Efficient Irrigation Planning

Pipe and Fittings

Larry M. Curtis, Extension Agricultural Engineer

Ted W Tyson, Extension Agricultural Engineer

The pipe and fittings that are used to join an irrigation system to the water source should be evaluated based on the energy required to push water through these components. The major factor affecting the energy required to move water through the pipe and fittings is friction loss. Three additional factors that should be considered in pipe selection are: captured air, water velocity; and pressure rating. Water velocity and pressure rating are important design considerations for a safe, dependable system. You should also consider the fixed cost of the pipe when evaluating a particular irrigation design.

 Friction Loss

As water flows through irrigation pipe on level ground, the pressure decreases as the distance from the pump increases. This pressure change is called friction loss. It is normally given in design tables in terms of pounds of pressure (reported as pounds per square inch, psi) lost per 100 feet of pipe for different pipe diameters. Other components, such as fittings, elbows, etc., that cause a restriction in the diameter of the pipe will also contribute to the pressure loss.

The higher the pressure loss in a piping system, the greater the amount of energy or horsepower needed to push the water through the pipe to overcome the loss. Several factors influence how great the pressure loss in a pipeline will be.

Type of Pipe. One factor that influences the amount of energy required to move water through a pipe is the material the pipe is made from. Different pipe materials have different friction-loss characteristics. For example, PVC (polyvinyl chloride pipe) has less resistance to flow than aluminum pipe. Therefore, for a given length of pipe, PVC would require less energy than aluminum pipe with the same internal diameter. Other pipe materials like steel and concrete also have different friction-loss characteristics. You must determine which type of pipe to use before you can evaluate friction loss.

Volume of water. The volume of water flowing through the pipe also influences friction loss. For a given flow rate, the friction loss will decrease if a larger pipe size is selected. Therefore, after a flow rate has been determined for your system, several different pipe sizes should be evaluated to see which is the best selection for minimizing the amount of pressure lost in the pipe.

Length of pipe. The total length of pipe also influences the amount of energy required to move the water through the system. For a given pipe diameter and flow rate, a longer pipeline will have more total pressure loss due to,pipe friction than a shorter one. Therefore, the designer must calculate the total pressure drop or friction loss in the entire length of pipe to determine the energy requirements to overcome resistance to flow.

Fittings. Other portions of a pipe network such as fittings, elbows, valves, and risers increase the pressure requirements of a system by increasing the turbulence in the piping system. The amount of pressure lost due to these components depends on several factors that can be determined from design handbooks.

Generally speaking, fittings should be about the same diameter as the pipe used. Fittings that are too small can require considerable input of energy because of the flow restriction.

Captured air. One source of pipeline restriction that requires attention from the designer is captured air. Captured air can come from several sources. All of the air present in a pipeline before it is filled with water may not be forced out. Or, during start up, the pump may force air into the pipe. Also, turbulence from a partially open gate valve may trap air in solution. The trapped air is then released at points along the pipeline where pressure is lower.

Air bubbles from any of these sources tend to collect at high places in the line. These bubbles actually reduce the inside diameter of the pipe and cause a restriction in the line. This restriction increases turbulence by restricting the flow. Bubbles of air moving in a pipeline get larger at high points in the line (lower pressure) and smaller at low points (higher pressure). This variation in size causes a surging action. Surging is also caused when large air bubbles suddenly break loose and move rapidly down the line.

Surging contributes to water hammer in the line. Water hammer is what causes household pipes to shake and rattle when a faucet is turned off quickly. Water hammer may damage the pipe, and, in a large agricultural irrigation system, it could blow the line apart.

Controlling air problems. Air relief/vacuum relief (ARVR) valves, continuous acting air vents (CAV), and pressure relief (PR) valves are used to control problems associated with air in irrigation piping systems. Long pipelines over gently sloping land can be particularly vulnerable because they are so easily neglected. Follow manufacturer recommendations to size these valves properly.

The location of these valves in the system is also critical. ARVR valves should be located as follows:

1) Immediately downstream of any in-line valve.

2) At all high points in the line.

3) At all turns of 90 degrees or more.

4) One hundred feet upstream of all mainline pipe ends.

5) Upstream of pump check valves.

6) At 1/4-mile intervals, along with continuous acting vents (CAV).

Place CAV vents with vacuum relief (VR) just downstream of any sudden downturn or decrease in pipeline slope.

Place pressure relief (PR) valves where pressure surges may occur. The most likely places include:

1) Downstream of the pump discharge check valve.

2) At low points in the piping system.

3) At the ends of submains and mains.

4) At the low point of any length of pipe that remains partially filled when the rest of the pipe empties.

Water Velocity

Although the water velocity in a pipe does not directly contribute to irrigation energy consumption, it is an important criterion to consider in selecting the pipe size. The velocity of the water moving through the pipe is an important criterion for determining the minimum pipe size that can be used.

National standards for PVC pipe recommend water velocities of 5 feet per second or less. Following these recommendations will generally reduce the possibility of water hammer and contribute to more economical operating costs over the life of the pipeline.

Pressure Rating

Finally, another criterion that must be considered in pipe selection is the pressure rating of the pipe. The pipe should have an operating pressure rating high enough to. ensure the long-term and reliable operation of the pipeline. The typical working pressure of PVC pipe used in irrigation is about 72 percent of the pressure rating when surge pressures are not known. For example, the working pressure rating of Class 160 PVC should be about 115 psi when the surge pressure characteristics of the installed system are not known.

For further information, consult your county Extension agent or refer to the following Extension circulars: ANR-545, "Energy And Irrigation"; ANR-546, "Energy And Irrigation: Elevation"; ANR-548, "Energy And Irrigation: The Pumping Unit"; and ANR-549, "Energy And Irrigation: The Power Unit"

Issued in furtherance of Cooperative Extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, and other related acts, in cooperation with the U.S. Department of Agriculture. The Alabama Cooperative Extension System (Alabama A&M University and Auburn University) offers educational programs, materials, and equal opportunity employment to all people without regard to race, color, national origin, religion, sex, age, veteran status, or disability.

This document is author-produced (unedited).

 

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