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  1. #1
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    Default Nozzle Tip Relationship to Hose Diameter Rule

    The nozzle tip may not be more than 1/2 the diameter of the hoseline. This is an old rule of thumb I have heard many times. I have looked through 3 of my books today and cannot find the rationale that explains this. I am assuming it has something to do with the amount of backpressure and formation of the stream, but I am looking for the formal/hydraulic explanation. Also, I am questioning if velocity has something to do with this, but I am having trouble putting it all together. Help?


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    Quote Originally Posted by MG3610 View Post
    The nozzle tip may not be more than 1/2 the diameter of the hoseline. This is an old rule of thumb I have heard many times. I have looked through 3 of my books today and cannot find the rationale that explains this. I am assuming it has something to do with the amount of backpressure and formation of the stream, but I am looking for the formal/hydraulic explanation. Also, I am questioning if velocity has something to do with this, but I am having trouble putting it all together. Help?
    The only thing I can come up with is in Fornell's book, and it seems to be as you note, the larger orifice has less back pressure at the same discharge pressures/target flow resulting in "limp" hose which may kink. He notes that generally you can go one size above the half the diameter though with little change.

    I'm betting KuhShise has the answer.

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    Forum Member FWDbuff's Avatar
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    I heard this same one myself from an old timer- When I asked what the rationale behind it was, or for a source, he shrugged and said "that's the way we've always done it." Lacking a source, I merely passed it off as "old fart thats the way we've always done it" crap.
    "Loyalty Above all Else. Except Honor."

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    Forum Member Rescue101's Avatar
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    Oh lay off the Old Fart Schit. When your pup azz gets in a jam,where do you turn? Oh,Ah,Lessee.....YES I have it! An OLD FART with the ANSWER and a CORRECT one at that. Fookin' amazing,isn't it? hehe T.C.

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    Quote Originally Posted by MG3610 View Post
    The nozzle tip may not be more than 1/2 the diameter of the hoseline. This is an old rule of thumb I have heard many times. I have looked through 3 of my books today and cannot find the rationale that explains this. I am assuming it has something to do with the amount of backpressure and formation of the stream, but I am looking for the formal/hydraulic explanation. Also, I am questioning if velocity has something to do with this, but I am having trouble putting it all together. Help?
    I haven't heard that one before, but it does make sense. In my mind it would refer to the relationship between what a nozzle tip of that size flows and the friction loss of a line that size flowing that amount of water. And since friction loss comes from velocity, your right on track.

    For example: A 1-1/4" tip at 50 psi nozzle pressure would flow approx. 328 gpm. The friction loss in a 2-1/2" line supplying it would be about 25 psi/100'. That's right at ratio of the tip being 1/2 the size of the hose line. Easily done, even at 400' or 500'. But do the same flow with a 2" line and look at what happens. Now you're at over 65 psi/100'. Still doable with a short line, but look what would happen with a 300' line. You'd need in excess of 245 psi at the point that the hoseline connects to the discharge port to provide it.

    If you're working from a standard 1-1/2" preconnect discharge, after you get done factoring in the internal friction loss behind the pump panel, you're into some pretty high numbers.

    So while that ratio might be some old billy goat's rule, it's got good science behind it. Probably not taught in any fire school or in any book, because it's waaay too simple.

    Second opinion and additional insight, Kuh?
    Last edited by chiefengineer11; 01-01-2010 at 12:25 PM.

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    I've always been taught this also and it's in our department pump manual but
    not crystal clear. I liken it to holding an open butt; Not enough back pressure to produce an effective stream and causes the hose near the end to kink and become unmanageable like RFDACM02 said.

    I dusted off an old book and this is from page 306 of "Fire Service Hydraulics," originally printed in 1941.

    "As a general rule, nozzle diameter should not exceed one-half the diameter of the hose used and should be reduced depending upon the length of the stretch. A short stretch is considered to be about six lengths; a medium stretch about 12 lengths; and a long stretch, 18 lengths or over.
    A nozzle which is too large for a particular diameter hose gives a weak ineffective stream. A large nozzle results in a large flow of water and a large flow of water produces large friction loss. With a long hose stretch and a large flow of water, the nozzle pressure weak because of excessive friction loss. In such case, an increase of nozzle pressure requires the use of a smaller nozzle, assuming engine pressure, hose diameter and length of stretch remain constant. Since the velocity of flow remains the same in the hose, the water must flow faster through the smaller nozzle, thereby increasing pressure and reach at the tip.
    If the nozzle is too small, and the engine pressure high, the stream will form a spray pattern on leaving the nozzle and its effectiveness is lost.

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    Default Practical Limits

    Wow! Guys, you are right on track. I put this together off line, and when I got around to looking at the thread this morning, Chief Engineer and R1Alum were right on target. Same rule of thumb appears in the IFSTA Fire Stream Manual and Hydraulics for Firemen.

    Sometimes when examining a system, it is most useful to investigate the limits to a ridiculous level. Such is the case with your question concerning practical limits of nozzle delivery compared with hose diameter. If we begin with increasing hose diameter, we rapidly reach a point where it is impractical to attempt to put a 1” nozzle on the end of a 5” preconnect. You simply could not move the hose once it was charged. On the other end of the spectrum is a 1” hose line with a 1” nozzle, or even more deviant is a large bore nozzle on a very small line. (3/4” booster hose with a 2” master stream)
    For most questions concerning fire streams, it is best to start at the fire and work your way backwards toward the engine and water supply. At every step in the process, one must decide what are the outside forces acting on the system and what are the practical alternatives that can allow you to reach the objective of fire extinguishment. Now that we have covered the B.S. portion, here is the scoop.
    Because the water volume (gpm) must extinguish the fire, there is a practical minimum that can be used. It also needs to reach the seat of the fire despite the effects of gravity. This means that a certain velocity is required before the stream is pulled back down to the ground or floor. Since the effect of gravity is a constant downward acceleration of 32 ft/sec/sec, the water will drop 16 feet in the first second, or after 2 seconds will fall 64 feet. (Average velocity times time) Working inside of a building, and angling the nozzle toward the ceiling, it becomes apparent that a water velocity over 2 times the effect of gravity is probably needed to reach the seat of a fire at a reasonable distance from the nozzleman. If you do the numbers for a 1” tip at 50 psi. (0.7857 sq in) the velocity turns out to be about 86 ft./sec. Pretty close to our seat of the pants guess.
    Say we want to have this water velocity (reach), but decide to use a 1” hose line and a 1” nozzle. The required velocity is 211 gpm, but 211 gpm through 1” booster from friction loss equations will need a little over 2,000 psi to stuff this amount of water through just 100 feet of booster line. Thus we begin to understand the practicality of the nozzle being about ½ the diameter of the hose line. A 1” nozzle on 100 ft. of 2” hose has a friction loss of about 33 psi. Much better than the 2,000 psi loss with booster line.

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    Default Friction loss vs tip size

    1.5" = .75 or 3/4" 37.5 FL .875 or 7/8" Effective 54.0 FL
    1.75"= .875 or 7/8" 34.8 FL .975 or 15/16" Effective 53.0 FL
    2.0" = 1.0 or 1" 32 FL 1.125 or 1 1/8" Effective 50.0 FL
    2.5" = 1.25 or 1 1/4" 21.1 FL 1.125 or 1 1/8" 14.0 FL


    These are all calculated friction loss per 100' foot for each size of common hose. I did not include 3" as I don't feel it should be included as an attack hose. Just something to think about as there is not a direct linear relationship as you jump up in size between hose and tips size but it is pretty close until you get to 2.5". I also included the next tip size up as discussed previously. I included 1 1/8" as it is commonly used for 2.5" hose. Food for thought.
    Last edited by Truck_3; 01-03-2010 at 01:25 AM.

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    Default

    please consider that the use of a 15/16ths does not necessarily break this rule.

    (look at it that 1/2 of 1.75 is 7/8ths) the modern 1.75" hose is usually overbuilt to the i.d. of 1 and 7/8ths or nearly 2"'s to cut down on the FL numbers per the mfg. roughly 40 psi fl per 100' is not over taxing on the pump or hose even at 300 ft.

    if you have a hale pump fl chart you'll notice they exclude 1.75 hose due to the varied FL calcs. (IMO)
    Originally Posted by madden01
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    Quote Originally Posted by Rescue101 View Post
    Oh lay off the Old Fart Schit. When your pup azz gets in a jam,where do you turn? Oh,Ah,Lessee.....YES I have it! An OLD FART with the ANSWER and a CORRECT one at that. Fookin' amazing,isn't it? hehe T.C.
    Relax elderly gentleman, I didn't say I wouldn't use it to my advantage, I merely said that since the quoter could not cite a credible source that "I would pass it off as old fart crap." Doesn't mean it doesn't work!
    "Loyalty Above all Else. Except Honor."

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    Um, well, okay then I won't tell you that we have been using both 1 1/8" and 1 1/4" smothbores on our 2 inch hoselines for about 10 years now.

    We underpump the 1 1/4 inch to flow 300 gpm. The only "problem" we have seen in using the slugtip is some loss of cohesivenes of the stream past about 50 feet. Other wise it has worked well for us.
    “The person who risks nothing, does nothing, has nothing, is nothing, and becomes nothing. He may avoid suffering and sorrow, but he simply cannot learn and feel and change and grow and love and live.” Leo F. Buscaglia

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    Cool

    It's all about the spritzer pressure. You want a tip size that will produce around 80 - 100 psi back pressure to get a stream with decent reach. If the tip is to large, you may get a lot more flow but if it is at 30 psi, will not do you much good as far as reach of the stream.

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