Paper pulp tank explosion

In a paper plant, an explosion occurs in a paper pulp tank while 2 workers were carrying out pipes cutting with a grinder on the roof of the tank.
Detailed analysis of the event was performed by two independent experts. The paper pulp stored under certain conditions can ferment and produce hydrogen.
The final expert report establishes the explosion process as follows:
- The pulp stored in the tank released hydrogen at a rate such that the LFL (lower flammability limit) could be reached within a few hours.
- Hydrogen mixed with the air in the tank top to formed an exploice atmosphere.
- The ignition source of the mixture was an incandescent particle produced during cutting with grinder of the pipe connected to the upper gaseous part of the tank.
- The overpressure developed by the explosion led to the rupture of the tower roof at the level of the connection welding with the cylindrical flange.


[ARIA]

The INITATING CAUSE was the creation of a flammable mixture in the surrounding of the tank due to leak, ignited by a spark of the pressure gauge, possibly assisted by static electricity shock.

All filling operations, including their controls, were performed manually. Except for a measure of the internal tank total pressure by means of a portable pressure gauge, no other monitoring system was used, despite the specific hazards related to handling hydrogen and oxygen. In particularly, no flammable gas detector is mentioned having been used. Moreover, leak tests after assembly had not been performed at the nominal (rated) tank pressure, but at less than 2/3 of it.

The ROOT CAUSE was related to lack of proper risk assessment, lack of preventing and mitigating safety systems, and lack of duly supervision by university management of the activities if a visiting fellow.

pulp storage tank

Maintenance works where being performed, which contributed to the severity of the consequences.

The incident occurred near the paper pulp preparation workshop, whose implementation process was purely mechanical and used no chemicals.

The process steps are as follows:
(1) Crushing involves suspending PCR (polymerase chain reaction) in hot water at 50°C contained in a pulper, to obtain paper pulp with a dry matter content of 4 to 5%.
(2) Refining, carried out in several stages, aims to eliminate unwanted solid matter. It is performed by centrifugation and passage through calibrated holes.
(3) Fractioning separates short fibres (SF) from long fibres (LF) by passing them through a rotating basket with very fine slots.
(4) Thickening, carried out in parallel on the LF and SF lines using a disc filter, achieves a dry matter content of 10%. Two categories of paper pulp are produced, one with "long fibres" and the other with "short fibres," stored in two separate towers (FL and FC).

The explosion occurred at the tower of the long fibres.

1 fatality.
The FL storage tower was severely damaged and its use was immediately prohibited by inspection of classified facilities.
This decision led to the cessation of production, as the tank was essential to the operation of the plant.

This accident shows that pulp obtained from papers and cartoons without chemical processing can become the site of a microbiological activity. This phenomenon produces hydrogen which can form an explosive atmosphere in the storage tank under specific conditions (high filling level, agitation of the pulp after a long period of stop)

A study was performed, which showed that in large tanks (> 1,000 m3 ), such as the one exploded, the release of H2 can be enough to easily trespass the lower flammability limit (LFL). This happens when the filling rate is high, and the mixture is set in motion after a prolonged stop phase. The gas bubbles, which are likely to be trapped in the fermenting dough are released when the mixture is set in motion.
The conclusions of the study were that in this type of paper manufacturing plants the risk of an explosion has to be taken into account in the safety design and the operational risk analysis.

The incident and the following safety study motivated the design of new provisions aiming at ensuring the protection of employees, what required a e-definition of the ATEX zones. A mapping of the usual presence of workers in various locations of the site, cross-referenced with the zones of effects in case of explosion, showed that the installation of safety barriers was not required. Nevertheless, to avoid the presence of ignition sources during work, the response procedures were completed and updated to include the possible release of hydrogen, which was not disregarded before the accident. In particular, the "operator training in explosive atmosphere detection" aspect was ensured that personnel is trained in the use of portable gas detectors.

In addition, large storage and mixing tanks located close to an outside wall in the workshop were equipped with additional vents.