Risk analysis of jet fires thermal effects

(English) The historical analysis performed has shown that jet fires have been the origin of a high number of domino effect accidents, both in the process industry and in the transportation of flammable fluids by pipelines and that, despite this, have been little studied. In order to improve the knowledge of these fires and be able to predict their potential effects, an experimental setup was designed to generate and study sonic and subsonic horizontal propane jet fires with a length up to 3 m. The facility was equipped with all the required measuring devices (flowmeters, manometers, thermocouples, radiometers, etc.), using as well visible image and infrared cameras to film them. Taking into account the potential danger associated to this type of experimental work, special attention was devoted to the safety aspects in both the design and construction of the facility, as well as during the experimental work. The main features and the behaviour of the jet flames have been studied as a function of the fuel flow rate (i.e., as a function of jet fires size): turbulence, geometry (lift-off, length, width, elevation and reach concerning the contact with some equipment), as well as the temperature distribution and the radiation intensity of the diverse flames zones identified. The treatment of the experimental data has allowed the obtention of expressions allowing the prediction of the lift-off and the probable reach associated to a possible flames impingement situation. In the second part of the tests a steel pipe was set up, containing water or air inside, with a series of thermocouples located both inside the pipe wall and outside it. This allowed measuring the jet fire effects in a situation in which the flames impinged on the pipe, i.e. the heating of the pipe wall as a function of time and of the flames zone impinging on it. It has been found that in the case of flames contact with the pipe wall very high heat transfer rates can be obtained, specially with sonic jets, reaching values as high as 275 kW·m-2 in the front pipe/flames position, these values being lower for subsonic jets. These high heating rates have originated very high temperatures at the pipe wall in very short times when the pipe contained air, registering heating rates as high as 19.5 19,5 ºC·s-1 and reaching temperatures of 600 ºC in 2-3 minutes and of 750 ºC in 5-6 minutes. These very high values clearly show that in the event of a jet fire impinging on an equipment without thermal protec