Explosion

The incident occurred at a storage tank of an organic chemical plant containing cyclododecane (C12H24). A transfer of this product from the stationary tank to a tank truck was taking place at the moment of the incident.
An explosion internal to the storage tank occurred, causing the partial opening of its 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 cause of the explosion was the accidental presence of hydrogen in the tank, due to desorption from the cyclododecane, when sending it to the storage tank. The low explosivity limit of the hydrogen in the vapour space was reached, when a under-pressure in the tank occurred during transfer, causing the opening of a burst dusk and access of air. The hydrogen-air mixture ignited, creating enough excess pressure (800 mbar) to rip open the top of the storage tank.

The INITIATING CAUSE was the accidental presence and ignition of hydrogen gas phase of the storage tank.

A series of malfunctioning of control and safety systems were intermediate causes. The post-incident investigation identified the following sequence of accidental sub-events:
1. The location of the nitrogen purging system on a balancing line hindered an effective purge of the tank’s vapour space and the elimination hydrogen traces.
2. When the transfer from the tank to the trucks started, 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 was foreseen for these cases, but it malfunctioned (most likely clogged) and did not stop the transfer.
4. The under-pressure in the tank caused its pressure-relief valve tank to open and to allow air to enter the vapour space.

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 considered in its design.
Moreover, the tank was equipped with a heating system aiming at avoiding product crystallisation. This system was inadequate to prevent in clogging of the safety systems (sensors, nitrogen injection) in cold weather (–1 to + 9 °C).

The ROOT CAUSE was an inadequate management of changes, which caused the malfunctioning of safety-critical systems.

cyclo-dodecane storage tank

The plant involved in this event is the same of the event HIAD_785.

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

The shift operators placed the unit in a safe state. This was enough to eliminate the possibility to continue to produce accidentally hydrogen.

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 same plant and the same unit had been affected by another incident 3 years before. Although the accidental mechanism was different, it had initiated by the same sensor failing to trigger an alarm by a situation of low-pressure/ low-level in the tank (HIAD_785).

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.