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GPM Calculator
- Thread starter retire09
- Start date
Dr. J
Silver Member
You mean like how much would flow out of a pipe if the end was cut off? That would essentially be a pressure loss of 60 psi/100 ft or 138 ft/100 ft. There is some loss at the sharp opening, so lets call it 125 ft/100 ft. My pipe sizing chart says 1 1/4" pipe at 125 ft/100 ft does 95 GPM. Pipe velocity is 20.4 fps.
Now, if you have a non-leaking pipe that is measured at 60 psi, and then cut of the end 100 ft away from the gauge, would 95 gpm come out the end? Probably not. You would need to know what is upstream of the pressure gauge. There are probably other pipes and components that were sized much less than 95 gpm that would drop the pressure lower than 60 psi at the gauge location when the unlucky pipefitter started hacking away. If the 100 ft pipe started at the bottom of a 138 ft tall reservoir dam, then you have all the pressure in the world, and yes, 95-ish gpm would come out. But if the water has to go through a meter, backflow preventer, another 400 feet of pipe and elbows, etc, before it even got to the 100 ft pipe of concern, then the upstream components would restrict the flow to much less than 95 gpm.
One concept to think about is a fire flow test. There is a static pressure measured with minimal/no flow, then a residual pressure with a hydrant open. In a highly restrictive water system, there is a large drop off between the static and residual pressures, and the resultant flow out of the flowing hydrant's 6" opening is relatively low. In an unrestricted water system, there is minimal residual pressure drop, and a buttload of water comes out the same size opening.
Now, if you have a non-leaking pipe that is measured at 60 psi, and then cut of the end 100 ft away from the gauge, would 95 gpm come out the end? Probably not. You would need to know what is upstream of the pressure gauge. There are probably other pipes and components that were sized much less than 95 gpm that would drop the pressure lower than 60 psi at the gauge location when the unlucky pipefitter started hacking away. If the 100 ft pipe started at the bottom of a 138 ft tall reservoir dam, then you have all the pressure in the world, and yes, 95-ish gpm would come out. But if the water has to go through a meter, backflow preventer, another 400 feet of pipe and elbows, etc, before it even got to the 100 ft pipe of concern, then the upstream components would restrict the flow to much less than 95 gpm.
One concept to think about is a fire flow test. There is a static pressure measured with minimal/no flow, then a residual pressure with a hydrant open. In a highly restrictive water system, there is a large drop off between the static and residual pressures, and the resultant flow out of the flowing hydrant's 6" opening is relatively low. In an unrestricted water system, there is minimal residual pressure drop, and a buttload of water comes out the same size opening.