The nozzle on the end of a fire hose is one of the most important pieces of equipment that a firefighter has at their disposal when combating a hostile fire. It’s the business end of what we do.
Understanding the nozzle involves more than knowing if a push or a pull opens the bale and if a left or right twist delivers a straight stream. Here are five nozzle questions whose answers you may not know or have forgotten. Either way, knowing your nozzle gives you an edge over your enemy in a fire attack.
1. How are automatic and conventional nozzles different?
Conventional fog nozzles have a fixed or selectable gpm setting. These settings correspond to a particular discharge orifice, or tip size. In order for a conventional nozzle with a fixed opening to operate at the correct nozzle pressure of 100 psi, the proper gpm flow must be supplied. For example, a selectable gallonage nozzle with settings of 30, 60, 95 and 125 gpm will only deliver those flows of 100 psi of nozzle pressure.
There are two possible results when the conventional nozzle is not supplied with the rated or selected flow. First, inadequate flow provides a weak, ineffective stream that fails to reach the seat of the fire. Second, too much water flow creates excessive nozzle pressure making the hose line more difficult to handle and potentially jeopardizing the safety of the nozzle crew.
With an automatic nozzle, the discharge orifice continually adjusts depending on the flow to the nozzle. This sets the flow being supplied to the proper nozzle pressure and correct velocity for maximum extinguishing capability.
2. How does an automatic nozzle work?
The automatic nozzle uses a principle very similar to that of a pumper relief valve. A highly dependable spring, connected to the baffle that forms the discharge orifice, is balanced against the water pressure in the nozzle.
The pressure-control spring senses any increase or decrease in pressure within the nozzle. It then moves the baffle in or out to maintain a particular tip size necessary to keep the nozzle pressure at 100 psi. In effect, the nozzle is constantly changing tip size to match the water being supplied at that moment.
3. What pressure should be pumped to automatic nozzles?
Automatic nozzles greatly simplify pump operation. Since automatic nozzles are designed to operate at 100 psi nozzle pressure, this becomes the minimum starting point for any operation.
The basic formula for calculating pump discharge pressure is PDP = NP + TPL — PDP is the pump discharge pressure, NP is the nozzle pressure and TPL is the total pressure loss (that’s hose line friction loss plus apparatus friction loss plus elevation pressure).
With an automatic, the nozzle pressure will remain constant and the formula can be rewritten as: PDP = 100 + TPL. So, for a 200-foot pre-connected 1 3/4-inch hose, what pump pressure will be required to flow 150 gpm? Friction loss in 1 3/4-inch hose for 150 gpm is about 28 psi per 100 feet of hose.
The answer: PDP = 100 + (2 x 28); PDP = 100 + 56; PDP = 156.
To flow 150 gpm in this scenario, a pump discharge pressure of 156 psi is required. The required pump pressure will vary depending on the friction loss produced, the amount of flow desired, and the length and size of the hose lay.
The advantage of using an automatic nozzle is that any flow can be delivered by the pump operator and still be controlled by the nozzle operator. Variable flow, constant nozzle pressure, and nozzleman flow control are three essential elements to successful fire streams and fire attack.
4. Can an automatic nozzle be used with foam and foam eductors?
If the eductor manufacturer’s recommendations for inlet pressure, maximum hose length and size are followed, the automatic nozzle will adjust itself automatically to the rating of the eductor. With any eductor system, the nozzle valve must be fully open to prevent excessive back pressure on the eductor, which will prevent foam concentrate pickup.
Certain guidelines, however, must be followed. Foam-making is simply the addition of a proper amount of foam concentrate to water. This solution of foam concentrate and water is then mixed with air (aeration) either at the nozzle with aspirating attachments or as the stream pulls air along with it in a non-aspirating application.
5. What are the trade-offs of low-pressure nozzles?
Reducing nozzle pressure does account for some reduction in nozzle reaction. But how much reduction in pressure is required to get a significant reduction in reaction? And while reduced reaction may be a positive aspect, what are the negative aspects of choosing a low-pressure nozzle delivery system?
Those advocating for reducing the fog nozzle pressure typically would reduce the required nozzle pressure downward from 100 psi to 75 psi. If the flow is kept constant, the reaction reduction from a 25 percent cut in nozzle pressure is 13 percent. It works out this way: a 200 gpm stream at 100 psi has 101 pounds of reaction; cutting the nozzle pressure to 75 psi reduces the reaction to 88 pounds.
Nozzle pressure is directly related to the velocity of the stream. For the given example, instead of a stream speeding through the fire’s super-heated gases at 80 mph, it goes through at 60 mph. Which one goes farther? Which splashes more when it hits? Which bores through the wood char to get to deep-seated heat?
Make a comparison to the trend of law enforcement in the United States. As the threat to police officers goes up, are they going to smaller guns with fewer bullets in the clip? Are they taking out the bullets and pouring out half the powder? If they cut out half the powder the gun would kick less, be easier to aim, and it wouldn’t hit as hard. Is that what the police want for their weapons?
Fire departments need to ask if this is really what they want for their primary weapon.
Or consider an example that comes closer to home: how many people wish for a shower with less pressure and how many would think that a shower with less pressure does a better job of getting the soap off?
This article, originally published on Oct. 28, 2015, has been updated.
