Piloted ignition calculations

This method is based on the Ohio State University Release Rate Apparatus. The specimen is injected into the environmental chamber through which a constant flow of air passes. The exposure of the specimen is determined by a radiant-heat source adjusted to produce the desired total flux on the specimen, which may be tested horizontally or vertically. Combustion may be initiated by non-piloted ignition, piloted ignition of evolved gases, or by point ignition of the surface. The changes in temperature and optical density of the gas leaving the chamber are monitored, and from the resultant data the release rate of heat and visible smoke are calculated. This aj aratus is also used to measure the rate of toxic gas release and consumption. 

ISO 5659-2 [108] determines the optical density of smoke generated and measured in a single test chamber. The test cabinet and smoke tneasuring equipment is that of the earlier NBS test defined as ASTM E662 and BS 6401. This ISO test uses a horizontal fire model in which the standard test specimen (75 mm square) is supported on a load cell. The specimen is exposed to radiant heat from a conical radiator positioned above the specimen holder see Fig. 19. Although a range of heat flux values can be used, the standard specifies that tests should be carried out at 25 kW.nr with and without a pilot ignition flame, at 50 kW m without a pilot flame. Calculation of results is by the method defined in the earlier tests. 

Pilots and Igniters - Two gas-fired pilots with igniters are installed adjacent to the inlet distributor. The igniter assembly and pilot gas valves must be located remote from the flare for protection of personnel and equipment. This restricts igniter selection to the forced air supply type. Location of these components should be such that the calculated radiant heat density at maximum load does not exceed permissible levels for personnel exposure. Because of the potential hazard of release of unignited hydrocarbons at ground level, a flame scanner (suitably shielded and aircooled and cotmected to an alarm in the control house), is provided for each pilot. 

Calculate the time to ignite (piloted) for the materials listed below if the irradiance is 30 kW/ m2 and the initial temperature is 25 °C. The materials are thick and the convective heat transfer coefficient is 15 W/m2 K. Compute the critical flux for ignition as well. 

SABS 961 - fire index offloor coverings. A specimen is placed horizontally on the floor of a tunnel furnace incorporating a radiant gas burner. It is ignited with a pilot flame and flame spread is observed. A Spread of Flame Index is calculated from the distance burned. SABS 960 describes a similar test for building materials, in which the specimen is mounted at 30° from the horizontal. 

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