As we mentioned in the previous article, the backward momentum of a fire hose is also known as nozzle reaction, which is a counterbalancing force against the waters forward action force.
Firefighters holding the fire hose lines can feel this fire nozzle reaction directly, and it causes 13,795 firefighting injuries on average every year, according to National Fire Protection Association (NFPA).
A firefighters water application rate can be limited or even reduced by such nozzle reactions while his or her air consumption rate increases, thus putting them in danger on the fire zone. Moreover, firefighters sometimes mistakenly believe that higher nozzle reactions also mean higher water application rates.
Firefighter nozzle reaction recently published in Fire Technology is the first paper to analyze nozzle reaction in terms of fluid mechanics. This interesting engineering mystery has been unsolved for more than a century.
Unfortunately, some of the assumptions made in this paper are found to be over simplistic that it fails in its attempt to solve the puzzle of how fire nozzle reaction works, namely that it ignores hose stiffness, ground/hose friction, fluid friction, gravity, etc.
Assuming that a steady, inviscid flow in a flexible hose is in frictionless contact with the ground, then nozzle reaction and hose tension can be analyzed in terms of fluid momentum conservation. A hose becomes straight when the hose tension is beyond normal due to anchor forces regardless of the bend angle.
The jet momentum flow rate will be equal to the nozzle reaction, which remains unchanged even when the hose and the nozzle are connected by an elbow. That means a firefighter holding the hose line must exert a forward force equivalent to the one exerted by the jet on a vertical wall.
Nozzle reaction force can be determined by three expressions based on such factors as hose/jet diameters, flow rate, and static pressure upstream of the nozzle. The fire service adopts the nozzle reaction predictions as 56% to 90% of those specified here in particular for firefighting hand lines.
Our main purpose is to share such information with the firefighting community to improve firefighters’ safety. Simply put, the nozzle reaction can be expressed as the following equation:
R = ρ Q 2 / A2
Where R represents the magnitude of the nozzle reaction, ρ is the water density. Then the nozzle reaction is proportional to the flow rate (Q) squared, divided by the nozzle discharge diameter (A2)
From this notion derived the NFPA requirements for spay nozzles suitable for the use of fire suppression. Therefore, the maximum nozzle reaction force that a firefighter can handle on his own can be predicted accurately.
And by increasing the flow rate and/or decreasing the nozzle diameter, a firefighter’s capability to handle the nozzle reaction force will be greatly increased.