Explosion on a hydro-treatment unit

The incident occurred at the hydrofining unit, refining a mixture of petroleum and diesel into Euro 3 and Euro 4 diesel.
A pump feeding the reactor with the process mixture failed. Following the shutdown of this pump, a series of other events occurred which led to an explosion followed by fires.
Detailed series of events:
(1) After the pump stopped, the flow in the system dropped and the sense valve cracked.
(2) The failure of the sense valve led to a reverse flow, filling the supply of the tank with a mixture of diesel and hydrogen at high pressure.
(3) A series of communication alarms followed and at the same time the explosion occurred.
(4) The explosion caused three fire flames in three directions: one at the exploding tank, one at a cracked pipe in the gas fractionation area and one on the ground in the tank area.

There was one fatality and 5 injuries.

The INTIATING CAUSE was the failure of a pump.
The ROOT CAUSE was shortcoming of the safety design: a series of single failures bought to an escalation of the initial incident, due to the absence of redundancy of the critical component. Moreover, the tank which failed due to over-pressure should have been equipped with a correctly dimensioned pressure relief device.

pump, valve

DESCRIPTION of the DIESEL HYDRO-TREATEMENT PROCESS
The feedstock, diesel, was pumped directly from the DA plant at a temperature of 100 °C into a feed tank. From the tank, the feedstock is pumped into the unit at a working pressure of 49.5 bar.
The hydrogen required for catalytic sulphur conversion was supplied from the refinery, compressed to a working pressure of 45 bar. The mixture of raw material and hydrogen was preheated and partially vaporised in the heat exchangers to 264-312 °C, after which it is heated in the furnace to the temperature required for the hydro-refining reaction, 316-359 °C.
Diesel - 109 tons/h
Hydrogen: 1.32 tons/h.

One fatality and 5 injuries due to burns or over-pressure.
Approximately 50 tonnes of Diesel burned. The eMARS incident does not quantify the quantity of hydrogen released/burned. For the information on the flows of feedstocks, one could assess an order of magnitude of hydrogen of 0.7 tons.
Destruction to the installations: the hydro-refiner was heavily damaged, with costs above the 2 MEuros.
The accident remained confined within the boundaries of the site.

A redundancy of the safety measures is an important element of the risk-reduction strategy. In this event, a series of failure of elements available only once has brought to an escalation of the original incident. Moreover, the unit was equipped with a back-up pump, but it was not available because under repair.
The operation should require a minimum of two functional pumps to ensure the flow.
The pressure relief valve of the tank must be designed taking into account the pressure which could occur under accidental scenario as the one occurred.
The lesson learnt from this event should be used to perform an improved HAZOP analysis.