Thread: Water flow question?

12172001, 09:50 AM #1
 Join Date
 Mar 2001
 Location
 NF / Canada
 Posts
 103
Water flow question?
If you have 500 gpm flowing from a hydrant and you run 500 feet of 2.5 line. what would be the flow at the end of the line?
Now, everything is the same but 5 inch line?
Is this enough info to figure it out?
[ 12172001: Message edited by: GPM ]

12172001, 11:15 AM #2
 Join Date
 Nov 2001
 Location
 Southeast
 Posts
 110
Good question!
500 gpm out of a 2 1/2" line requires 9 psi outlet pressure (500/167)
So you have 9 psi to work with.
Flow out of 500 feet of 2 1/2" hose = 9 (psi) / 5 (500') x 2 = get the square root / 2 .25 x 100 = 70 gpm or 14% of what was available. That is reversing the formula FL = 2Q squared times 2.
500 feet of 5" hose = 9 (psi hydrant pressure) x 52 (the constant for 5 inch hose / 5 (500 feet) x 2 = square root / 2  .25 x 100 = 500 gpm. 100% of the ability of the hydrant.
However, in the above example you don't need 9 psi out of the 4 1/2" steamer connection to move 500 gpm, you only need 0.8 psi, so you'd get 649 gpm or 130% of the hydrants ability and 9.4 times more water than a 2 1/2" line.
[ 12172001: Message edited by: the7tower ]

12172001, 11:39 AM #3
Nope.
*Must know:*
 Pressure at the hydrant when discharging 500gpm
 Pressure desired at the end
(Traditionally, 100psi for fog nozzles, 80psi for solid bore master streams, 20psi for delivery to a pump)
*For best accuracy, should know:*
 Make & Model of the hose your using
(Yes, friction loss varies dramatically from the published charts...they generally significantly underestimate what modern, quality fire hose can deliver)
How your hydrants perform. Maybe the pressure flowing 500gpm is the maximum pressure you see, since your next to a 500gpm booster pump on the system. But maybe as the flow reduces, the PSI goes up  this can affect especially the calculations for the 2.5" hose!
Also, if we where looking to determine if more GPMs where available, we'd also need the Residual Hydrant Pressure, typically taken from the next hydrant down the road, which run through a calculation helps estimate that. Or we just hook up the 5" and see how much we can actually move
MattIACOJ Canine Officer
20/50

12172001, 01:01 PM #4
 Join Date
 Nov 2001
 Location
 Southeast
 Posts
 110
500 gpm flowing is not a function of discharge pressure that can be calculated?
...*Must know:*
No other way?
...... Pressure at the hydrant when discharging 500gpm
It can only be 9 psi!
Course we could say the residual is, 25, 50, 80, certainly not 250, 1000, or 10,000 psi. The ratio would stay unchanged.
.... Pressure desired at the end
His question seemed to indicate an open butt flow out of 500 feet of 2 1/2" and 5 inch hose, pretty easy to calculate isn't it? He said, "Now, everything is the same..." So who cares about variables, calculate one way and apply it evenly.
Now may be you read it as a hose boost or hose line. So, let's do that. Pumper at the hydrant 9 psi and 500 gpm coming in pumping 500 feet with 2 1/2" hose you'll get all 500 gpm if you pump 275 psi to the next rig or pump 25 to 35 psi with the 5 inch. Or add 40 to 100 psi to either PDP to get the flow out of a nozzle at the end.
When you are starting with 9 psi, you better not have much of an expectation on pressure out the end of a forward lay.
Any pressure at the end would apply across the board to both sizes of hose so, the ratio would remain unchanged.
>>>>(Traditionally, 100psi for fog nozzles, 80psi for solid bore master streams, 20psi for delivery to a pump)
If any of those numbers are used the flow is Zero if a pumper is not on the hydrant or a relay is not used.
0 to 250 psi residual could be used but the author clearly stated: "Now, everything is the same... " So any additions or changes don't effect the end number.
.....*For best accuracy, should know:*
 Make & Model of the hose your using
(Yes, friction loss varies dramatically from the published charts...they generally significantly underestimate what modern, quality fire hose can deliver)
Some charts are simply made up too. So you'll need to go out and conduct a flow test off the hydrant and pumper you intend to use. If you have threaded hose it will perform much better than storz.
.....How your hydrants perform. Maybe the pressure flowing 500gpm is the maximum pressure you see, since your next to a 500gpm booster pump on the system. But maybe as the flow reduces, the PSI goes up  this can affect especially the calculations for the 2.5" hose!
Lets say the pressure goes up 400% for the 2 1/2" line, the flow would only change by 69 gpm. Of course we know a hydrant flowing 500 gpm at 9 psi, is off a water line of a certain size, we can use the 10 fps AWWA rule and calculate, the main size, the loss in the main and be right on, too. AWWA states 60 to 80 psi is the balance point in water system design. So if this hydrant dropped to 9 psi, it is obvious the diameter of the main or the length of the dead end.
We know for a fact the pressure for the steamer port goes up 188% minimum due to the difference in loss in the port. So, what you end up with is what the author asked: "Now, everything is the same "
,,,,,Also, if we where looking to determine if more GPMs where available,
THERE AREN'T GOING TO BE ANY MORE FOR THE 2 1/2" LINE. There will be at most a 30% increase as demonstrated for the 5" line, assuming a complaint ldh steamer.
I think I'll stand by my calculations based upon the data supplied and the question asked especially in lieu of "Now, everything is the same... ". If it becomes a relay, or boost, then anything to a point is possible. I just didn't try to read anything extra into the questions.
The question was difference, whether it is 9.4 times or 4.7 times, the 2 1/2" flow is essentially unusable.
If the intent is to split decimals, then we need to know what discharge you'll use, are you using an intake valve, a 4 way, where the pressure gauges is plumbed, the loss in the plumbing the, diameter of your discharges, the type and style of adapter off the hydrant, the last time your gauges were calibrated, the altitude of the fire hydrants, the time of day, size of the pump, level of your water tanks and whether any booster pumps are on, of course all this stuff is a bunch of bull. In the whole scheme of things the answer is the answer. If any of these really mattered they'd be in all the books as essentials.
I think you provided plenty of data, certainly enough to answer your question.
The original question was quite clear: "Now, everything is the same"

12172001, 02:08 PM #5
 Join Date
 Mar 2001
 Location
 NF / Canada
 Posts
 103
The reason I asked was, that I was sitting down with my water works dept head. There is a section in town that he says he can only give the FD 500 gpm more flow above "normal demand" if needed.( not much!!)
He could not nail down at what pressure because it depends where you are in the system.
then in talking about 2.5" versus 5" I went to show him on paper what the difference would be baised on 500 gpm alone, to show the advantage of 5"... Got stuck there.

12172001, 02:39 PM #6
Well, I'll admit I worded my first reply very poorly in regards to residual pressure.
But Larry, are you saying residual pressure can be disregarded in the question?
Yes, 9psi is the pressure needed to deliver 500gpm through a 2.5" orifice.
But can't hydrant systems, depending on their design, have more pressure available while flows are still limited? Specifically I'm thinking of wellfed systems with no aboveground storage. You're limited to the (water system) pump capacity, where you may have only 500gpm available, but an extra XX amount of pressure left to use to overcome friction.IACOJ Canine Officer
20/50

12172001, 04:22 PM #7
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 Nov 2001
 Location
 Southeast
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 110
But can't hydrant systems, depending on their design, have more pressure available while flows are still limited?
Sure, but how much more can they have and hit 9 psi flow pressure? The residual in this example would be irrelevant. The water guy was quite clear only 500 gpm is left.
The math:
Say: 100 psi static, flow 500 gpm, residual 20 psi however that could be possible.
100  20 = 80 or 10.66
100  50 = 50 or 8.27
10.66 / 8.27 = 1.29% or the 5" will flow 649 gpm. It won't help the 2 1/2" flow more.
But how far will the 2 1/2" will flow 140 gpm?
100  20 = 80 10.66
100  5 = 95 11.69
= 92% or flow will be 460 at 95 psi residual so: It could flow 289 gpm. Good luck finding a 9 psi flow and a 100 psi residual though.
Nice theory but in this case, not likely.

12232001, 06:21 PM #8
 Join Date
 Jun 2001
 Location
 ST.PETERSBURG,FL,USA
 Posts
 202
Both you guys really know your stuff, I'm impressed, but to answer this question I would send this gentleman to the IFSTA water supplies redbook. This details how to do the flow tests and gives him some tables to wok with. It should also be pointed out that if you test a hydrant you may also want to test another hydrant on that main (at the same time)just to make sure of the residual. Dead end hydrants can look good but flow real bad. You also nedd to determine if the flow will be augmented by pumps at some point, and what the pressure will be at that time. IFSTA goes into all of this in great detail, and I think you both bring up great points, Remind me not to argue with you guys.
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