A fire broke out affecting the heavy fuel purification plant of the refinery.The incident could have originated by the failure of one of the following plant components:- pipes leading to the pressure gauges of the reactor - recycled gas pipe at the bottom of the reactor having a quench function,- diatermic oil pipe (hot oil) entering or exiting the exchanger- flanged joints exchanger and connection lines.On the base of and data logging and video evidences, the company excludes a release from the hot oil circuit as the triggering factor of the fire. The release started 30 minutes before the fire. Also a release from the hydrogen pipes is not considered likely, as the records demonstrate that the hydrogen pipe failed 7 min after fire began. Concerning the flange joints of exchangers, after dismounting the exchanger flanged joints, the gaskets resulted not to be damaged. The company concluded that the failure of a pipe from the pressure measurement gauges of reactor is the most likely accident triggering factor. This assumption is supported by the following facts:1) this part is located in the area corresponding to the epicenter of the fire,2) the area corresponds to the area visually identified by the witnesses,3) the product release (hydrogen and fuel oil) from one of these pipes can cause a 6 m long jet flame as occurred.4) the product supposedly released would have had a high enough temperature and pressure to self-ignite or ignite against a hot spot of the plant, like the hot oil circuit.5) the damages recorded were caused by overheating (flame exposition) and were not caused by overpressure or explosion - the pressure measurement records confirm significant pressure changes at the beginning of the event. The company does not have any element allowing to identify the failure that caused the pipe to rupture.