ARE YOUR EMERGENCY SHUTDOWN PROCEDURES ADEQUATE TO PREVENT DAMAGE?

 On February 15, 2021, at 3:05 p.m., a hydrogen and hydrocarbon gas mixture was accidentally released into the firebox of a fired heater, where it ignited and exploded. It was estimated the incident resulted in $2.5 million in property damage.

The company's investigation found that the facility shut down multiple fired heaters when extreme cold weather from Winter Storm Uri caused several freeze-related operational issues. Due to emergency conditions that included utility losses and supplier-driven gas shortages, the company shut down its fired heaters without removing the solidified carbon deposits on the interior walls of the heater tubes (decoking).

The company's investigation revealed that further utility upsets caused the facility to stop all fuel gas supply and fully shut down the remaining fired heaters. During the shutdown, a valve between one of the ethylene unit’s fired heaters and 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 after this happened, a tube within the fired heater ruptured, allowing the flammable gas to enter the firebox. The gas accumulated, ignited, and exploded, causing extensive damage to the fired heater (Figure 2).

The company's investigation determined that the tube failure likely occurred because the fired heater tubes were shut down 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. The company's investigation concluded that hot insulation inside the firebox likely ignited the gas (autoignition).

Probable Cause
Based on the company investigation, the CSB determined that the probable cause of the incident was a fired heater tube failure from thermal stress due to the rapid shutdown. When the tube broke, a hydrogen and hydrocarbon gas mixture from downstream equipment flowed into the firebox. Hot insulation within the firebox likely ignited the flammable gas, resulting in an explosion. Inadequate winterization of multiple valves, instruments, and control systems contributed to the incident.

Source: CSB.gov

HAVE YOU CONSIDERED FREEZING OF WATER IN INSTRUMENT AIR HEADERS DURING LOW AMBIENT WEATHER CONDITIONS?

 On February 15, 2021, at 9:53 p.m., a hydrogen and hydrocarbon gas mixture was accidentally released into the firebox of a fired heater, where it ignited, creating a fire at a facility in Texas. The estimated property damage from the incident was $5.8 million.

The company investigation determined that freeze-related operational issues due to extreme cold weather from Winter Storm Uri caused the facility’s boiler units to shut down. This shutdown decreased the supply of steam and water to an ethylene unit at the facility. A safety system automatically shut the fired heater down due to insufficient steam and water flow. Although the automatic valves closed to prevent feed from entering the tubes, the fired heater’s fuel gas isolation valve failed to close as intended. This failure allowed the burners to continue operating. Without any flow through the tubes to remove heat from their walls, the tubes in the fired heater reached excessively high temperatures. Some tubes ruptured, likely due to short-term overheating, but a metallurgical analysis was not performed to confirm the cause of the tube failures. When the tubes ruptured, hydrocarbons downstream from the fired heater were released through the broken tubes and ignited by the burner flames inside the firebox, causing a fire.

During post-incident testing by the company in temperatures above freezing, the fuel gas isolation valve successfully closed as designed. The company's investigation concluded that the fuel gas isolation valve did not close because water in the instrument air supply had frozen or ice formed on the external actuator components (related to inadequate winterization), preventing the isolation valve from closing.
Probable Cause
Based on the company's investigation, the CSB determined that the probable cause of the incident was the rupture of fired heater tubes from short-term overheating. When the tubes were broken, a hydrogen and hydrocarbon gas mixture flowed from downstream equipment into the firebox. Flames from the burners ignited the flammable materials, resulting in the fire. Inadequate winterization of flow control equipment contributed to the incident.

HAVE YOU IDENTIFIED ALL POSSIBLE SOURCES OF CORROSION?

On October 1, 2020, at approximately 9:00 a.m., approximately 554 pounds of chlorine gas were accidentally released at a facility in Louisiana. Exposure to the toxic chlorine vapors seriously injured one employee.

At the time of the incident, four workers (two employees and two contractors) were trying to stop a chlorine leak (described by as a fugitive emission) from a drain valve in the company’s methyl diisocyanate production plant. Because a chlorine leak was detected, the company assumed that at least one of the drain valve’s connections in its flanges or bonnet was loose.
The four workers had a safe work permit to retighten the bolts using hand tools. However, the leak persisted after the bolts were retightened by hand. One of the contractors then tried to retighten the bolts using an impact wrench, a power tool designed to tighten and loosen bolts with short bursts of high torque. The vibrations from the impact wrench caused the connections on the drain valve’s bonnet to fail catastrophically, forcefully disconnecting the top half of the drain valve (closing element, stem, and handle) from the bottom half (body and seat) and releasing the toxic chlorine vapors.
The workers were wearing supplied air respirators to perform the job. After the chlorine release, they evacuated the area. However, one employee did not switch to the “escape bottle” for their respirator before detaching the respirator from the stationary air supply source during the evacuation. The subsequent lack of supplied airflow in the respirator caused the employee to remove their respirator mask during the evacuation, resulting in the employee being exposed to the toxic chlorine vapors. The employee was transported and admitted to a hospital for medical treatment.
The company's investigation found that the drain valve was severely corroded. Over time, chlorine being released from the drain valve had reacted with condensed water that had accumulated in the piping system’s insulation blanket, creating hydrochloric acid that had corroded the valve and deteriorated the bolts in the valve’s bonnet. After the incident, the company discovered that three other valves were similarly corroded.

The investigation also revealed that one of the contractors tried to use their stop work authority after seeing the dilapidated valve. The contractor brought concerns to one of the employees. While the  employee went to get a supervisor’s opinion, there was a miscommunication between the two parties about whether or not to continue working on the corroded valve. As a result, the work continued, ultimately leading to the chlorine gas release.

Probable Cause
Based on the company's investigation, the CSB determined that the probable cause of the incident was the catastrophic failure of corroded bolts in the drain valve’s bonnet. The corrosion was caused by prolonged exposure to hydrochloric acid, created by the reaction between chlorine and condensed water.
Contributing to the incident was the company's mechanical integrity program. Had the company  inspected the valve before authorizing this job, the extent of the corrosion could have been identified, and a shutdown could have been initiated to replace the drain valve instead of attempts to repair it. Contributing to the severity of the incident was the company's respiratory protection program, which did not ensure its workers could effectively transition to the escape bottle during an emergency. Had the company effectively trained its workers to switch to their escape bottles during an emergency, the employee should have not been exposed to the chlorine.

Source: CSB.gov

DISTILLATION TOWER COLLAPSE DUE TO INTERNAL FIRE

On September 5, 2023, at 11:50 a.m., a distillation tower (“Tower”) collapsed (Figure 1) at a Petrochemical in Texas. The collapse was the result of a fire inside the distillation tower. The incident resulted in approximately $194 million in property damage.

A few weeks before the incident, the company shut down its Pyrolysis Gasoline unit, including its extractive Tower. The company followed its procedure to remove solvent and hydrocarbons from within the Tower to prepare for maintenance work. On the day of the incident, a maintenance crew began opening manways to prepare the Tower for entry. At 9:30 a.m., temperatures with the Tower’s stainless steel structured packing beds began to rise. The company sounded an emergency alarm at 10:44 a.m. after smoke and flames showed a fire within the Tower. Personnel were evacuated after adding nitrogen and trying to stop air from entering the Tower. At 11:53 a.m., the Tower folded over (collapsed).
The company investigation found that the Tower fire resulted from a chain of undesired reactions. About a month before the unit shut down, equipment leaks in another area of the process allowed water to enter the extractive distillation unit, leading to internal corrosion that removed iron from the metal walls. This iron formed iron sulfide (a pyrophoric material) from hydrogen sulfide that is usually present in the process equipment. When the Tower’s manways were opened, oxygen (air) entered the Tower, starting an exothermic iron sulfide oxidation reaction. The company concluded that the heat from this iron sulfide oxidation reaction resulted in localized heating (1,300 to 1,800 degrees Fahrenheit) within the structured packing that weakened the Tower’s walls and ultimately caused the Tower to collapse.
In addition, The company’s investigation found that a 2016 incident had revealed the potential for iron sulfide to accumulate within the Tower. The investigation team determined that this knowledge had not been effectively transferred to the existing site personnel. Had the site’s operations team known about the potential for iron sulfide within the Tower, changes to the procedure to remove solvent and hydrocarbon could have been made to mitigate the heating from the iron sulfide oxidation.
During the incident, approximately 160 pounds of sulfur dioxide and an unknown amount of other reaction and combustion products were accidentally released.
Probable Cause
Based on The company’s investigation, the CSB determined that the probable cause of the fire was smoldering iron sulfide oxidation within the structured packing of the Tower. Contributing to the incident was that the company did not effectively maintain its knowledge from its 2016 incident finding that iron sulfide could accumulate within the Tower.

Source: CSB.gov

TANK OVERPRESSURE INCIDENT

On June 22, 2023, at about 9:10 p.m., a 5,200-gallon polyethylene storage tank (“tank”) ruptured at a facility in North Carolina. The rupture fatally injured one employee.
At the time of the incident, the tank needed to be refilled with aluminum chloride; however, sulfuric acid was added instead. Approximately 80 gallons of 93 percent sulfuric acid were added to the 60 gallons of aluminum chloride remaining in the tank, resulting in a reaction that caused the tank to rupture. The tank rupture fatally injured the employee who was refilling the aluminum chloride tank.
The company's investigation appeared to focus on the incorrect reaction rather than a reaction scenario consistent with the available evidence. Its research materials pointed to the reaction between sulfuric acid and aluminum metal that generated hydrogen gas. The company concluded that the hydrogen gas was released at its autoignition temperature (over 1,000 degrees Fahrenheit (°F)) and ignited, triggering an explosion. Post-incident photographs of the damaged tank do not appear to support a scenario where the internal tank temperature reached 1,000 °F, however. The melting temperature of the polyethylene is approximately 260 °F. Therefore, the company’s reaction scenario appears inconsistent with the available evidence.
The CSB concluded that the reaction most likely generated hydrogen chloride vapors. The more likely scenario was that the reaction between aluminum chloride and sulfuric acid produced enough hydrogen
chloride vapor to increase the pressure within the polyethylene tank, likely resulting in an overpressure that separated the tank’s body from its base.
The CSB estimated that approximately 130 pounds of hydrogen chloride vapors were accidentally released.
Probable Cause
Based on the factual information from the company's investigation, the CSB determined that the probable cause of the overpressure was the increase in the tank’s internal pressure resulting from the hydrogen chloride vapors created by the reaction between sulfuric acid and aluminum chloride.

Source:CSB.gov