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Thread: Drafting capacity question

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    Default Drafting capacity question

    We did some drafting training yesterday and we came up with a weird result. When drafting off of a particular dry hydrant, we were flowing around 750gpm out of the deck gun with a 2" tip. We maxed out the pump (cavitation) at around 50psi at the pump and 40 at the tip. When we switched to a smaller tip (due to limited reach), we not only got better reach, but we calculated we were flowing around 920gpm. Our pump maxed out at 70psi for that evolution. We're using a single stage 1500 gpm pump with about a 12' head.
    Would having a smaller tip result in some backpressure that actually helped deliver more flow?? I just thought it was kinda odd to get that much more flow from a smaller tip.
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    Did you have the chance to repeat the experiment? About the only thing I can think of right now is that you may have had some air/debris blockage somewhere within the plumbing of that dry hydrant? If you could try the same thing over, I am curious if you would now experience the better flow? Other than that I do not have a clue. HB of CJ (old coot)

    Also....could it have been an air or physical blockage/leak somewhere inside the pump or gates? Dunno again. Sometimes down tipping helps with velocity....thus appliance range, but you have me stumped. Not the first time.

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    What was the size of the smaller tip

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    Isn't head pressure positive pressure? Shouldn't that be "12' of lift"? And I may not have a clue what I'm talking about.

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    JohnSB: I'm guessing that you reduced your tip from 2" to 1 3/4" and found the pitot pressure to be 60 psi. If this is the case, the person using the look-up table simply took the 2" column reading instead of the 1 3/4" column. If you were using the 1 3/4" tip and had 60 psi pitot reading the flow was 709 gpm. There is one other thing that can affect the apparent readings on a master stream device. Some deck gun designs plumb the base pressure gauge into the side of the gun, but don't direct the opening toward the flow of water. This can act as a partial venturi, and lowers then reading of the gauge. When working high flows in pipe, there is a significant amount of "Velocity Head" energy in the flowing water that does NOT appear as "Pressure Head" at the base of the nozzle. Always try to use a Pitot with the pick-up tube in the flow coming from the nozzle. If you took the readings from the gauge on the gun, then I would suspect flows above 750 to be the case, because you were getting abnormally low pressure readings with the 2" tip. Either way I still think someone used the wrong look-up column.

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    Quote Originally Posted by kuh shise View Post
    JohnSB: I'm guessing that you reduced your tip from 2" to 1 3/4" and found the pitot pressure to be 60 psi. If this is the case, the person using the look-up table simply took the 2" column reading instead of the 1 3/4" column. If you were using the 1 3/4" tip and had 60 psi pitot reading the flow was 709 gpm. There is one other thing that can affect the apparent readings on a master stream device. Some deck gun designs plumb the base pressure gauge into the side of the gun, but don't direct the opening toward the flow of water. This can act as a partial venturi, and lowers then reading of the gauge. When working high flows in pipe, there is a significant amount of "Velocity Head" energy in the flowing water that does NOT appear as "Pressure Head" at the base of the nozzle. Always try to use a Pitot with the pick-up tube in the flow coming from the nozzle. If you took the readings from the gauge on the gun, then I would suspect flows above 750 to be the case, because you were getting abnormally low pressure readings with the 2" tip. Either way I still think someone used the wrong look-up column.
    Well, to be honest, when I think of who I was asking to give me the readings off of the gauge and the flow chart...Sometimes you just assume that people know which chart to use...
    We had just flushed the dry hydrant, so I know that wasn't an issue.

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    Back pressure doesn't affect the drafting capacity of a pump from a given source. Something went wrong wither in the data collected or the calculations. If you did truly achieve a higher flow rate with the second, smaller, tip, then it is because an intake side obstruction disappeared.

    The pump doesn't care whether it is flowing a given volume through one size tip at a higher pressure or another size tip at a lower pressure. The back pressure change due to the outlet restriction is what resulted in your higher pump discharge pressure, but it doesn't affect available capacity from the source.

    Drafting is all about friction loss & elevation loss. Every pump has a NPSH (Net Positive Suction Head) required to flow a given volume of water. That NPSH varies by pump design and can be calculated by the manufacturer. If the pump is rated at 1500 GPM with an NPSH of 4.0 psi that means that you have to have 4.0 psi of atmospheric pressure remaining to achieve the 1500 GPM flow rate for the pump.

    So, how do you tell how much atmospheric pressure is remaining? You calculate it. So, where I live the "normal" atmosperic pressure is about 14.3 psi. From that we subtract lift (.434 psi per foot, typically rounded to .5 psi), friction loss through any intake hose and dry hydrant plumbing, and loss for any valves or appliances. So, I know your lift was approximately 12 feet, that's about 5.2 psi of loss (6 if you round up). What I don't know about is how much hard sleeve you were using (.28 psi loss per 10' section at 750 GPM) and the dry hydrant you were using.

    Dry hydrants are problematic, usually because when people install them they do it on the cheap without an understanding of how they should be built. In many cases folks will run 6" PVC for the whole system. If you have a very short run (10' or so) that works OK, but in most cases you should really use at least 8" for the horizontal run if it's of any distance. At 1000 GPM 6" PVC has a friction loss of 3.1 psi per 100 ft, whereas 8" is only 0.8 psi at the same length and flow. If you have 100+ feet of pipe in the dry hydrant that adds up pretty quickly.

    Probably more than you wanted to know, but what good in the internet without educational material.
    Last edited by Johngagemn; 05-13-2014 at 06:33 PM.
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    I'm thinking it was an error in the data collection, ie: the dufus that was reading the chart on the wrong tip.

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    So what size was the second tip

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    Quote Originally Posted by johnsb View Post
    I'm thinking it was an error in the data collection, ie: the dufus that was reading the chart on the wrong tip.
    It happens to us all from time to time.
    Just a guy...

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    Disclaimer: The facts and opinions expressed above are mine, and mine alone, and are not intended to represent the views of any company I have ever worked for, past or present.

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    1 3/4" tip is what we had going the second round. Much better reach. But I'm also for the idea of using a larger tip with lower pressure to lob water into a building. So many times you see guys blasting away with a deck gun at high pressure and the water is just blowing right over the fire and onto the crews on the other side

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    johnsb: I'm not so sure that your analysis of "Lobbing" verses a good 80 to 100 psi nozzle pressure is correct. Many times you will see an aerial or a ground monitor dumping on the roof of a well involved building. This tactic makes for wonderful press photos, but is a very ineffective use of the available water. The roof continues to do its designed job of shedding water (rain or fire stream) and the fire continues to perform in the predicted manner consuming all combustibles in an upward direction until the roof falls into the fire. If possible an elevated stream should be directed through openings (windows, doors, or breached walls) and up onto the underside of the involved ceiling or roof. Even a one story shopping mall can benefit from clearing out the front windows and applying a heavy caliber stream from an aerial basket directing water from sidewalk level into the underside of the roof. Of course it goes without mentioning that NO, ZERO, NADA personnel in the building when directing large caliber streams into a structure. There are all sorts of things that can be done with a heavy caliber stream, like removing parapets or hanging cornice to make a safer condition for the eventual necessary entry by firefighters to do the final extinguishment.

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    Quote Originally Posted by kuh shise View Post
    johnsb: I'm not so sure that your analysis of "Lobbing" verses a good 80 to 100 psi nozzle pressure is correct. Many times you will see an aerial or a ground monitor dumping on the roof of a well involved building. This tactic makes for wonderful press photos, but is a very ineffective use of the available water. The roof continues to do its designed job of shedding water (rain or fire stream) and the fire continues to perform in the predicted manner consuming all combustibles in an upward direction until the roof falls into the fire. If possible an elevated stream should be directed through openings (windows, doors, or breached walls) and up onto the underside of the involved ceiling or roof. Even a one story shopping mall can benefit from clearing out the front windows and applying a heavy caliber stream from an aerial basket directing water from sidewalk level into the underside of the roof. Of course it goes without mentioning that NO, ZERO, NADA personnel in the building when directing large caliber streams into a structure. There are all sorts of things that can be done with a heavy caliber stream, like removing parapets or hanging cornice to make a safer condition for the eventual necessary entry by firefighters to do the final extinguishment.
    I'm talking about ground moniters, large handlines, and deck guns, not aerial devices. And of course, if the roof is still pretty much intact it will shed water. I'm specifically talking about when crews are using a low angle to hit fire coming through a roof, NOT putting it in from a window or wall opening. Many times they are just blasting a stream through the fire and smoke, not actually hitting what's burning. If you use lower pressure and a larger tip, at a higher angle, you get a broken stream with larger drops of water falling into the fire from above. A lot of times the nozzle being used is too close to the fire building, so breakover is actually occuring on the far side of the building and very little water is reaching the fire. This is typical when there is a building with a flat or low pitch roof. When the nozzle is too close, firefighters tend to aim at the visible fire, and the water actually goes right through the flame without actually hitting the burning material. By elevating the stream, lowering the pressure, and using a larger nozzle size, you can still deliver a large amount of water right where it's needed, instead of blasting the crew on the other side of the building.

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