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How do automatic fire hose nozzles work?

  1. Jun 18, 2010 #1
    How do "automatic" fire hose nozzles work?

    I'm hoping someone can explain to me how "automatic" fire hose nozzles work. As in TFTs (Task Force Tips). I have a background in physics and since becoming a volunteer firefighter for my own interest have been working through some of the physics behind firefighting. I am happy with fixed-diameter nozzles, which I understand as a device that converts (static) pressure to velocity (dynamic pressure) -- Bernoulli's principle. But these selectable-flow-rate nozzles baffle me (pun intended). TFT have long descriptive article:


    but it doesn't make much sense to me. It talks as if pumps are constant flow devices, not constant pressure devices (as operators usually regulate them to be). It also talks about producing a constant "nozzle pressure" of 100 psi and seems to indicate that this is the pressure of the ejected stream (Fig 13) when I would have thought the stream's static pressure was atmospheric (0 gauge) and neglecting friction loss the difference between this and pressure immediately upstream of the nozzle is equal to the constant pump pressure.

    So, is the above document very confused, or am I, or both?

    Is the nozzle a lossy device, that simply dissipates power through turbulence around a sharp obstruction (Fig 12) so as to uphold the promise of a "constant nozzle pressure"? If so, this doesn't really live up to the hype of providing the best stream possible for the available water.

    Or is it a conservative device, that regulates flow past a nearly-lossless spring-loaded obstruction (fig 16a)? If so, how does that work out-- what is the relationship between pressure and flow rate? I am struggling with ram pressure vs static pressure and the effects of angled faces and pressure gradients.

    Hoping someone can help :).
  2. jcsd
  3. Jun 19, 2010 #2
    Re: How do "automatic" fire hose nozzles work?

    FWIW I have nutted out half of the answer.

    They do maintain a constant pressure across the nozzle, by adjusting the outlet effective aperture and relying on upstream friction losses to reduce pressure as flow rate increases. Effectively it maximises flow rate for the system and pump pressure given the constraint of 700 kPa nozzle pressure.

    That's when the flow selector is a full throttle. I haven't yet worked out how the flow selector works or whether it really does ensure a constant flow.
  4. Jun 19, 2010 #3
    Re: How do "automatic" fire hose nozzles work?

    OK, I *think* the other half of the story is the flow control throttle.

    What I'm wondering about is the claim that the throttle slide valve "reduces flow" (drops pressure) without creating turbulence.

    How is this actually achieved at the particle level? The lost energy must go into the internal energy of the water by raising its temperature. This occurs by giving particles random kinetic energy. If there is any spatial coherence of the velocity field then you have turbulence. Likewise if particles lose forward kinetic energy to the valve walls, again surely this would happen with some spatial coherence, inducing eddies and so forth. Or is it the case that turbulence will be occurred but it stabilises over a short path length and is not evident some distance downstream of the throttle?
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