Irrigation 101

To regulate pressure, a differential of about 10 psi, depending on the manufacturer, must exist between the incoming pressure and the pressure needed for the zone. For example, the zone pressure you need is 40 psi. The incoming pressure is 70 psi at the valve. Because there is a 30 psi differential across the pressure regulator, the regulator will work properly and the pressure immediately downstream will be 40 psi. However, if the incoming pressure is 46 psi, the valve will not regulate as there is only a 6 psi pressure differential. The system would operate at 46 psi and the cost of the pressure regulator would be wasted.

When designing an irrigation system, you can group the electric valves together in one location (manifold), place them in individual locations (remote) or put them in several manifold groups throughout the system.

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Where the irrigation system is situated and how large it is will determine the type of pipe to use. Most irrigation systems, especially larger systems (over 2 inch), use PVC pipe predominantly, but systems in many of the northern climates use polyethylene (PE). Each has different characteristics, but from a design standpoint, either is acceptable when properly sized and installed.

Once you determine the amount of water that each zone uses, you can size the pipe. Based on the amount of water flowing through each pipe, you can determine the velocity and friction loss. In irrigation system design, you size the pipe based first on velocity and then on friction loss.

PVC pipe should have velocities less than 5 feet per second (fps), polyethylene less than 6 fps. For metal pipe-copper, galvanized or ductile iron-7.5 fps is the rule. To calculate velocity, use a formula or a chart. The formula for water velocity through a pipe: V=0.408 times the flow, divided by the inside diameter (d) of the pipe squared (V=.408xQ/dxd).

Most designers use charts instead of calculations, which require only the pipe size and the flow required, to get both the velocity and the friction loss. Friction loss equations are hard to use, and charts are easy and quick. For example, the velocity through a 1-inch piece of Schedule 40 PVC pipe flowing 15 gpm is 4.82 fps. This would be the maximum limit for this size of pipe. You can calculate maximum velocities for each size and type of pipe. The friction loss through the same pipe at 15 gpm is 3.90 psi per 100 feet. Friction losses are, for all practical purposes, given in psi per 100 feet

Pipe is not the only element that results in friction loss. Backflow preventers, shut-off valves, fittings, electric valves and swing joints all have friction loss. You need to calculate all the friction losses in a system and keep them to their specific limits so that the sprinklers operate at their intended pressure.

You can use a chart to look up friction losses for each specific piece of equipment. Recognizing friction loss can be intuitive and will come with experience. For example, on a residential system, friction losses will be 10 to 15 psi with a pressure-vacuum-type backflow preventer and 15 to 20 psi with a reduced-pressure-type backflow preventer. However, on systems with low pressure, you need to carefully calculate friction losses. A good irrigation text or basic irrigation class will take you through the procedures for properly calculating friction losses and velocities.

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