In the August edition, we looked at visual stream comparisons and test methods related to current nozzle designs, i.e., smooth-bore and combination nozzles. We also examined the effects of turbulence on such nozzles at the same flow and pressure. In this article we will discuss reach, impact force, nozzle reaction and the comparative knockdown ability of streams flowing from smooth-bore and combination nozzles.

Let’s start with reach and impact force, which are related somewhat. Again, we turn to science in addition to what we can visualize. Reach is a result of pressure that is converted to velocity as the water leaves the nozzle. The impact force (striking power) is determined by the weight of the water (mass) multiplied by its velocity (speed). Both reach and impact forces are related to the speed at which the water exits the nozzle. The higher the pressure, the higher the speed; thus, the reach is greater and the stream hits harder, provided that the flows (mass) are equal.

It has been said repeatedly that a smooth-bore nozzle hits harder with greater knockdown power than a combination nozzle. The truth is that many conclusions have been drawn by looking at the large flow smooth-bores operating at a lower nozzle pressure, compared to 100 psi combination nozzles, which flowed less volume. This is not a fair comparison. Doing so would be like comparing a .22-caliber rifle with a .44 Magnum.

It is important when comparing streams to ensure that the nozzles being evaluated are producing equal flows at equal pressures. Over-pumping from 50 to 75 psi increased the flow rate, reach and the impact force of the stream, with no adverse effect on its performance. As the pressure on either nozzle is increased, a little more water drop-off will occur near the edge of the stream due to friction with the surrounding air, which is not moving. When compared properly, both nozzles still delivered a good stream with good characteristics until the break-over point, where they both fell equally into showers of heavy rain.

The eventual break-up of the streams is caused by a “piling up” of air in front of the stream as well as by the displacement of the surrounding air. Both nozzles were affected equally by winds, which cut down the reach of the streams. Even a very slight cross breeze will cut the reach of either stream by 20 to 30 feet. Most nozzles performed equally well at lower pressures down to about 40-psi tip pressure. Keep in mind however, if one maintains the flow rate but operates at lower pressures (50 psi or less), hose kinking will be much greater than at the higher pressures (75 or 100 psi). Crews must be diligent in correcting any kinks should they occur.

Examine the photo above. Note that both streams appear to have the same reach. In fact, at equal flows and inlet pressures, the stream from the combination nozzle went further than the smooth-bore stream by about 10 feet, unless a stream straightener was placed on the latter. Then, the smooth-bore gained a slight advantage. Regardless, both nozzles had a reach of more than 100 feet. The bottom line is that they both performed in a very similar manner and delivered a nicely shaped stream that landed in a relatively tight area.

Under Pressure

Some argue that the smooth-bore stream hits harder. To determine whether this is true, we can use formulas to convert the gallons into pounds per second (lbs/sec) and the pressure into miles per hour (mph). In the photo on page 28, it is evident that the impact force of the streams at the same flows and pressures are, for all practical purposes, equal.

This has been confirmed by impact testing. One such test used plates that transferred force to a compression load cell at 12.5-foot, 25-foot and 50-foot distances, and then compared a combination nozzle to a 7/8-inch smooth-bore nozzle. Both nozzles were flowing 160 gpm at 50 psi. The impact was equal, with the combination nozzle hitting slightly harder at the 50-foot distance.

A pitot tube normally is used to ensure that a smooth-bore nozzle is operating at the proper pressure and thus delivering the proper gallonage. A pitot also can be used on a combination nozzle to determine the exit pressure. It is placed at the edge of the stream — recall that the stream exits the nozzle in the form of a donut — but this is difficult to do and requires a fine-tipped pitot gauge. Instead, manufacturers recommend using a line gauge placed at the base of the nozzle to ensure proper pressures on combination nozzles.

Overall, we found that nozzle reactions almost are identical, with the exception being that there is a slight drop between the base pressure and the exit pressure on the combination nozzle, a disparity that increases as the flows increase. In other words, the combination nozzle loses a slight amount of pressure and thus nozzle reaction due to the water pushing forward on the baffle, as well as the water changing direction as it exits the nozzle, and as flow rates increase, the difference increases. At 250 gpm, the difference was about 10 pounds, resulting in a slightly easier nozzle to handle — though most firefighters would not notice the difference.

Another test employed scales to check nozzle reactions and confirmed that nozzle reaction formulas are very accurate and that at equal flows and pressures, the reactions almost are identical for both smooth-bore and combination nozzles.

We have now demonstrated that at equal flows and pressures, the streams from both the smooth-bore and low-pressure combination nozzles:

• Look alike
• Have the same reach and reactions
• Strike with equal impact force
• Are equally turbulent
• Are of a similar droplet size

But what about their capabilities to extinguish fire, i.e., their knockdown ability? Some experts believe that both nozzles will deliver water to the seat of the fire in a similar fashion. As demonstrated above, both streams are identical once they are a few feet from the nozzle. Both are equal in quality, reach and impact force, and are affected equally by wind. Moreover, neither stream is solid; instead, both are composed of large droplets.

The volume of water (gpm) determines the amount of BTUs it can absorb and thus the amount of fire heat that can be extinguished. So, providing the flows and pressures are equal, the only differentiating factor between a smooth-bore and a combination nozzle is the form that the water is applied-straight stream or fog pattern. According to Andy O’Donnell, retired district chief for the Chicago Fire Department, there is very little difference, if any. “In the real world of interior structural firefighting, the difference between a smooth-bore and straight stream in terms of direct attack effectiveness is negligible,” he said.

However, the combination nozzle can absorb greater amounts of heat if set to a fog pattern, where the water begins to break up offering a greater surface area for heat absorption, and those who attended fire school in the early 1980s were taught to use this indirect method of fire attack. But this method has lost support for interior attacks due to the amount of steam generated and the resulting thermal upset, though a high-pressure variation of this method, dubbed “3D,” is being used successfully in Europe.

Push Over

Another highly debatable topic concerns whether the combination nozzle will “push” fire. Indeed, while it might be possible to push fire in the fog setting due to the massive air movement it develops, generally neither the straight-stream nor the smooth-bore pushed fire any more or less than the other, provided that the flow rate is adequate for the amount of fire. Neither the smooth-bore nor the combination nozzle in a straight stream setting has any great effect on air movement. This is not to say that neither nozzle will push the products of combustion when an overabundance of steam is created. Remember, for every gallon of water required to extinguish the fire, about 200 cubic feet of steam is produced. In situations where stream production is more of a concern — such as when victims are inside the structure — using short, intermittent bursts will minimize this effect.

Here’s a simple test to demonstrate this phenomenon: Set a garden-hose nozzle to the straight setting and shoot the stream through the flame of a small campfire or bonfire. The stream will travel through the flames and emerge on the other side with virtually no impact on the flame. Then change the stream to a fog pattern or move the nozzle in a circular pattern — you will begin to see the flames dance in response to the air movement. The same test can be duplicated using a pump-can or pressurized extinguisher with its solid-stream.

The purpose of this article was not to discuss attack methods but rather establish that low-pressure nozzles will perform equal to a smooth-bore tip when used in the straight-stream setting at the same flow and nozzle pressure. But while the smooth-bore nozzle is not a bad nozzle, in the majority of fires the combination nozzle will be used in a straight stream setting in the same way as the smooth-bore nozzle. This leads to an important question: Why, if both nozzles provide an equally effective straight-stream pattern, would firefighters not want to use the nozzle that offers the greatest versatility? This answer is obvious, according to O’Donnell, who was one of the early pioneers of the low-pressure combination nozzle, and who in the early ’80s pushed manufacturers to produce a lower-pressure combination nozzle that would still offer versatility but operate at pressures more consistent with the smooth-bore streams.

“If the situation changes and we find ourselves fighting a flammable liquids fire in a structure, or if a window in a high-rise breaks and we have to deal with a wind-pushed fire, it’s nice to know that I have the option to change my nozzle pattern right at my fingertips,” O’Donnell said.

In our department, we have set up our crosslays to offer the same flows and pressures, one with a smooth-bore nozzle and the other using a combination nozzle. The decision is now up to the officer as to which nozzle to choose. If your department is still using the older-style 100 psi nozzles, it may be time to do some testing at different flows and pressures to see how the design of your nozzle adapts, or upgrade to a newer-style low-pressure nozzle. Whatever flow that you decide on, there are a variety of options to achieve similar results, and most manufacturers have charts in their catalogs that list the gallons delivered at various pressures from their nozzles. For example, at 185 gpm a 15/16-inch tip compares to a 250@100 psi combination pumped at 50-psi tip pressure. At 250 gpm, a 11/8-inch tip compares to a 250@50 psi combination. On rapid attacks the 500-gpm, low-pressure combination tip performs similar to the 1K-inch smooth-bore nozzle pumped at 75-psi tip pressure. If your department does not have the equipment needed to conduct nozzle testing, manufacturers usually are willing to come out with their equipment and assist.

The next time someone tells you that the smooth–bore nozzle must be the nozzle of choice or that it is a better nozzle, question them as to what they are basing their conclusions upon. Are they talking about the old 100-psi nozzles? Have they tested the newer low-pressure combination nozzles at equal flows and pressures compared with the smooth-bore? And, more importantly, have they done live side-by-side comparisons using valid testing equipment and methodology? Most often, firefighters are relaying experiences from years ago that compared a 100-psi combination nozzle to a smooth-bore nozzle that had a greater flow and certainly less nozzle pressure. As this article has demonstrated, pressure and the resulting turbulence directly affect nozzle performance. But at equal flows and nozzle pressures, which are obtainable with today’s nozzles, we see almost identical stream patterns and firefighting effectiveness. Employing physical science to this age-old debate reveals indisputable facts that clearly can assist in understanding how combination and smooth-bore nozzle streams truly compare. n

Ron Eilken is a 29-year veteran of the fire service and currently serves as a deputy chief for the Des Plaines (Ill.) Fire Department. His past duties have included service in the department’s training division as well as its engineering program. He has been an instructor at the fire academy, at FDIC and at various fire colleges and fire departments across the country.

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