Task Force Tips has engineered a test method to accurately measure the impact of a fire stream.
Which nozzles hit hardest?
“A fire stream can be defined as a stream of water or other extinguishing agent after it leaves the fire hose and nozzle until it reaches the desired point. During the time a fire stream of water passes through space, it is influenced by its velocity, gravity, wind, and friction with the air. The condition of the stream when it leaves the nozzle is influenced by operating pressures, nozzle design, nozzle adjustment, and the condition of the nozzle orifice.” This is the opening statement from the IFSTA Pumping Apparatus Driver/Operator Handbook Chapter 7 – Fire Hose Nozzles and Flow Rates and clearly summarizes the rules that affect fire streams and their characteristics.
When testing or evaluating nozzles, the fire service generally measures flow and pressure as its primary yardstick for comparisons. Much controversy has surfaced recently concerning the ability of one type of nozzle and fire stream to outperform another. It is common to read about hard-hitting types of nozzles, however much of the currently available comparative data surrounding these issues is subjective in nature with little or no scientific evidence to support any claims made.
One specific characteristic of a fire stream that has not been measured until now is the “hit” or “punch” of a stream. This particular characteristic clearly separates the smooth bore advocates from those in the adjustable pattern “combination” nozzle camp. Proponents of smooth bore fire streams claim that for greater impact, a smooth bore nozzle must be used. Others claim that a combination nozzle set to the straight stream position, and flowing the same amount of water at the same nozzle pressure will have similar results.
Because of this controversy, Task Force Tips engineered a test method (noted in the set up diagram) to accurately measure the impact of a fire stream. The device consisted of a test stand that used interchangeable 38 cm, 15 cm, and 8 cm diameter circular “targets” attached to a vertical member of the test apparatus. A compression load cell was located on the horizontal leg a distance that is equal to the distance from the pivot point to the center of the target. The impact of the stream pushed the target, and the vertical support transferred the force to the load cell, which in turn indicated the impact of the stream in kgs/force on a digital readout.
The Test Setup
A flow rate of 600 lpm was chosen for comparisons since it represented a typical fire attack in North America. Each nozzle tested was operated at, or very near, this flow rate. This provided a constant measurement to which the different operating pressures and nozzle types could be compared. The smooth bore and combination nozzles were both operated at 3.5 and 7 bar at a similar 600 lpm flow rate. By comparing the impact with different types of nozzles at the same flow and pressure, the performance capability of one type over another could be easily determined. A 22mm (7/8”) smooth bore was used to deliver 600 lpm at 3.5 bar, and a 19mm (3/4”) smooth bore was used to deliver 600 lpm at 7 bar. Three individual TFT 265-750 lpm automatic handline nozzles were gated to 600 lpm at 3.5 bar, 5 bar, and 7 bar respectively. The impact measurements were taken at distances of 5m, 8m, and 15m. The measurement was from the target face to the point at which the water exited each nozzle. At the 5m distance, the 8cm target was used. This was done to address the claim of smooth bore advocates that “solid” streams are more compact than “hollow” streams from combination nozzles and, therefore, have more impact. At the 8m distance the 15cm target was used, and at 15m the 38cm target was utilized. To secure the nozzle, a fixed monitor was used. This assured accurate readings by keeping the nozzle stationary once water was flowing and eliminated error caused by handholding the nozzle. The monitor was fed with a 15m section of 44mm hose. The pump used was a Hale 3785 lpm model, and all flows were verified using a calibrated electromagnetic flow meter cross referenced with pitot readings on the smooth bore nozzles. The flow meter data was collected using a 30-channel chart recorder. One set of tests was undertaken at each distance with the five different nozzles and yielded a total of 15 sets of data. Each test ran approximately 3 minutes to insure sufficient data was collected to average the results.
The first series of tests used an 8cm target at a distance of 5m (Chart A). At that distance, there was very little difference in the impact from either the smooth bore tip or the combination nozzle operating at 3.5 bar. At 7 bar nozzle pressure, the combination nozzle developed 10% more impact than its smooth bore counterpart at the same flow and pressure. The 5 bar nozzle developed impact values as expected between the 3.5 bar and 7 bar nozzles. It is interesting to note (and logical based on Newton’s theory) that, at this distance, the impact values are very close to the reaction forces.
The second series of tests used a 15cm target at a distance of 8m (Chart B). Again, either at 3.5 bar or 7 bar nozzle pressure, there is no noticeable difference in impact for either the combination nozzle or the smooth bore tip. The 5 bar nozzle again performed as expected, with impact forces between the tests for high and low pressure nozzles at this distance.
The third series of tests used a 38cm target at a distance of 15m (Chart C). At the 15 m range, the combination nozzle developed a slightly greater (5%) impact force than the smooth bore nozzles at either 3.5 bar or 7 bar. The 5 bar combination nozzle developed impact forces between the 3.5 bar and 7 bar combination nozzles.
Referring to the bar graphs of each series of tests (Chart D), it is obvious that there is little difference between the “punch” or “hit” of a fire stream based on the type of nozzle from which the fire streams exits. Rather, the impact (“punch/hit”) is more a function of the velocity of a fire stream (pressure) and its mass (flow). If the flow rate and pressure of the two fire streams are similar, the impact force will be similar also. The smooth bore nozzle has NO ADVANTAGE over the combination nozzle when it comes to impact capability. Conversely, if the impact forces are similar, then the combination nozzle is preferable due to its ability to vary the pattern to meet the needs of the firefighting team. If the best of both worlds is desired, an automatic combination nozzle provides maximum impact for any given flow at 7 bar and has the obvious advantage of pattern control and selection.
For more information, go to www.tft.com