Explosion in a fired heater of an ethylene plant

This incident occurred at a fired heater of an ethylene unit of a chemical plant. The fired heaters was shutdown. A mixture of hydrocarbons and hydrogen flew back from the unit to a tube of the fire heater, where it accumulated, ignited, and exploded, causing extensive damage to the fired heater.
The plant had decided to shut down all fired heaters due to the extreme cold weather caused by a winter storm. This had already caused several freeze-related operational issues, such as utility losses and supplier-driven gas shortages. However, a valve between one of the fired heaters and the downstream equipment remained open. The open valve allowed downstream flammable hydrogen and hydrocarbon gas to flow backward into the fired heater tubes. Approximately 30 minutes one of the tube ruptured, allowing the flammable gas to enter the firebox.

[Note of the HIAD validator: for events triggered by the same storm, see event #1168, occurred at a different plant, and #1056, which affected a pipeline providing hydrogen to the plants.]

The INITIATING CAUSE was the failure of a valve to isolate the ethylene reactor, allowing flow back of a flammable mixture. The fired heater tube filed due to thermal stress due to the rapid shutdown.
The IGNITION was probably caused by the hot insulation within the firebox likely ignited the flammable gas, resulting in an explosion.

CONTRIBUTION and ROOT CAUSES were
(i) The decision to shut-down the fired heater tubes without decoking. Coke fouling can insulate the tube surface, resulting in local hotspots and increasing the risk of thermal shock and tube failure in fired heaters. These conditions stressed the tube’s walls when the metal cooled faster than the internal coke, breaking the tube.
(ii) Inadequate winterization of multiple valves, instruments, and control systems contributed to the incident.

process valve, heater tube

Extreme cold weather from a winter storm caused several freeze-related operational issues. Due to the emergency conditions such as utility losses and supplier-driven gas shortages, the refinery shut down its fired heaters without removing the solidified carbon deposits on the interior walls of the heater tubes (a process called ‘decoking’).

The damage to fired heater was extensive

This event is very similar to the next one #1168, which happened at another plan of the same company. The both occurred at the same time as the event #1056, affecting a pipeline. They all were triggered by the same extreme environmental conditions caused by a winter storm and by the measures adopted by the plans operators to face lack of feedstock supply. They happened at approximately 100 miles distance from each others.
According to CBS, to the material or component failures characterising these incidents contributed an inadequate preparation of multiple valves, instruments, and control systems to extreme weather conditions.