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Water Rules - Summaries of the recent UL studies Part 1 - Water Mapping

The nozzle is the fire service’s iconic and essential tool, like the rifle to the soldier, the hammer to the carpenter, or the knife to the chef. While we have many other tools, and vital roles that don’t even involve water, when on the fireground most of them support or depend upon use of this appliance. Forcible entry clears its way, and ventilation clears its results. The protection of life and property relies on its expert use, and salvage protects the building and contents from the effects of that use. It is the foundation of any fire attack strategy, and all firefighters must be intimately familiar with its many functions, capabilities, and limitations, regardless if their specific assignment involves even touching a hoseline. Now, thanks to the researchers at the Underwriters Laboratories Firefighter Safety Research Institute, and our tax dollars, we have more information than ever about this most basic of our utensils.

Not that we had ever been lacking for details about how water streams work. The findings of nozzle studies performed in the 1800s are still applicable today, and there are innumerable formulas for calculating the movement of water through hoses, as well as its heat absorption abilities. Anyone so inclined could pursue a college degree in hydraulics engineering. What the Impact of Fire Attack Utilizing Interior and Exterior Streams on Firefighter Safety and Occupant Survival series has provided is additional insight regarding the actions and effects of the water once it exits a nozzle, specifically how it lands, pulls air along, and affects the environment in a burning structure. The new significance of these parameters arose from the renewed interest in the use of exterior water streams and the increased awareness of the effects of airflow on fire growth.

The first two parts of this project, Water Mapping ( and Air Entrainment ( are what I would characterize as hose stream behavior studies, and were intended to provide background information for Part III, the Full Scale Experiments ( that measured the effects of those streams when used by real firefighters in real burning buildings. The results of each of these components will be here outlined in a series of blogs, but, rather than rely on this commentator’s summary and perspective, I would encourage everyone to read this material first hand, at least to review the findings and develop an appreciation for the attention to detail and accuracy to which the researchers’ strived. And, for those less inclined to sift through reports of experimental processes, UL has created interactive training presentations, accessible at (My blog might be the CliffsNotes, but there are also movies.)

The Water Mapping experiments served to fill in knowledge gaps we had regarding hose stream use indoors. There is historic, but still accurate, data about the ideal angle of elevation to achieve the greatest reach (32 degrees); and how far, and at what pressure and flow rate, a water stream remains “effective”, or minimally “broken up” (40 to 145 feet, depending on the type of nozzle, flow, and pressure). Except, when we are attempting to extinguish a structure fire, the water from our hoselines impacts a wall or ceiling long before losing velocity or shape. Nozzles used by the American fire service may be designed for long distance trajectory, but are most often used for close quarters combat.

The UL researchers constructed a test structure that consisted of a 17 foot 4 inch x 11 foot 4 inch room, with an 8-foot ceiling, the floor of which consisted of a grid of 20-inch by 20-inch square collection funnels connected to individual flow gauges, allowing for precise measurement of where on the floor water ended up after entering the room from the end of a nozzle. A 2-foot wide by 4-foot high opening 3 feet above the floor in a long side of the room simulated a window, and a 3-foot wide by 6-foot 4-inch high opening along a short end served as a door. The entire structure was elevated almost 8 feet above ground level, a necessity to accommodate the water collection system below, with access via a moveable staircase and landing that could be positioned at the window or door sides, or moved away. This allowed for measuring water entry angles from both the level of the test room, as if into a first-story window or door, or to simulate flowing into the window from one story below. Water dispersement patterns were measured from different angles of entry of the streams, ranging from nearly vertical and bouncing off the lintel, to directly horizontal and bouncing off the opposite wall, with multiple angles in between. A total of 83 experiments were conducted, varying the angles, flow, and amount of stream movement.

While the data collected from the experiments was as voluminous as the water flowed, the practical results are rather easy to summarize. My primary takeaway from these studies is that hose streams were found not to bounce off of surfaces they impact as much as they follow those surfaces. That is, rather than the water that leaves a nozzle striking a ceiling and raining upon the floor below, its momentum instead causes it to continue to travel along the ceiling until reaching the opposite wall, and then to follow along that wall, until another impact, a loss of velocity, and/or gravity finally causes it to fall to the floor along that wall. Water aimed directly at the opposite wall spread out in a radial pattern, generally parallel to the surface. Moving the streams from side to side deposited more water in the far corners of the room, but still along the walls. These findings were pretty consistent, regardless of the angle of entry.

The stream direction that resulted in the greatest amount of water bouncing back onto the floor of the room was that which struck the lintel, or top horizontal edge, of the opening. When the water stream was aimed entirely into the window, the more vertical (steeper) the angle, the more water ended up on the floor, with that amount decreasing, and the amount striking the opposite wall increasing, as the stream was directed lower. Additional findings included the propensity for solid streams to hold together more after initially impacting a surface, resulting in more water dropping to the floor directly opposite the nozzle position, while that from the less dense straight and fog patterns tending to deposit more water in the far corners of the room. They also looked for any differences in the distribution related to nozzle pressure and flow rate, but none were found. It was possible manipulate those parameters in order get more water into a room, but where it went changed little. Gating the nozzle bale down to 50%, thereby decreasing water turbulence and momentum, did have the effect of more uniformly distributing water onto the floor but, of course, resulted in less water flowed overall. A similar, though less pronounced, finding was found when water was directed from one floor below due to the decreased velocity of the stream at the time of impact, and, hence, less momentum to carry it across the ceiling.

So, now we know that the steep, still, and straight recommendations for exterior streams results in mostly wetting the ceiling and walls. That doesn’t mean, of course, that they are any less effective than previously demonstrated, as cooling the hot gases that the stream moves through, and the surfaces that surround the fire, apparently serves to reduce room temperatures significantly. (This was again made evident in the Full Scale Experiments when exterior streams directed at the ceiling resulted in temperature inversions, where the ceiling temperatures became lower than those nearer the floor.) What it does mean is that maximizing the wetting of burning contents using exterior streams, which is a necessity to maximize the duration of cooling until interior hoselines can be positioned, will require specific intent. For a ground floor fire, this can be accomplished by placing the nozzle into the window once the room is sufficiently cooled to allow approach, thereby eliminating the entrainment of air from the exterior, and allowing for the application of water directly onto the burning contents. For a fire on the second story or above, aiming for the lintel, keeping the stream as steep as possible (by placing it as close to the structure as possible), flowing water for as long as practical (real buildings don’t have collection drains on the floor, and water will eventually accumulate and spread across the floor), and gating down the nozzle after initial knockdown are all reasonable strategies for improving conditions pending arrival at the involved room by crews from the inside direction.

The researchers’ admitted limitations for this study included the single size and simple geometry of the test room, and the lack of furniture and fire, any of which will effect the disbursement of water streams. Still, though in no way revolutionary, the information provided adds to our understanding of water stream behavior, and will enhance our ability to maximize the efficiency of our most important implement.


Next: Part 2: Air Entrainment


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