


Thread Tools  Display Modes 
12112009, 02:51 PM  #1 
Member
Join Date: Dec 2009
Posts: 100
Rewards Points: 75

Need a Formula
How do I figure out gpm with pressure? How do I set this formula up?
Advertisement 
12112009, 03:18 PM  #2 
Civil Engineer
Join Date: Mar 2009
Location: Boston
Posts: 5,178
Rewards Points: 3,936

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.
Advertisement 
12112009, 06:18 PM  #3  
Member
Join Date: Dec 2009
Posts: 100
Rewards Points: 75

Quote:


12122009, 01:57 AM  #4 
Member
Join Date: May 2007
Posts: 406
Rewards Points: 356

Bucket test. put 1 gallon bucket under pipe, turn on valve. Time until bucket is full

12122009, 10:19 AM  #5 
Water quality'n pump guy
Join Date: Sep 2008
Location: Wherever I park the motorhome.
Posts: 358
Rewards Points: 250

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.
__________________
Gary Slusser 23 years in water treatment and well pumps, 13 years on the 'net helping others to help themselves. 
12122009, 05:15 PM  #6 
Member
Join Date: May 2007
Posts: 406
Rewards Points: 356

Do search for HazenWilliams.
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? 
12132009, 07:07 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.

12132009, 08:19 PM  #8 
Civil Engineer
Join Date: Mar 2009
Location: Boston
Posts: 5,178
Rewards Points: 3,936

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. Advertisement 
Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
my kitchen reno  complete remodel (with concrete countertop)  Knucklez  Project Showcase  311  07072014 05:07 PM 
bundle formula  timothy918  Roofing/Siding  3  02162009 05:36 PM 
Formula for Main upgrade  hogger9  Electrical  6  09282008 11:02 AM 
Formula for changing lights to recessed bulbs?  Jemo  Electrical  12  01262007 05:02 PM 
stone veneer formula  tom1st  Landscaping & Lawn Care  1  07082006 04:48 PM 