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Heat flux in jet fires: New method for measuring the heat flux levels of jet fires
RISE - Research Institutes of Sweden, Safety and Transport, Fire Research Norway.ORCID iD: 0000-0003-2164-940x
RISE - Research Institutes of Sweden, Safety and Transport, Fire Research Norway.ORCID iD: 0000-0003-0979-2369
2018 (English)Conference paper, Published paper (Other academic)
Abstract [en]

Jet fires are ignited leakages of pressurized liquid or gaseous fuel. In jet fire testing for the offshore industry, heat flux is the defining factor for the accidental loads. NORSOK S001 [1] defines two different heat flux levels of 250 kW/m2 and 350 kW/m2 depending on the leak rate of hydrocarbons. These heat flux levels are used in risk analysis and define what type of fire load bearing structures and critical equipment need to be able to resist in a given area. Examples of such ratings can be “250 kW/m2 jet fire for 60 minutes”, “350 kW/m2 jet fire for 15 minutes” or any other combination based on calculations in the risk assessment. Combined with critical temperatures this defines the performance criteria for the passive fire protection. Each configuration of the passive fire protection needs to be tested and verified. Manufacturers of passive fire protection request fire tests to document their performance against jet fires with these various heat flux levels. The challenge is that the standard for testing passive fire protection against jet fires [2] does not define any heat flux level or any method to define or measure it. We have developed a method for defining and measuring the heat flux levels in jet fires. This method can be used when faced with the challenge of testing passive fire protection against specific levels of heat flux. The method includes a custom test rig that allows jet fire testing with different heat flux levels. A large number of tests have been performed to verify the reproducibility and repeatability of the method. Heat flux is defined as the flow of energy through a surface. The heat flux from a fire to an engulfed surface of an object is dependent on both the engulfing flame and the properties of the surface. The properties of the surface may change during the exposure to the flame as it heats up and changes its surface properties. At some point the object inside the flame will reach a thermal equilibrium with the flame where the net flow of energy into the object is balanced by the energy emitted from the object. The heat flux for an object can be calculated as incident heat flux, emitted heat flux or net heat flux. A definition of heat flux needs to include parameters of the receiving object. These variations give a lot of degrees of freedom when calculating heat flux in a fire. Special water cooled gauges are designed to measure heat flux to a cooled surface, but these have proved to be very unreliable when placed inside a large fire. A more robust and easily defined method is to measure the equilibrium temperature inside an object placed inside the flame. This is the principle used in plate thermocouples used in fire resistance furnace testing [3]. In our experience, these plate thermocouples are often damaged during high heat flux jet fire tests. This raises questions to how long into the tests such measurements are reliable. Several other types of objects have been tested and the most convenient and reliable type was found to be simply a small 8 mm steel tube that is sealed in the end and has a thermocouple inside. One key difference between this small tube thermocouple and the plate thermocouple is that the plate thermocouple is directional and the tube is omnidirectional. Current works and tests will optimize the measuring objects in order to get the most relevant equilibrium temperature while still maintaining the robustness of the sensor during the test. The suggested heat flux calculation is to follow the Stefan-Boltzmann relation of temperature and heat flux. For a black body this gives 350 kW/m2 for 1303 °C and 250 kW/m2 for 1176 °C. A lower emissivity may be defined for the surface of the sensing object giving higher temperatures for the same flux levels. This method gives a simple, robust and reproducible correlation between heat flux levels and temperatures that can be measured during jet fire tests. The method does not differ between the varying convective and radiative heat transfer in the flame, but it is a representative measurement for the temperature that an object would reach when placed inside the flame.

Place, publisher, year, edition, pages
2018.
Keywords [en]
jet fire, heat flux, standardisation, testing
National Category
Other Engineering and Technologies
Identifiers
URN: urn:nbn:se:ri:diva-37538OAI: oai:DiVA.org:ri-37538DiVA, id: diva2:1281936
Conference
Nordic Fire & Safety Days, Trondheim, 7-8 June 2018
Available from: 2019-01-23 Created: 2019-01-23 Last updated: 2019-01-29Bibliographically approved

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Stolen, ReidarFjellgaard Mikalsen, Ragni

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