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. Meanwhile, instruction was given for removal of a 40-inch spool piece in
the column overhead line to facilitate overhead exchanger blinding. 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
(SO2) 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 cover 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 reboiler (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 build up in the
reactor due to a pyrophoric iron fire. The temperatures went as high as 500 oC.
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 N2 must be kept
opened during blinding and deblinding of the upstream and downstream flanges in
exchangers.
Pyrophoric Iron fire in a petrochemical unit producing Nitriles
This case study relates to a pyrophoric fire
incident in an East Asianpetrochemical plant
producing Acrylonitrile along with Acetonitrile and Hydrogen Cyanide as byproducts.
This was shared in an annualmeeting of
licensees of this technology.
During one of the turnarounds, the flare
header was taken for cleaning by steaming along with connected columns and exchangers.
When a cold cut was made on the flare header, a minor fire was observed.
Thorough investigations revealed that the flare header had a major choking with
polymeric cyanides and sulfur compounds. The exposure to open atmosphere resulted in
pyrophoric iron oxidation fire. The sulfur compounds got accumulated from the excess
sulphur dioxide injected into the connected column upper portion to prevent hydrogen
cyanides from polymerization. This SO2 entered from the from the vapor space of
certain equipment which is connected to general flare and not to separate HCN Flare.
Lesson learned: A highly safety-conscious
participant in this meeting, subsequently provided a flanged spool piece on the flare
header after the last entry point of vent gases to the flare header in his plant.
The dropping of the spool piece enables proper inspection for any deposition of
sulfur compounds in the flare header. Usual precautions of steaming and nitrogen purging
are essential before taking a “Cold Cut” on flare headers along with a “water
curtain” to prevent fires and explosions.
Contributed by : G.Vishwanathan (vishtech03@yahoo.co.in ) , a freelance consultant
on Energy Audits , Process Engineering and Trouble shooting operations. He also works as
Associate Consultant with M/S. Devki Energy Consultancy Pvt. Ltd., Baroda. He has
more than 25 years of experience in petrochemical plantoperations.
General
Precautions to Avoid Pyrophoric Iron Fires
The scraps and debris collected from cleaning of filters in naphtha / crude service must
be kept wet and disposed of underground.
Tanks, reactors, columns, and exchangers in high-sulfur feed service must be kept
properly blanketed with N2 during idle periods.
All equipment and structured packing must be properly water washed and kept wet when
exposed to the atmosphere.
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 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.
