Study of factors influencing the formation of a fire signal when using a fire alarm.
Research of factors influencing the formation of a fire signal when using a fire alarm
Of all the facilities where fires occurred, only 853 were equipped with a fire alarm. In 62.3% of cases, the alarm went off, in 18.52% it was out of order, and in 12.42% it was off.
The initial stage of a fire in a room is characterized by significant non-stationarity of both the parameters of the fire source (burnout rate, size of the source, etc.) and the values of hazardous fire factors (HF) at different points in the volume of the room. The non-stationary field of HF distribution is characterized by high heterogeneity due to the presence of jet streams.
The operating mode of the automatic fire alarm depends not only on the type and design features of the sensors, but also on the conditions of fire development, which determine the change in environmental parameters in the sensor installation area over time. To determine the response time of fire detectors, it is necessary to know the dynamics of the environmental parameters, the impact of which leads to the triggering of the detectors, and the reaction of the sensitive element of the detector to this impact.
Due to the variety of space-planning solutions for premises and possible variants of fire development in them, the physical and chemical parameters of the material (heat output during combustion, smoke-forming capacity, burnout rate, or other flame propagation rate, etc.) are of great importance when forming a signal. During combustion, high-temperature combustion products rise upward, forming a stream of ascending gas, the surrounding cold air is entrained and mixed with the gas stream, rising to the ceiling. As a result of this process, the total mass velocity of the flow continuously increases, and the average temperature and concentration of combustion products in the stream decreases with increasing height, which increases the detection time of the OFP by fire detectors. When the gas stream hits the ceiling, it spreads along it, forming a relatively thin layer of heated gas. In this case, heat exchange with the enclosing structures occurs, the gas temperature decreases, the detection time becomes longer, the further the ignition source is from the detector axis.
To study the factors influencing the formation of a fire signal in rooms equipped with point thermal fire detectors of maximum and differential action, a study was conducted of the dependence of the temperature of the thermal element of the detector on the characteristics of the fire source (mass burnout rate, linear flame propagation rate, etc.), the placement of the fire detector (height, distance from the fire axis), and the inertia of the fire detector. A differential equation for the heat balance of the sensitive element of the detector under the conditions of thermal impact of a ceiling jet of combustion products was used.