontaneously in air when
rubbed, scratched, or struck, e.g. finely divided
metals". Iron sulfide is one such pyrophoric material
that oxidizes exothermically when exposed to air. It
is frequently found in solid iron sulfide scales in
refinery units.
It makes no difference whether
these pyrophoric sulfides exist as pyrite,
troilite, marcasite, or pyrrhotite.
ABSTRACT
In almost all the process involved in refinery
sulfur remains present in one form or other. Still
today downstream companies are striving hard
and introducing new and latest technology to
bring down the concentration of sulfur to
minimum as per regulatory norms. At one time or
another, most refineries experience spontaneous
ignition of iron sulfide either on the ground or
inside equipment. When this occurs inside
equipment like columns, vessels, and tanks and
exchangers containing residual hydrocarbons and
air, the results can be devastating. Most
commonly, pyrophoric iron fires occur during
shutdowns when equipment and piping are
opened for inspection or maintenance. Instances
of fires in crude columns during turnarounds,
explosions in sulfur, crude or asphalt storage
tanks, overpressures in vessels, etc., due to
pyrophoric iron ignition are not uncommon.
Often the cause of such accidents is a lack of
understanding of the phenomenon of pyrophoric
iron fires. This article aims to explain the basics of
pyrophoric iron fires and to provide ideas for
developing safe practices for handing over
equipment for inspection and maintenance.
It is formed by the conversion of iron oxide
(rust) into iron sulfide in an oxygen-free
atmosphere where hydrogen sulfide gas is present
(or where the concentration of hydrogen sulfide
(H
2
S) exceeds that of oxygen). The individual
crystals of pyrophoric iron sulfides are
extremely finely divided, the result of which
is that they have an enormous surface area-
to-volume ratio.
When the iron sulfide crystal is subsequently
exposed to air, it is oxidized back to iron oxide
and either free sulfur or sulfur dioxide gas is
formed. This reaction between iron sulfide and
oxygen is accompanied by the generation of a
considerable amount of heat. In fact, so much
heat is released that individual particles of iron
sulfide become incandescent.
This
rapid
exothermic
oxidation
with
incandescence is known as pyrophoric oxidation
and it can ignite nearby flammable hydrocarbon-
air mixtures.
Basic chemical reactions: Iron sulfide is one of
the most common substances found in refinery
distillation columns, pressure vessels, etc. It is
formed by the reaction of rust or corrosion
deposits with hydrogen sulfide as shown below:
There is a greater likelihood of this reaction
occurring when the process involves a feedstock
with high sulfur content. This pyrophoric iron
sulfide (PIS) lays dormant in the equipment until
the equipment is shutdown and opened for
service, exposing the PIS to air, allowing the
exothermic process of rapid oxidation of the
sulfides to oxides to occur, as shown in the
equations below:
The heat usually dissipates quickly unless there is
an additional source of combustible material to
sustain combustion. The white smoke of SO
2
gas,
commonly associated with pyrophoric fires, is often
mistaken for steam.Page 62

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 53
Pyrophoric iron oxidation in Distillation Columns
In petroleum refineries, the equipment most prone
to
pyrophoric
combustion
induced
fires
is the distillation columns in crude and vacuum
distillation units.
Deposits of iron sulfide
are formed from corrosion products that most
readily
accumulate at the trays,
pump
around zones, and structured packing.
If these
pyrophoric iron sulfide (PIS) deposits are
not removed properly before the columns are
opened up, there is a greater likelihood of
PIS spontaneous ignition.
The trapped
combustible hydrocarbons, coke, etc. that do not
get adequately removed during steaming/washing
often get ignited, leading to fires
and
explosions inside the equipment. These fires not
only result in equipment damage but can
also prove fatal for the personnel who are
performing inspection and maintenance work
inside the columns.
The accidents due to pyrophoric iron oxidations
are entirely avoidable if safe procedures
for column handover are followed. The targets of
these procedures should be twofold:
¢ First, to remove all the combustibles
¢ Second, to remove or neutralize pyrophoric iron
sulfide deposits
The basic distillation column oil-cleanup procedure
is discussed in steps below.
Distillation Column Oil Cleanup Procedure
1. Steaming: The steaming is done after all liquid
hydrocarbons
have
been
drained
from
the column and associated piping. The objective of
steaming is
to make
the column
and
associated piping free of residual hydrocarbons.
The column vent and pump strainers in the side
draw piping are de-blinded and steaming is started
from
utility
connections
at
the
bottom of the column.
Generally, steaming is
continued for about 20 to 24 hrs, ensuring
the column top temperature remains more than 100
°
C throughout the operation.
2. Hot Water Washing: When clear steam is
observed exiting the column vents, water washing
of the column should be started. With steam still
in
commission,
water
is
sent
to
the column, usually via reflux lines, and it is
drained from the column bottom, associated
pump strainers, etc. The water flow rate should
be adjusted so that steam still comes from
the vent (i.e. water should not result in
condensing of all steam before it reaches the
column top). Water flow should be stopped for
2-3 hrs and then resumed. This cycle of
steaming and washing should be repeated several
times for a total of about 15 to 20 hours.
Injection of a turpene-based detergent into
the steam can also be considered.
The
condensate-soap solution can be collected and
circulated through the various side cuts.
3. Blinding: When clear water is observed at
side draw pump strainers, etc., associated piping
should be isolated by installing blinds wherever
isolation is possible.
4. Cold Water Washing: The hot water wash
should be followed by a cold water wash
(i.e. steam should be fully closed). The cold
water washing is done for about 20-24 hrs.
5. Chemical Injection for Removal and
Neutralization of PIS Deposits: During the
cold-water wash or after washing is over,
chemical injection for removal of pyrophoric
sulfides should be considered.
The various
options for chemical treatment are discussed
below:
¢
Acid cleaning - This procedure involves
pumping in an acid with some corrosion
inhibitor. The acid dissolves sulfide scale and
releases hydrogen sulfide gas. It is effective and
inexpensive; however, disposal of Hydrogen
Sulfide gas can be a problem, as can corrosion
(when the system contains more than one
alloy).Dilute hydrochloric acid solutions may
also be used. The resulting iron chloride turns
bright yellow, acting as an indicator for removal
of the iron sulfide.
¢
Acid plus hydrogen sulfide suppressant -
Additional chemicals can be added to the
acid solution to convert or scrub the hydrogen Page 63

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 54
sulfide gas.
¢
Chelating solutions - Specially formulated, high
pH,
chelating
solutions
are
quite
effective in dissolving the sulfide deposits without
emitting
hydrogen
sulfide,
but
this is an expensive application.
¢ Oxidizing chemicals - Oxidizing chemicals
convert sulfide to
oxide.
Potassium
permanganate (KMnO
4
) has been used commonly in
the
past
to
oxidize
pyrophoric
sulfide.
Generally the potassium permanganate
is
added
to
the
tower
during
the
cold water washing as a 1% solution. At various
intervals,
samples
are
taken
and
checked for color. The colors of the fresh KMnO
4
and
the
spent
MnO
2
are
purple
and brown respectively. If the color of the solution
becomes brown, additional KMnO
4
is needed.
The reaction is judged complete when the
solution
color remains purple. It
takes
approximately 12 hours to complete the job.
The cost of potassium permanganate treatment
is more expensive than acid cleaning and
traditional oxidizing agents such as sodium
hypochlorite or hydrogen peroxide. Nevertheless,
it is less corrosive to equipment than acid cleaning
and used properly can be safer than other oxidizing
agents.
The following conditions should be avoided when
using potassium permanganate:
¢ Do not add KMnO
4
to acids or use in a low pH
environment
¢
Combustible materials should not be
allowed to contaminate KMnO
4
stocks
¢
Residual MnO
2
may remain in vessels after
treatment and cause combustion or flammability
issues
in
equipment with large surface
areas such as packed towers
¢ KMnO
4
cannot be used in conjunction with most
detergents
¢ KMnO
4
may have a bad reputation in some
processing plants, but this is often times the
result of misuse by contractors or plant
personnel.
Alternative oxidation technologies are being
developed with a focus on
¢ increasing safety in application
¢ saving water
¢ Eliminating odor problems
¢ minimizing wastewater problems
¢ reducing wastes
One such alternative is Zyme-Flow®. Zyme-Flow®
offers unique chemistry which is patented and
offered by license from United Laboratories
International, LLC as Zyme-Flow® and related
products. The Zyme-Flow® chemical applications
are administered by a highly trained staff of
technicians provided by United and sold only by
license from United Laboratories International,
LLC.
The Zyme-Flow® generic Vapour Phase® method
is apparently unique in that the de-oiling and
oxidizer composition that is being dispersed actually
may
be
vaporized
in
the
steam (instead of being just atomized). This allows
the Zyme-Flow® composition to travel (in easily
measurable concentrations) extensive distances
and throughout equipment with high efficiency for
contacting and condensing on internal surfaces.
The composition may expend quickly, but the
application technicians can measure its progress.
This prevents over-dosing.
A very generic Vapour Phase® procedure may
include:
1. Stop feed and de-inventory the unit per normal
procedures.
2. Perform initial isolation per the pre-established
plan with the Zyme-Flow® specialists.
3. Establish a thorough steam path throughout
the equipment. Page 64

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 55
4.
Add Zyme-Flow® to the incoming steam
(commonly
only
1-3
points
are needed
even for very large units).
5. Continue the injection and steaming for 8-12
hours.
6. Perform isolation per pre-established plan with
the Zyme-Flow® specialists.
7. Possibly perform a final rinse with cold water to
cool the columns quickly.
8. The unit is then ready for opening, ventilation, and
hot work.
One major advantage for oxidizing pyrophoric iron
sulfide is that the distribution dynamics of the
Vapour Phase® applications are often more
efficient than atomized distribution methods. This is
why Zyme-Flow® is often used for decontamination
of flare lines and overhead systems where few
injection points can be utilized. The same dynamics
allows for full treatment of most tight packing
structures in refinery columns.
In some situations, Zyme-Flow® applications may
be combined simultaneously or in sequence with
compatible solvent and oxidizer products to target
specific challenges such as monomer / polymer
coatings and other challenges. These applications
require specialty design consideration from the
Zyme-Flow®
specialists.
This is especially
important in structured packing situations where
polymerization tends
to coat and
protect
the pyrophoric deposits from contact with oxidizers
until
cooling
promotes
cracking
of
the
polymer.
Solvent/Surfactant Steam Dispersion Methods
There are alternatives to the steam, wash, blind, and
wash again technologies. These include steam
dispersion technologies which are sometimes
combined with oxidizer washing technologies.
These alternatives may include steam dispersion of
organic solvent products and can be very good to
excellent de-oiling and degassing compositions
(which expose pyrophoric iron sulfide to subsequent
oxidizer treatments).
For critical path process units in a turnaround, a very
generic procedure may include:
1.
Stop feed and de-inventory the process
equipment
per
normal
procedures.
Sometimes this involves steaming and sometimes
water displacement.
2.
Establish a steam flow throughout the
equipment.
3. Add chemicals to the incoming steam to
promote de-oiling
and degassing of the
equipment (this may involve numerous injection
points).
4. When the outflow vapors are within safety and
environmental specifications, blow-down the
equipment to the atmosphere for a short time (or
continue to flare or condenser as needed).
5. Perform isolation as needed prior to final
washing for
oxidation (per
pre-designed
isolation plan).
6. Perform a thorough water wash with oxidizer
until the oxidation requirement of the fluid path is
complete. Sometimes this must involve total fluid
fill of the equipment to obtain positive contact of
pyrophoric surfaces.
7. The unit is then ready for opening, ventilation,
and hot work (unless other chemical treatments
are required).
This generic procedure may allow for 24-48 hours
of savings over the extended steaming, isolation,
and washing approaches, and may be safer to
perform. There are several products currently
offering this type of service. Many of them are
strong encapsulators and require secondary
treatment to break the emulsions.
Case Studies: Pyrophoric Iron Fires
The history of refining is replete with cases of fires
and explosions due to pyrophoric iron ignitions. A
few of these cases are discussed below (details
like the location and date of the incidents are not
included), to give the reader an idea of the nature
of pyrophoric iron fires and the lessons learned.
Pyrophoric fire/explosion inside a Vacuum
column in a Crude Unit
During a turnaround in the Crude Unit the vacuum
column was being prepared for handover to
maintenance. The oil was removed from the
column and the column was steam purged. A
water washing connection was made in the light
vacuum gas oil (LVGO) reflux pump suction.Page 65

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 56
Meanwhile, instruction was given for removal of a
40-inch spool piece in the column overhead line to
facilitate overhead exchanger blinding. The design
of the flow path is critical. Air ingress occurred from
this open flange, leading to auto-ignition of
pyrophoric iron sulfide inside. An explosion took
place
causing
damage
to
the
internals. White smoke (SO
2
) was also observed at
the open end. Nitrogen injection and water washing
were immediately begun to quench the heat and halt
the oxidation reaction inside the column.
Lessons learned: Before carrying out any
maintenance activity on overhead exchangers,
proper water washing and blinding must be
completed. Full-face blinds should be provided
wherever spool pieces are dropped.
Pyrophoric Fire inside the floating head covers
of a Naphtha Stabilizer Reboiler
During a maintenance and inspection (M&I)
shutdown, after steaming of the reboiler loop, the
floating head cover of the naphtha stabilizer (S&T
exchanger) was opened so the bundle could be
pulled for cleaning. The head cover was left in the
open position. After about 2 days, fire and smoke
was observed from the head cover. It was
determined that the fire occurred because of the
PIS ignition of residual hydrocarbons. The fire was
immediately extinguished with water. The cover
was thoroughly flushed with water and kept wet.
Lesson learned: Whenever exchangers in
naphtha service (containing sulfur) are opened for
maintenance, the exchanger areas must be
properly water washed for PIS removal. No amount
of steaming can ensure full removal of PIS or
residual hydrocarbons.
Pyrophoric Fire inside a Naphtha Tank
A naphtha tank (floating head type) was emptied
out for maintenance. It was left unattended for one
month. One day, flames and smoke were
observed
coming
from
the
tank.
Upon
investigation, it was found that PIS had ignited
leading to combustion of residual naphtha in the
tank.
Lessons learned:
Tanks
in
high-sulfur
hydrocarbon service, such as naphtha, crude,
etc., must be properly emptied and washed
before allowing them to remain idle for
maintenance.
Also, such tanks should be kept under adequate
nitrogen blanketing.
Pyrophoric Fire inside a Hydrotreater Reactor
During a maintenance shutdown, a naphtha
Hydrotreater
reactor
feed/effluent
heat
exchanger was to be opened. The reactor gas
loop was thoroughly nitrogen purged. During
deblinding of the exchanger air ingress occurred
to the reactor causing excessive heat buildup in
the reactor due to a pyrophoric iron fire. The
temperatures
went as high as 500
o
C. Heavy smoke was
observed from the open flanges and the
reactor platform area became hot. The heat was
immediately
quenched
by
purging
with nitrogen.
Lessons learned: Whenever piping associated
with a naphtha Hydrotreater reactor has to be
opened, purging N
2
must be kept opened during
blinding and deblinding of the upstream and
downstream flanges in exchangers.
General Precautions to Avoid Pyrophoric Iron
Fires
1. The scraps and debris collected from cleaning of
filters in naphtha crude service must be kept wet
and disposed of underground.
2. Tanks, reactors, columns, and exchangers in
high-sulfur feed service must be kept properly
blanketed with N
2
during idle periods.
3. All equipment and structured packing must be
properly water washed and kept wet when
exposed to the atmosphere.
4. In processes where catalyst handling is
required (such as in Hydrotreating and fluid
catalytic cracking) caution must be taken during
catalyst
recharge
or
disposal.
When unloading any spent coked catalyst, the
possibility exists for iron sulfide fires. If the spent
catalyst is warm and contacts oxygen, iron sulfide
will ignite spontaneously and the ensuing reaction Page 66

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 57
may generate enough heat to ignite carbon
deposited on the catalyst. Therefore catalyst must
be stripped of all hydrocarbons, cooled to about 50
o
C and wetted with water to prevent it from igniting
vapors. Once cooled, the used catalyst may be
emptied into drums for later shipment to a
regenerator or a disposal site. As the catalyst may
be highly pyrophoric (containing iron sulfide, etc.),
it should be dumped into drums containing an
internal liner for shipment. The drum and liner
should first be filled with inert gas, which is then
displaced by the catalyst. The liner should be tied
off
and
a
small
chunk
of
dry
ice
placed inside the drum before sealing. These
precautions
should
protect
against
catalyst auto ignition.
Reference
1 Troubleshooting Process Operations By
Norman P. Lieberman.
2. Safety Rules for Use in the Chemical
Works By Association of British Chemical
Manufacturer

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