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12-11-2009, 03:51 PM   #1
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## Need a Formula

How do I figure out gpm with pressure? How do I set this formula up?

12-11-2009, 04:18 PM   #2
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The simplest way is to check tables for pipe flow. The actual mathematics to compute the flow through a pipe or out of a nozzle is too complex to present in a post like this. If you wish to pursue the math angle, check on the web for "orifice flow" and "pipe flow". You will find that the flow through a pipe depends on the diameter of the pipe, roughness of the pipe, length of the pipe, and head loss through the pipe. Computation of head loss is the most difficult, requiring careful mathematical computation, unless of course you use tables.

12-11-2009, 07:18 PM   #3
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Quote:
 Originally Posted by Daniel Holzman The simplest way is to check tables for pipe flow. The actual mathematics to compute the flow through a pipe or out of a nozzle is too complex to present in a post like this. If you wish to pursue the math angle, check on the web for "orifice flow" and "pipe flow". You will find that the flow through a pipe depends on the diameter of the pipe, roughness of the pipe, length of the pipe, and head loss through the pipe. Computation of head loss is the most difficult, requiring careful mathematical computation, unless of course you use tables.
I know that their is 231 cu in. = 1 gallon, and 1/2 inch pipe would take 98' to make a gallon. I know that equation but, how do I figure Pressure say 40psi. applied, so what does that make my rate of flow in GPM? thats the formula I need. thanks

 12-12-2009, 02:57 AM #4 Member   Join Date: May 2007 Posts: 413 Rewards Points: 370 Bucket test. put 1 gallon bucket under pipe, turn on valve. Time until bucket is full
12-12-2009, 11:19 AM   #5
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Quote:
 Originally Posted by Livewire78 I know that their is 231 cu in. = 1 gallon, and 1/2 inch pipe would take 98' to make a gallon. I know that equation but, how do I figure Pressure say 40psi. applied, so what does that make my rate of flow in GPM? thats the formula I need. thanks
You need a friction loss chart for the type pipe or tubing you are using, and the type and number of fittings you are using. Each foot of pipe/tubing and each type of fitting has a pressure loss that must be calculated at whatever psi you are going to use. So there is no set formula.
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Gary Slusser
23 years in water treatment and well pumps, 13 years on the 'net helping others to help themselves.

 12-12-2009, 06:15 PM #6 Member   Join Date: May 2007 Posts: 413 Rewards Points: 370 Do search for Hazen-Williams. Why do you need this information. What is your project? You are not doing calcs for a fire sprinkler system, fire sprinklers do not use 1/2 pipe. Is this for a potable water supply? If so is this gravity flow or pressure fed?
 12-13-2009, 08:07 PM #7 Mold!! Let's kill it!   Join Date: Feb 2008 Location: Central Pennsylvania Posts: 2,849 Rewards Points: 2,012 A 1/2" bore will discharge 46 GPM at 40 PSI. The 40 PSI is measured at the discharge. How much pressure you need to apply at the source to reach 40 PSI discharge will depend on type of supply piping, length of run, number of fittings, plus or minus elevation, etc.
12-13-2009, 09:19 PM   #8
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If you must have a formula, here it is. Manning's equation for flow through an open channel, can also be used for closed pipes.

Q = (1.49/n)*A*R^(2/3)*S^.5

where Q is the flow in cubic feet per second, n is Manning's n (the roughness coefficient), A is the area of the pipe in square feet, R is the hydraulic radius of the pipe, and S is the slope of the hydraulic grade line.

If some of these terms are unfamiliar to you, you can Google them and learn all about hydraulics.When used with a closed pipe, the slope of the hydraulic grade line is measured as the total head loss in feet divided by the length of the pipe in feet.

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