Offensive fire attack – Variables that interfere with the fire gases outlet: Part 1
Offensive fire attack, or transitional attack, is an increasingly technique used in Fire Services around the world with some differences based on the degree of training and the equipment used. That is why we have set out to analyse the variables that can improve implementation, application and effectiveness.
The offensive fire attack technique aims to reduce fire power by directing a straight stream of water against the ceiling of the room on fire from outside. With this form of projection, the gas flow dynamics of the fire are minimally affected due to two factors. On the one hand, it prevents blocking the exhaust by using a straight stream and, on the other, it generates less vapour than with the application of fog pattern water, affecting the gas layer and its dynamics as little as possible, thereby not jeopardising the survival conditions of potential victims nor endangering the fire-fighting teams as they advance further into the building.
The choice of hose diameter is one of the first decisions to make after assessing the incident, but this decision can have major consequences for Fire Services which are accustomed to the use of 25 mm water lines at high pressures to increase flow, as opposed to lines with a greater diameter working at lower pressures.
The variety of options available creates the need to assess which line is recommended for this technique, as well as the factors which influence their effectiveness. The aim of our trials is to determine the influence of the variables in relation to the type and position of the installation to interfere the fire gas coming out, the way in which the water is dispersed inside, and to where and under which conditions it is more efficient to aim the water. In this article we will focus on the variables that can interfere with escaping gas.
Drag cone effect on the outlets
As we know, if we project a water fog pattern at a outlet, the flow of water and air will block the fire gases coming out, reversing the flow of gases by pushing them inwards, thereby aggravating the survival conditions. Any water stream drags around the jet and coaxially an airflow due to friction with the water, which we can call “drag cone”. To interfere as little as possible with the fire gas flow path coming out of the exahust where the water is being introduced, either by the jet of water itself or by the drag cone generated, we use a straight stream. This type of stream, on moving away from the nozzle, gradually opens out due to the dispersion effect of the water body as a result of the friction with the air, also increasing the size of the drag cone. In this equation, friction is proportional to the square of the stream speed, configured as the key variable, so the system pressure and the speed with which the water leaves the nozzle play a fundamental role in the formation of this drag cone and in the interference that causes in the outlet at a given distance.
The angle at which the stream enters the window is another variable to be considered, as the interference with the fire gas flow path is lower the greater the angle at which the water enters the horizontal plane. The hose operator can see how the visible surface of the window (and thus the area available for the air inlet) is smaller the closer he stands to the façade. Furthermore, the pressure of the airflow into the room (on X axis) is less because the angle of its movement is greater.
To approach these issues, it is not necessary to have sophisticated measuring devices, but decent ones are required to make comparisons to tell us whether our hypotheses are correct or not.
Two trials were designed and repeated with different configurations in a multi-storey building constructed specifically for firefighter training at the Málaga Fire Service. The variables for the hydraulic system were:
- Pump pressure. A Rosenbauer NH30 centrifugal pump was used. The pressures used were 10, 20 and 30 bar for the 25 mm hoses and 5 bar for the 45 mm ones.
- Nozzles. Three Firestar devices were used: two 25 mm models with maximum flow rates of 150 L/min and 230 L/min, and one 45 mm model capable of delivering up to 475 L/min, which was used with the flow meter positioned at 360 L/min.
- Position on the façade. Two projection positions were set from the ground, perpendicular to the window, placed at a distance of 1.5 and 4 metres from the façade.
This was conducted in a room on the second floor with all openings closed except for the projection window and a known surface outlet. From the outside a straight stream of water was aimed at the window, whilst the output speed of the air through the exhaust was measured by an anemometer to determine the airflow, which allowed us to estimate the amount of air that had entered due to the effect of the stream projection. It should be noted that the site was not completely sealed, with there being the normal space between the frames and sashes of windows and doors.
This involved estimating the flow of water from the different configurations by measuring the projection time of a known amount of water.
Findings and discussion
The results of trials 1 and 2 are shown in the table below. The data provided by the results confirm the baseline scenario, verifying the following statements:
- The higher the pressure in an installation and the output speed of the water, the greater the airflow dragged into the room.
- The closer we stand to the façade, the lower the airflow dragged into the room.
- ressure has more influence on the drag cone than the actual water flow.
The data obtained shows that, when deciding how to apply this technique, it is preferable to use hydraulic installations with a larger diameter at a lower pressure and that the operator stands as close as possible to the façade within security conditions in order to interfere with the fire gas flow path as little as possible.
In the next article we shall discuss issues related to water dispersion inside the room and vapour generation.
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