# pressures

Posted by jack fairchild on January 14th, 2003 02:30 PM
In reply to Water Pressure by Raymond VinZant Plm Prof on January 13th, 2003 06:10 PM [Go to top of thread]

I do not agree Raymond, different friction losses occur based on pipe sizes, coefficient of friction, and velocities. These variables are well understood and freely published. The diffrence between 1/2 and 3/4 is substantial, see http://www.copper.org/tubehdbk/tables/table4.html when you add in the loss from the fittings you can have big problems. Assuming 100' of pipe length and refering to the above table, the friction loss through 1/2" at 10gpm is 48.8 psi. If the pipe were all 3/4" the loss would only be 8psi. THATS A 38 PSI DIFFERENCE! Brian should also check the size of the water meter. A 10gpm flow through 5/8" is a loss of 6psi, while a 3/4" meter causes only a 2psi loss. Remmeber this is all at 10gpm.

Brian, call up your water company and ask for hydrant flow test data at the hydrants closest to your house, this should give you a good idea of the pressure available to you. If they don't have it then request one, they may ask why you want this, just be honest. The static pressure (the pressure at no flow) should be the same as what you have in your house assuming no difference in elevation. The residual pressure (the pressure at some (x) flow) will also be on the test data. If the static pressure is around 50-60psi then the problem is in your home or the incoming line. A few questions: How far is it from the water line in the street to your house? Is there a change in elevation is your house on a hill? What size is the meter? How many valves are on your water line and what kind are they? Good luck

Jack Fairchild
Fire Sprinkler Designer