Explosion in a refinery

This accident took place in different phases and in two different units of a refinery:
It consists first in a non-hydrogen accident, followed by a hydrogen-related accident.

Before the hydrogen-related accident took place, a local fire occurred near the atmospheric vessel, containing a solution of NaOH in water used for neutralizing the acidity of the raw oil contained in the desalter. The fire caused damage to the tank and to the electrical cables running near the vessel. This fire was completely extinguished before the second accident.

Half-hour later, an explosion occurred at the exit of the tube from the preheating oven of gasoil + hydrogen of the desulphurisation plant. This explosion damaged, besides the oven, various equipment in a radius of about 50 m from the explosion point. The fire following this explosion was of short duration.

The incident occurred in a zone in which numerous personnel, either internal or external, were present for the modifications going on the near-by plants. Its consequences could have been more serious not only to structures, but also to people, if the "all clear" alarm was given immediately after the end of the first accident; in that case many persons would have returned to their job and would have been taken by surprise by the second accident. This conclusion is supported by the considerable damages received by the materials (crane with extensible arm, van for material transport, hut from sheets of metals, materials for changes of parts, etc...) in a radius of 50 m from the explosion point.

The INITIATING CAUSE of the first accident was the release of the hydrocarbons contained in a tank.
Its root CAUSE was probably a human error, during the manual operation of the dilution of the sodium hydroxide solution (routine operation), protracted for an excessive time, leading to the overflow and emission of hydrocarbons in the canals and their flushing shafts collecting white waters.
One of the consequence of the first accident was the interruption of the electrical supply which provoked the loss of the gasoil supply flow to the oven in which the second accident occurred. The increase in the temperature and pressure inside the pipes in which gasoil + H2 should have circulated occurred due to this supply loss, with consequent overheating of the pipes resulting in rupture and explosion.
Also in this second case, the root CAUSE was probably a human error consisting of the undue silencing of alarms due to insufficient supply and of high temperature and pressure and the subsequent omitted interventions as foreseen by the existing procedures. As also stated by the eMARS report, a CONTRIBUTING CAUSE would be inadequate training on processes and procedures.

chemical storage tank,
electrical power,
pipes in a heat-exchanger

The accident occurred in two different phases, in successive times and in two different units.
The operational condition for the second accident, involving hydrogen, was abnormal because of the first accident (local fire occurred anomalous temperature and pressure increase).

In the second incident the hydrogen circulating in the serpentine of the preheating oven in the
desulphurisation plant trespassed the temperature limit of 1000oC (maximum temperature
foreseen for the serpentine less than 500oC) when the gasoil supply loss occurred (the preheating oven, in normal conditions, operates whit a combined charge of liquid gasoil and gaseous hydrogen and is heated by burners supplied by fuel gas.)

DESCRIPTION OF THE PROCESS
Desalter = a unit removing salt from the crude oil. The salt is dissolved in the water contained in the crude oil, not in the crude oil itself. The desalting is usually the first process in crude oil refining.

For plant workers and external worker were light injured.

The first fire provoked material damages around the sodium hydroxide recipient. The second event provoked besides the damages to the oven and to the connecting pipes, also damages to
mobile devices like a crane truck, a light lorry required for the modification works in progress and glass breakage and other damages to various objects inside a radius of 50m from the point of the explosion.

An action of the judicial authority (taken by from the jurists police on behalf of the Attorneys Office of the Republic of Italy).

8.45 the pre-alarm concerning the first accident is given to external firemen, Carabinieri, prefecture, mayor, port authorities).
In the same time the internal emergency plan was put in action with the internal fire brigade intervention with foam and water extinguishers of various kinds. The emergency procedure required the evacuation from the nearby plants of all the persons not involved in the extinction efforts, which resulted in sustaining only material damages and no personnel injuries.

9:25 The persons assigned to the emergency, believing the incident has concluded, gave an "end of pre-alarm" signal to the external authorities, which in the meantime have not intervened. In the meantime, for prudence, the state of emergency was maintained inside the establishment.

9:45, having verified the 2nd event, a 2nd pre-alarm to the external authorities was issued: in this case the firefighting forces of Cagliari intervened; however when they have reached the establishment the 2nd incident was also concluded.

As stated in the eMARS report, in both accidents the human error was of great importance.

A first consideration is to make the procedures of the training more stringent, repeating them at regular intervals, as well as the updating of the responsible personnel to the operations in the plant, even for operation considered "routine", emphasising the necessity of following to the rule, the instructions received and above all the written procedures.

Another consideration consists of the opportunity of making more rigid the security processes prescribed for the modification and /or improvement of the plants, in particular when external companies are used, when theses operations take place in the vicinity of functioning plants, or with operations under work (routine or not), adopting a more stringent system for the working permits. However, because the experience teaches that human errors can happen, although everything is always possible, it would be opportune to avoid the possibility to bypass, ignore or silence key safety intervention measures.