Dust Explosion in a Hydrogenator

The Incident

The hydrogenation vessel had been under maintenance for internal vessel inspection. On the first day after hand over to production, 1,8-Naphthalene-dicarboxylic-acid-anhydride was added to the vessel by a powder screw conveyor. Shortly after lunchtime the addition was completed. At that instant, a 'pop' sound came from the vessel. There was no fire or explosion with effects outside of the vessel.

Cause

Immediate on-site investigation was carried out

Findings:

The vessel was earthed/grounded properly. It was re checked and found OK.
Although the vessel was inerted, there might have been an oxygen pocket inside the vessel.
A 'POLY' collar was tied to end of screw conveyor for discharging powder into the vessel.
The wheels of screw conveyor were made of PVC.
Relevant Safety Data of the powder:
1. Minimum Ignition Energy : >3mJ
2. Electrical Resistivity: 4x10^12 Ohm·m
3. Particle Size: <0.1mm

The cause of dust fire was attributed to static discharges in combination with the incomplete inerting. Two types of discharge could have caused the incident:

Cone discharges due to charge accumulated on the powder (high resistivity):
The critical heap diameter (see note below) for a cone discharge of 3mJ is only about 2.2m. The vessel diameter is 2.35m, but in view of the rather limited amount of powder (125kg) it is rather unlikely that the critical heap diameter was exceeded.
A spark discharge between the conveyor and the grounded vessel:
Charge separation occurred between the moving powder on one hand and the collar on the other hand. The charge was accumulated on the collar and on the conveyor, which was not properly earthed/grounded. Eventually there was a spark discharge between the conveyor and the grounded vessel, igniting the explosive mixture formed by the combustible dust and the residual oxygen.

NB) The critical diameter for cone discharges is d[m] = 0.6 x M[mm]-0.435 x MIE[mJ] 0.298

Measures taken

A metallic funnel was provided for discharging powder from the conveyor to the vessel.
An earthing connection was provided between screw conveyor, funnel and vessel.
In addition metallic wheels were provided for the screw conveyor. (floor is conductive to a sufficient degree)
Continuous purging/inerting of the vessel will be carried out during powder addition.
The material supplier of the powder was contacted for the possibility of supplying flakes instead of powder.
An electrostatic meter was procured and will be used to monitor static charge generation when piloting such processes.

Lessons Learnt

When handling combustible powder, all equipment made of conductive material must be properly grounded to earth, in order to avoid accumulation of static electricity.
Where inerting is used as a protective measure, it must be ensured that the required reduction of the oxygen level is maintained during the entire operation. This can be achieved e.g. by verification with an oxygen sensor during the first operation and subsequent strict observation of the approved inerting procedure.