In the article published in the previous issue, we did a description of the fundamentals of the exterior offensive attack technique, establishing that its effectiveness depends on three different aspects: Interfering as little as possible in outgassing, thus avoiding changing the dynamics of gas flow, to generate as little water vapor as possible, to effectively cool solid fuels. The variables related to the obstruction of fire gases outlet by water stream (and the air it drags) projected inward have been discussed in the previous article. Now we will try to describe what happens inside the room on fire.
Water dispersion and effectiveness
In a confined fire we can make a simplification of the elements involved: the fuel that feeds the fire, the layer of hot gases from the combustion and the enclosure that surrounds and contains them. We start from the premise that in order to be able to decrease the intensity of fire, the water should reach the fuel feeding that fire. Furthermore, we should avoid changing the dynamics of hot gases flow, minimizing where possible the generation of steam, namely the one generated when water traverses the gas layer or when getting in contact with structural elements that do not contribute to fire (walls and ceiling, provided they are not covered or coated by combustible materials).
The straight stream projected into a room against the ceiling goes through the gas layer without suffering significant vaporization due to the small surface that the water body exposes. Once it breaks against the ceiling, it loses kinetic energy by drawing a dispersion pattern that varies depending on the speed and the angle at which it reaches the ceiling.
Once again we have the influence both of the delivery pressure, which shows up in the speed with which the water leaves the nozzle and of the entrance angle of the jet into the room. When water reaches the ceiling, it spreads and slips under it, being this movement more horizontal the faster the speed of the jet and the smaller the angle of entry, what means more time in the layer of gases, and therefore a greater vaporization in this area. In addition, faster speed causes greater dispersion of water and drops of smaller diameter.
As a result of gravity and disaggregation, the water ends by rushing towards the ground dropwise. The size of these drops, as they traverse the layer of gas while falling, is decisive in the degree of vaporization, being preferable large drops against mist, since the smaller size, the bigger surface exposed to heat and the bigger residence time in the gas layer, which means that less liquid water will reach the solid fuel at lower levels.
Finally, water will fall vertically on fuels or on the ground, or it will impact on the walls. In this respect, we should minimize the amount of water reaching walls, as water will use its cooling capacity on them by generating steam and then it will drain into the lower planes, where it will accumulate without any effect on fuel.
In order to study all these aspects, several trials have been conducted in the facilities described in the precedent article.
Trial 3. In the same room, we project water inwards, with different configurations of pressure, flow and operating position in respect to the façade, making an observation of the water spraying on a water sheet on the floor.
Results and discussion
Results of Trial N.3 allow us to make the following statements:
- The higher pressure and the lower entry angle, the greater distance reaches the water projection.
- The greater angle of entry, the greater dispersion in the axis perpendicular to the projection.
- The lower pressure and the higher actual water flow, the thicker drops confirmed by direct observation.
In respect of water dispersal patterns, we check that they vary considerably, mainly moving between 3 and 7 meters in front of the window, extending in the transverse axis with configurations with lower pressure and higher actual flow.
Type of hydraulic system
From the results obtained in the framework of the study it emerges that, among the facilities studied, the most effective and producing less interference in the exhaust is the one of 45 mm against that of 25 mm, by applying the appropriate pressure to the height we have to achieve, and adjusting the flow to the intensity of the fire.
Although the 45 mm hose is a priori more difficult to handle, in practice it does not pose any particular problem because the operator has a basically static position and, by having a high projection angle, he can place the hose so that the reaction is absorbed by the ground.
In a real situation, it is difficult for the operator to know the size and distribution of the room into which he or she projects the water beyond what he or she might guess basing on the use of the building. However, we can say that it will always be preferable to start in a position as close to the front as possible and starting the projection of water by focusing a point near the window, because in this way we will reduce the air flow that interrupts the gas output. Furthermore, by using an angle closer to the vertical water projection has a greater radius of dispersion and prevents the water from reaching the opposite wall if this was nearby.
The operator should also be placed in a position allowing him/her to avoid walls, insofar as possible, if these are visible. In any case, it is always useful to the operator to know the dispersion pattern of the jet used, because a more effective implementation can be achieved.
In summary, we can establish the following general rules for the application of this technique in order to improve its effectiveness:
- Use preferably hydraulic systems that allow us an adequate projection of flows based on their diameter and not on their pressure. In the case of Deparment using small diameter lines, 45 mm hoses instead of 25 mm ones, adjusting the pressure to the height of the fire and the nozzle flow to its intensity.
- The operator must be located as close to the façade as security conditions allow, directing the water jet against the ceiling immediately below the window frame.
- Project the water to a fix point, moving the operator between one touch and the other in order the water projection to reach different areas into the room, avoiding walls if possible.
For more information, go to www.fundamentosparabomberos.es/