Understanding Your Nozzle Reaction

As a firefighter, understanding nozzle reaction is a simple way to take more control over nozzle choice and operation. In your day-to-day, you contend with nozzle reaction and use it to your advantage or struggle to get past it. So, how can you better understand it to be more in control?

In this article we’re going to examine nozzle reaction, why you should really care about it, look at the things that affect it, and study a couple of misconceptions that permeate the fire service.

Why Should You Care About Nozzle Reaction?

Before we dive into the factors that impact nozzle reaction and how to calculate it, let’s spend a few minutes talking about why you should care about nozzle reaction.

Think about this: Do you want to fight the fire, or your nozzle reaction?

Nozzle reaction is always directly opposite of the stream, so understanding the amount of force you need to manage is important. The amount of nozzle reaction you and your crew must manage directly impacts how mobile you can be. Think about the last time you managed a line. How much force was pushing back against you? How much effort did you put into controlling the nozzle and directing the water?

Even an endless supply of water is no good if you can’t direct the stream to where it needs to go.

Understanding your nozzle reaction can help you plan and minimize the reaction you need to fight. This can help you be more mobile, better advance your lines, and minimize your fatigue on the fire ground.

Safety

Nozzle reaction also brings safety concerns front and center. If you are struggling with a high amount of nozzle reaction, you may hunch your body inward and use it to hold the nozzle in an unnatural or uncomfortable position. Over time, this can cause added strain and fatigue in your muscles and lead to injury.

In the worst-case scenario, you may lose control of the nozzle. Imagine the consequences if you lose control of the nozzle inside a structure with limited visibility. You or your crew could be injured in a preventable accident.

So, how do you get a better understanding of nozzle reaction? Let’s explore.

What is Nozzle Reaction?

Nozzle reaction is the amount of force directed back to the person or device holding a nozzle due to the volume and speed of water exiting that nozzle.

Nozzles are intended to create velocity in your water stream to project your stream the required distance. This builds pressure. We know from physics that any action has a reaction. In this case, the reaction to water building pressure and exiting the nozzle at a higher speed is the nozzle reaction that pushes back on the operator.

Nozzle reaction is a function of flow and pressure. This means that the two factors that most directly affect your nozzle reaction are the mass of water leaving your nozzle and the velocity or pressure of it exiting the nozzle.

Flow and Nozzle Reaction

One of the variables in understanding your nozzle reaction is flow. Flow can affect your nozzle reaction differently depending on the type of nozzle you are using. In either case, you should know that as you increase your flow, you will increase nozzle reaction, it is just a matter of how much that reaction increases.

When you use a fixed flow nozzle, an increase in flow means an increase in nozzle pressure. Your nozzle reaction here is proportional to the increase in pressure. Typically, with a fixed flow nozzle, if you double your flow, you will quadruple your exit pressure.

With an automatic nozzle, or a nozzle that has fixed flow with pressure release (like the TFT Working Fire nozzle), your nozzle reaction is proportional to your flow rate increase.

Nozzle Pressure and Nozzle Reaction

The other variable that directly affects nozzle reaction is nozzle pressure. An increase in nozzle pressure is the direct result of an increase in flow. As we know, flow builds pressure.

In your fixed flow nozzle, any increase in nozzle pressure will create an increase in nozzle reaction.

With an automatic nozzle, it is difficult to increase pressure because the baffle moves to release excess pressure. Because of this, with an automatic nozzle the nozzle reaction is proportional to the flow rate instead of nozzle pressure.

Nozzle Reaction Formulas and Equations

Luckily, since understanding nozzle reaction is so important, there are formulas to help you calculate your nozzle reaction.

Looking for other hydraulic formulas? Visit our Hydraulic Calculations for Firefighting article!

Smooth Bore Nozzle Reaction Formula

The smooth bore nozzle reaction formula lets you accurately calculate the nozzle reaction you will experience for the given exit pressure of a smooth bore.

NR = 1.57 X D2 X NP

D: Diameter of the nozzle opening in inches
NP: Nozzle exit pressure measured by a handheld pitot gauge in PSI
NR: Nozzle reaction in pounds of force

Combination Nozzle Reaction Formula

The combination nozzle, often called a fog nozzle due to its ability to achieve both a straight stream and fog pattern, has a separate formula for calculating nozzle reaction. Remember, this formula gives you a high estimate because it uses nozzle inlet pressure and does not account for friction loss.

NR = .0505 X GPM X √NP

NR: Nozzle reaction in pounds of force
NP: Nozzle inlet pressure in PSI

A common rule of thumb for nozzle reaction in a combination nozzle is that the nozzle reaction at 100 PSI is half the flow in pounds. This is not a reliable method for figuring out your nozzle reaction, though.

Common Nozzle Reaction Misconceptions

As we’ve worked through demonstrations and talked with firefighters around the world, we’ve noticed a couple of common misconceptions about nozzle reaction. Let’s address two right here:

The Combination Nozzle Reaction Formula is Your Actual Nozzle Reaction

We’ve been trained our entire life to trust mathematical formulas, and for the most part, we should. Unfortunately, when it comes to the combination nozzle reaction formula, what you get may not be completely accurate.

In many cases the formula is going to give you a high estimate or worst-case scenario since you are using the nozzle inlet pressure rather than the nozzle exit pressure in the formula. This may seem like a small distinction, but friction loss and other factors can make a big difference.

Lowering Nozzle Reaction Makes the Job Easier

It isn’t uncommon to hear fire departments wanting to lower their nozzle reaction to make the job easier. And they aren’t completely wrong! As we discussed above, having a high nozzle reaction can add to fatigue, muscle strain, and impact mobility on the fire ground. However, to lower your nozzle reaction, you are lowering your nozzle pressure, and this can come with its own set of complications.

Lower nozzle pressure can affect how firm your hose line is, which affects how maneuverable it is. Imagine having a nozzle pressure low enough that your hose line becomes soft. That line can be difficult to manage due to the increased potential for kinks. In these cases, it may be better to have a little bit more nozzle pressure to ensure the hose line is as firm as needed.

This also highlights the importance of hose and nozzle pairings. If your hose is not intended for use with lower pressure nozzles, it may kink more easily.

Conclusion

As a firefighter, it can benefit you to understand your nozzle reaction and the factors that affect it. Getting a strong grasp of the relevant formulas and the relationship between flow, pressure, and nozzle reaction can set you up to make the best choices for your department and minimize the unintended consequences of unmanageable nozzle reaction.