FIREGROUND HYDRAULICS: STRAIGHT-STREAM REACH
Fourth in a series.
Last month we discussed nozzle reaction and saw how, like flow rate, it varies with the square root of nozzle pressure and how the maximum reaction can be calculated in your head. In practice, the actual reaction felt by the nozzleman is always less, because some is transmitted along the hose to the floor. Reaction felt by the nozzleman is very difficult to calculate because it is highly dependent on technique. For example, one small woman using an efficient technique can manage a larger flow than three large men using brute force.
Let’s discuss how nozzle reach is measured and how nozzle pressure affects it. The discussion is limited to horizontal reach in interior firefighting with smooth-bore and fog nozzles for both 100 psi and low pressure. For interior work in closed compartments, vertical reach usually is not a limiting factor, except for fires in churches and other tall structures, and these structures rarely are higher than they are long.
REACH IS A COMPLEX TOPIC
Reach is an even more complex subject than nozzle reaction. Many variables affect reach, and it can’t be measured easily. Despite this, since the decision whether to use the smooth-bore or the fog nozzle should be based on numbers—not just subjective impression — comparative reach should be evaluated.
Since it is widely assumed in the fire service that smooth-bore tips give longer reach than fog nozzles, many departments use fog nozzles for interior firefighting and then switch to smooth bores for defensive exterior firefighting—“surround and drown” operations —believing that this method will expedite punching through fire to extinguish the seat of the flames. Assumptions like this one should be reexamined.
What is reach? Many definitions are possible. Elkhart Brass (in Catalog T, page 60) gives reach figures for its fog nozzles, but these show only the “point where the final drops of water land in still conditions.” Although this “footprint” criterion is easy to measure and is useful for comparing spray patterns, it is irrelevant to straight streams, since they are used to punch a solid column of water through high heat to a target. Droplets will be vaporized before arrival.
The center of the “footprint” sometimes is considered the reach. This, too, is irrelevant, since the fire service usually doesn’t sprinkle water unless it’s bounced off a ceiling.
A hundred years ago John R. Freeman did a series of exhaustive hydraulic tests on hose and smooth-bore nozzles, which form the basis for fire service hydraulics even today. His definition of reach is still relevant to practical firefighting. According to Freeman, “…an effective solid firestream is one that:
- at the limit named has not lost continuity of stream by breaking into showers of spray;
- up to the limit named appears to discharge nine-tenths of its volume of water inside a circle 15 inches in
- diameter and three-quarters of it inside a 10-inch circle;
- is stiff enough to attain in a fair condition the height of distance named even though a fresh breeze is blowing; and
- at a limit named will, with no wind, enter a room through a window opening and just barely strike the ceiling with force enough to spatter well.”
HORIZONTAL REACH VS. PRESSURE
Smooth-bore reach tables normally relate to nozzle diameter. In modern firefighting, flow rate is of paramount importance, so it is instructive to look at reach as a function of flow rate. In Table 1, I list Freeman’s reach tables ith his discharge tables’ to show horizontal reach as a function of pressure of a constant flow rate. The calculated reach is based on the empirical formula: R = 65 X /(P/50), since at 50 psi the reach is about 65 feet. For the smooth-bore handline range shown, the error is less than 8 percent. In practice, horizontal reach increases roughly as the square root of pressure if the gpm is held constant. The flow is kept roughly constant by changing the tip size.
Reach increases with increasing gpm at the same pressure because the constant velocity core of the stream is larger in a nozzle of larger diameter; a smaller proportion of water curls off because of velocity differences at the outside of the stream.
Mental reach calculation. To mentally calculate reach using Freeman’s data for smooth bores on 2’/2-inch hose, remember that at a 50-psi tip pressure, the horizontal reach is roughly 65 feet. Although this figure is for a flow rate of 200 gpm, reach doesn’t change much with gpm; this estimate, therefore, can be used. For example, at 30 psi, using the approximate square root method, the reach would be 65 feet x /(30/50) = 65 feet x v’0.6 = 65 feet x 0.8 = 52 feet. The measured reach for a 1 ¼inch tip at 30 psi is 52 feet.
Measuring reach. Since reach depends on the size and type of nozzle and the wind speed and direction, the best way to compare nozzles is to measure reach directly.
Although the four Freeman criteria are highly relevant to the fire service, it is very difficult to estimate the second and third criteria by eye. An objective test system should be set up to do it properly.
The first criterion is relatively easy to estimate, though. Before breaking into spray, the solid stream breaks into visible “slugs” of water. The effective reach can be approximated by the horizontal distance at which these slugs are still readily visible.
Here is the method 1 use to estimate reach: For a 1 ½-inch system with a 1 ½-inch waterway, I clamp a 1 /2-inch Task Force Tips (TFT) pistolgrip adaptor into a heavy machinist’s vise. The vise is bolted to a piece of Viinch steel plate to make a stable base that can be held down by a firefighter. I use the level protractor on a machinist’s combination square to set the pistol grip’s barrel to 32°, the angle for maximum reach found by Freeman. The pistol grip waterway is drilled and tapped for ¼-inch National Pipe Thread so that I can attach a pressure gauge that will measure the inlet pressure of a fog nozzle and estimate the discharge pressure of a smoothbore stream. A pitot gauge, if available, should be used on smooth bore. It is difficult to use a pitot gauge on a fog nozzle because the wall thickness of the cylinder of water leaving the nozzle is too thin for an accurate reading.
A deluge set and stream straightener are used to test a 2 Vi-inch system. A 100-foot tape measure is stretched in a line from the nozzle tip on a level area such as a parking lot, and markers are positioned every 10 feet.
The farthest point at which slugs of water are clearly visible is estimated by firefighters who have been instructed in what to look for. At least three observers are asked to note their estimated reach on paper without conferring with one another. This keeps the observations independent. Estimates are then averaged and a standard deviation calculated to estimate the “spread” of the observations. Wind speed and direction also should be noted.
Preliminary results on a 1¾-inch handline with a gentle tail wind are given in Table 2.
MENTAL ARITHMETIC
Applying the mental arithmetic square-root law for comparing the reach of the above ⅞-inch smooth bore with the fog nozzle on straight stream seems simple at first glance. The fog nozzle pressure is 100 psi, twice that of the smooth bore, so if they respond the same to pressure differences, the reach should be 1.4 times as long, since the square root of 2 is 1.4. This is calculated as 48 feet x 1.4 = (48 x 1) + (48 X 0.4) = 48 + 19= 67 feet.
The measured reach is 62 feet, so the rough estimate is 8 percent high. The discrepancy is due to the pressure drop from the inlet of the nozzle to the outlet or tip, which is roughly 15 psi for the TFT handline nozzle, flowing 160 gpm. This means that the actual stream pressure is about 100-15 = 85 psi.
Performing the calculation with the actual tip pressure of 85 psi gives a reach of 48 feet x 7(85/50) = 48 X 1.30 = 63 feet, which is almost the measured value of 62 feet. A smoothbore nozzle flowing at 85 psi will also have a reach of 62 feet, since at the same gpm, psi is the major factor in determining reach.
Calculating the square root mentally might be a challenge, but there is a simple way to calculate the square roots of numbers between 0.5 and 2 (see sidebar at right).
To participate in the low-pressure fog vs. the 100-psi fog nozzle debate, we can estimate the reach of an equivalent low-pressure fog nozzle. Suppose the inlet pressure is 75 psi. The outlet or tip pressure would be approximately 60 psi, since there is a 15-psi friction loss within the nozzle. A higher degree of accuracy is attained when reach is calculated at the tip instead of the inlet pressure. The measured reach at 85-psi tip pressure was 62 feet. Using mental arithmetic, therefore, the reach at 60-psi tip pressure should be about 62 x /(60/85) = 62 x /(12/17) = 62 x /07= 62 x .85 = 53 feet. The exact calculation gives a reach of 54 feet.
Table 3 compares the horizontal reach and the nozzle reaction of standard and low-pressure fog nozzles with smooth bores, flowing 160 gpm The reaction was calculated by the method used in my February article, using tip pressure rather than nozzle inlet pressure for greatest accuracy. Since the reach and the nozzle reaction vary with the square root of pressure at constant gpm, what you gain by reducing nozzle reaction you lose in reach.
Also, comparing smooth-bore tip and the low-pressure fog nozzle at 160 gpm, you endure only five more pounds of nozzle reaction for six feet more reach and for the ability to blow fire out of a compartment or protect yourself from a flashover with a fog pattern.
To make an informed choice of nozzles, we must know how much reach is required. More isn’t always better. This and other important factors such as hose maneuverability will be discussed in subsequent articles.
The ability to calculate such figures, rather than guess, is needed to determine which nozzle and attack hose is best suited for each application. Using the right tool for the job is a major part of professionalism. Thumbing through catalogs is not good enough.
Endnotes
Elkhart Brass Mfg. Co., Catalog T, p. 60.
Eire Chiefs Handbook, fourth edition. Eire Engineering, 1978, p. 304.
Ibid., p. 306.
Fire Service Hydraulics, second edition. Fire Engineering, p. 396.
Handbook, fourth edition, Eire Engineering, 1978, p. 305.
