Pipe size question

17 years ago

My sprinkler system is all 1/2" pipe at the moment. The main manifold on the front of the house (service, regulator, hose bib, etc) is all built of 1/2", and it appears that the line from the meter is even 1/2".

I often have trouble getting enough pressure to pop all of the sprinklers, even when no other water is running. It all worked fine on initial installation, and I calculated all the zones to be within the max gpm. Sometimes when a zone has trouble, replacing the valve will fix it, but it's a recurring problem. I'm also not sure that the sprinkler heads are putting out their max flow, even if they are fully extended.

I was wondering, is there anything to be gained by using 3/4" pipe if it is coming from a 1/2" source? Also the sprinkler lines are connected after the regulator which provides about 70psi. I have a hose bib that is outside the regulator which gets about 120psi straight from the meter.

How do these factors relate, psi, gpm and diameter? What is most important for the sprinkler head, flow or pressure? If I connect 3/4" pipe to a 1/2" source, will I get a higher gpm than 1/2", or is it the same?

I'm thinking the front yard may need an overhaul, so I'm trying to decide, should I put the sprinklers before or after the regulator, and would I see any improvement in performance if I change all the pipe to 3/4"?

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Comments (3)

lehua49
17 years ago
Hi Ian,

At 70 psi you have enough pressure. How many heads per zone do you have? What is the Flow rating for the heads at 50-70 psi. What is the maximum pressure the heads are rated for? 1/2" pipe is fine for a low demand system. The demand is created by the number of heads you are operating for each zone. You can look up on the internet what a 1/2" maxes out at. The diameter and gpm are related Q (gpm) = Velocity x Area of pipe cross section. When the pipe is smaller the velocity increase to achieve the demand (Q). With smaller pipe diameters with high demands turbulent flow can occur and reduces flow. Pressure exists in the system as a static force caused by the weight of water from the large tank on the hill. This pressure can change with height of water in the tank (reservoir). What is experienced in your pipe system is the losses to that value by movement of the water through your system. Water traveling through your system causes friction loss to the static pressure value (psi). The faster the water travels through the pipe the greater the friction loss( diameter matters). The longer the pipe the greater the friction loss. Each irrigation head also produces a loss in pressure as well as flow. Your zone head design should be 80% of available flow and pressure. Shoot what do I know. Let me know if I just confused you more? When you experience a problem check your pressure. I think high pressure is reeking havoc with your valves and heads. Aloha.
ianjohnson
Original Author
17 years ago
I found an excellent tutorial here-
http://www.irrigationtutorials.com/sprinkler00.htm

It provides a means to figure starting PSI and flow, determine the PSI loss at each point of the system and figure out the proper size of valve and the needed size of each section of pipe between the sprinklers to deliver at least 80% of the needed PSI to the furthest sprinkler.

This is especially important for my backyard because the water has to go from the street, under the house, across the backyard, and through the detached garage to the valve manifold behind it. It's clear that 1/2" pipe and 3/4" valves are can't deliver enough water back there.
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lehua49
17 years ago
Congrats Ian,

The internet is a wonderful too. You are probably right, although you didn't say what that distance was. That is much farther than 1/2" line is designed to go. Too much friction loss and turbulent flow problems. The 1/2" source would be a problem if it is under your house. You will here the water's noisy high velocity and turbulence in that pipe section. If the valve shuts too quickly you will hear about it. Water-hammer is accentuated by high-velocity flows.