Experience Transfer Stories (01) –Hazardous Pyrophoric Substances !

Two years ago, on one of the offshore platforms processing oil and gas in the North Sea, a fire incident occurred during maintenance activities related to a large inlet separator vessel. The plant was on a revision stop (turnaround) at that time. Replacing the inlet internals of this vessel was a part of the turnaround schedule.

Before the fire broke out, the vessel had been depressurized, drained, isolated, and vented. The plan was to send maintenance operators into the vessel after it was fully vented. The sudden fire lasted about 3 hours before being fully extinguished, and was restricted to the vessel itself. No person was harmed. However, consequences could have been more severe if the fire occurred when the operators were inside the vessel.

Investigation revealed that the fire was caused by pyrophoric iron sulfide deposits present within the separator vessel. Iron sulfide is deposited out of the reservoir fluid and over time it builds scales or deposits on the vessel internals. As long as the vessel is closed to the outside environment, such deposits stay dormant. When the vessel is open (e.g. being vented for maintenance), air /oxygen reacts with dry iron sulfide deposits to start a highly exothermic oxidation reaction (FeS + O2 -----> Fe2O3 + S + Heat). This exothermic reaction can start a fire and it can be further fed by the remaining oil residues in the vessel. This kind of self-igniting Pyrophoric substances is a less well-known hazard in the process industry.

Safeguards include;
- Make operators aware of such self-igniting pyrophoric substances that can be found inside process vessels /process equipment.
- Do more thorough insitu cleaning of vessels before opening (e.g. using internal water jets).
- Keep vessels wet while they are open (do not let complete drying out if such deposits are suspected).
- Use a fire guard on watch, or use temporary fire detectors (flame detectors) during the maintenance interval.
- Risk assessment related to the vessel entering (by operators) should include this type of fire hazards and prepare accordingly.
- Additional firefighting means must be available if this type of fire risk is present during maintenance (e.g. overhead deluge nozzles, fire hoses, portable extinguishers, possibility to flood the vessel, quick closing and cutting off the air supply, etc.)
- Other necessary measures based on the risk level (such as ?).
Are you able to find some other pyrophoric substances that may be encountered in the process industry (like iron sulfide) ?