Explosion in a chemical plant

[Note of HIAD Validator: the presence of hydrogen and its role in the explosion is only one of the two possible explanations of this incident]
The explosion occurred in a chemical reactor tank of a plant (SEVESO low level) manufacturing products for medical imaging.

According to the investigation, the reaction causing the explosion concerned only the gas phase, at the dome of the reactor, consisting of oxygen (due to degassing), and a flammable gas a mixture.
The investigation has proposed two alternatives:
1. The reaction between di-chloromethane (CH2Cl2) and oxygen stated by a electrostatic charge.
2. Auto detonation of a mixture oxygen and hydrogen produced by the corrosion of a part of the axis of the enamelled stirrer that had been damaged during maintenance.

The building was partially blown, especially in the upper part, and fragments (flanges) were found up to several tens of metres away.

INITIATING cause was the formation of flammable gas mixture in the reactor, in presence of oxygen.
However, the accidental formation of hydrogen is only one of the possible explanations for the explosion. The incident highlighted a lack of measures aiming at monitoring the reaction, and a lack of proper risk assessment based on the possibility of development of run-away reactions.

reactor

The event occurred after an atypical reactor behaviour that required successive degassing to reduce the pressure.

One worker suffered light injuries.
The building was partially blown, especially in the upper part, and fragments (up to 1.2 kg) were found up to several tens of metres away.

The general message coming from this event is that even a frequently used process can still cause accidents. This is the truer, when the operations do not have adequate monitoring of the critical process parameters.

Another more specific lesson learnt is that enamelled walls, in place because of their excellent corrosion protection, are fragile and sensitive to mechanical and thermal shocks.

The risk assessment of the process must consider the risk that these walls soon may fail and cause loss of confinement. Among the preventive measures there could be regular integrity checks.

Another element of a proper risk assessment is the possibility of run-away reactions. The sizing of reactor protection measures and of the associated piping must be designed according to the worse accidental scenario, considering particularly rapid thermal transient or the formation of significant quantities of gaseous by-products. Burst disc or pressure relief valves must be designed considering the possible pressure rise dynamics of such accidental scenarios.

Among the corrective actions suggested by the investigations, these were related to the presence of hydrogen:
1. To avoid the condition for explosion in the reactor, by setting alarms on oxygen concentration in the gas phase
2. To avoid the formation of hydrogen , by ensuring the presence of enamelled protection coating on all the steel surfaces.
3. Control all the possible source static electric discharge.
The operator completely changed the process following the accident.