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.

 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.

 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.

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.

This case study relates to a pyrophoric fire incident in an East Asian  petrochemical plant producing Acrylonitrile along with Acetonitrile and Hydrogen Cyanide as byproducts.   This was shared in an annual  meeting 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 plant operations.