2.1 Sizing Water Pipes

Irrigation system designers use a friction loss table for sizing pipe.

The Friction loss for PVC class 160 Pipe table is an example of a friction loss table, and shows the relationships between pipe size (diameter), water velocity, and friction loss (pressure loss) . For each kind of pipe there is a friction loss table that illustrates the relationships of flow rate, water velocity, and friction loss.

Major irrigation manufacturers like Rain Bird (TM) and Toro (TM) have data books that include complete tables for common pipe types and sizes.

Proper selection of pipe size is needed to ensure that the pipe:

  1. is large enough to safely carry the quantity of water,

  2. has a reasonable friction loss so the operating pressure is largely preserved, and

  3. is economical for the water flow.

The Friction loss for PVC class 160 Pipe table is a simplified friction loss table. Click on this link and resize the window with this tgable, so you can read this text as you read the table....

Across the top are the nominal pipe sizes and actual pipe inside diameters. Pipe "inside diameters" vary with the pressure rating of the pipe because the wall thickness varies. Down the left side in the first column are the water flow rates, in gpm. Under each pipe size there are two columns. The first column is the water velocity that corresponds to the water flow rate and pipe diameter. Velocities are important because water velocities exceeding 5 feet per second should be used with caution. A line crosses the column at this point to show the caution. The second column is the friction loss value in psi per 100 ft of pipe length for the given flow rate. The friction loss column is the primary information used.

In summary, the velocity column shows the range of flows that are safe to use for each pipe size. The friction loss column shows the pressure energy lost per 100 ft of pipe length for a given flow rate.

  1. Select the size of PVC Class 160 pipe to carry 20 gpm of water a distance of 1000 feet.

  2. Guideline: As a rule of thumb, friction loss in a main line should not exceed 10 psi.

  3. Use the Friction Loss Chart. Go down the first column to 20 gpm and then move horizontally to the right to check water velocities (fps). The table shows that 0.5 and 1.0 in. pipe are both too small; their water velocities would exceed 5 fps. Available sizes are the 2, 3, and 4 in. pipes.

The friction loss for 20 gpm for 1000 ft in a 2 in. pipe is 1000 feet x (0.24 psi/100ft) = 2.4psi. Meeting the maximum friction loss criteria is the 2-in. pipe. The 3- and 4-in. pipes would cost a lot more. The 2-in. pipe is the best buy based on the information given.

How much friction loss should be allowed? There is no one answer, as several factors may be important. A friction loss under 1 psi per 100 ft is a guideline.

Friction loss within an irrigation zone of sprinklers or trickle emitters should be low because uniform pressure means uniform water application. The larger the friction loss, the greater the pressure a pump must create to overcome the loss. The difference in selecting one pipe size over another may be the difference between a higher initial investment in a larger size pipe with lower friction and more flow capacity versus the long-term cost of operating a larger pump to achieve the required pressure.

A wide variation in operating pressure within an irrigation system is a constant problem. Size the pipe to keep the friction losses small and the pressure uniform throughout the system. This makes the day-to-day operations much smoother. An experienced irrigation system designer must make the final judgment.