On-Site Sprinkler Irrigation of Treated Wastewater in Ohio

Bulletin 912

Step 6: Layout and Sizing of Pipe

Selecting pipe and laying out the system depends on the sprinkler placement (Step 4) and sprinkler selection (Step 5). Smaller systems of 15 gpm (4-5 heads) can be treated as one zone whereas larger systems should be separated into multiple zones. The total zone gpm will determine the size and cost of the pump required to deliver the water.

Layout

Polyvinyl chloride (PVC) and polyethylene (Poly) pipe are commonly used for irrigation systems. Connect all sprinkler heads in each zone using the most direct route possible. Keep turns and changes in direction to a minimum.

Sizing Lateral Lines

For small residential installations it is standard practice to use the same size of Poly or PVC pipe for all lateral lines. It is important to use the proper size. Using too small a pipe will result in unacceptable friction losses, while using too large a pipe will result in reduced pressure and insufficient pipe scouring. It is recommended that a velocity of greater than 2 feet/second is maintained throughout the pipe network to prevent the build-up of slime and bacterial growth. Using pipe that is too large or too small will impair sprinkler head function.

Example: Step 6 illustrates how to size pipe based on flow rates. The procedure for proper sizing is outlined here.

1. Identify the gpm requirement of the furthest head from the zone valve based on Step 5. For systems with only one zone use the head furthest from the main line point of connection (POC).
2. On a Friction Loss Chart (Figure 3) for the type of pipe selected, find the gpm amount from 1st item in the far left column.
3. In that row, move right across the chart until a velocity of less than 5 feet per second is reached.
4. Move up that column to find the minimum pipe size necessary to carry the flow to this head.
5. Add the gpm requirement of the last and the next to the last head together to size the next pipe.
6. Find the total gpm in the 1st column of the Friction Loss Chart and repeat steps 3 and 4.
7. Continue this process until you reach the zone valve or main line POC.
8. Select the largest of the pipe sizes for the entire zone.

Do not size pipe smaller than the chart indicates.

If the required pipe size becomes unrealistic (i.e., too large, too expensive, or not available), add more zones to the design.

Sizing Main Line

Size the main line using the Friction Loss Chart as described above. The main line must be large enough to handle the entire flow rate of the largest zone in the system without exceeding a velocity of 5 feet per second.

Figure 3. Friction Loss Chart

Example: Step 6
Lateral Lines

1st—gpm of furthest head = 2.4 gpm
2nd—3 gpm on Friction Loss Chart
3rd—3.16 ft/sec
4th—Minimum nominal pipe size is 1/2 inch.
5th—gpm total (2.4 + 2.4) = 4.8 gpm

Repeat 2nd—5 gpm on Friction Loss Chart
Repeat 3rd—3.0 ft/sec
Repeat 4th—minimum nominal pipe size is 3/4 inch.
Repeat 5th—gpm total (4.8 + 2.4) = 7.2 gpm

Repeat 2nd—8 gpm on Friction Loss Chart
Repeat 3rd—4.8 ft/sec
Repeat 4th—minimum nominal pipe size is 3/4 inch.

For this example, 3/4 inch PVC schedule 40 pipe will be used for all lateral lines in zone 1 and zone 2.

Main Line

Since the area is divided into two zones, the total gpm of each zone is 7.2 gpm. Using the Friction Loss Chart as described above, the minimum nominal size of the main line is 3/4 inch.

If the area was considered one zone, the total gpm would be 14.2 gpm. This design would require a minimum nominal pipe size of 1 1/4 inch.

For quick pipe sizing use the following summary chart based on flow rates of 5 feet per second.

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