Explosion

The incident occurred at a storage tank of an organic chemical plant. The storage tank was containing cyclo-dodecane (C12H24, melting point: 60 °C), and a tank trucks was being filled at the moment of the incident.
An explosion internal to the storage tank occurred, causing the partial opening of the roof. The shift operators placed the unit in a safe state. A driver of the truck was admitted to hospital for emotional trauma experienced during the accident.

The analysis of the accident attributed the incident to the ignition of hydrogen present in the gas phase of the storage tank. The hydrogen was formed accidentally via desorption when sending of Cyclododecane into the storage tank. The following elements contributed to the situation:
1. The position of the nitrogen purging system on the balancing line did not make it possible to purge the tank’s vapour space and eliminate the traces of hydrogen.
2. When the tank trucks were loaded, the liquid level inside the storage tank dropped, but the nitrogen injection system did not sufficiently compensate for this decrease (the nitrogen system was likely partially clogged).
3. Consequently , the internal pressure of the storage tank dropped. A low-pressure safety sensor is foreseen for these cases, but it malfunctioned (most likely clogged) and did not stopped the loading.
4. This caused the pressure-relief valve of the storage tank to open and allow air to enter the vapour space. As the explosivity limit of the hydrogen in the vapour space was reached on the day of the accident, the hydrogen ignited, creating enough excess pressure (800 mbar) to rip open the top of the storage tank.

The INITIATING cause was the unexpected formation and ignition of hydrogen during a transfer.
A series of malfunctioning of control and safety systems were intermediate causes.
The root cause was an inadequate management of changes

The nitrogen injection system had been modified shortly before, but the possible formation of hydrogen and its ignition in the tank’s vapour space had not been sufficiently taken into account.
In addition, the risks of the product crystallising and forming clogs inside the tank’s safety systems (sensors, nitrogen injection) in cold weather (–1 to + 9 °C) despite the heating system also had been underestimated.

cyclo-dodecane storage tank

The nitrogen injection system had been modified shortly before the incident.

The driver of the truck was admitted to hospital for emotional trauma experienced during the accident.
The storage tank roof was damaged.

This is a case where the plant operator did not succeed to assess the additional risks introduced by a modification of the process. Unfortunately, the report does not provide direct information if and/or how the new potential hazards were taken into account. Since a HAZOP study is mentioned as corrective measure, it could be concluded that this had not been done at the moment of the modification.

The operator implemented the following key corrective actions:

1. The safety dossier for the process was updated (HAZOP study: solubility of hydrogen in cyclo-dodecane, its kinetics and its degassing).
2. The nitrogen injection system was resized and made more reliable (separate injection and discharge lines, scavenging lines via the storage tank, separate purging lines via the storage tank, safe and instrumented control of injection).
T3. he diameters of the venting lines were changed and the reliability of their heating system was improved to prevent the formation of clogs.
4. The reliability of the pressure and level sensor ports was improved to prevent the risks of clogs (double jacket to maintain the sensing lines at the appropriate temperature).
5. As additional measure, the hydrogen degassing and storage/loading processes will be separated.