8.4: Pipelines

Irrigation pipelines are made of many materials. Currently, the most common materials used for aboveground sprinkler systems and gated pipe systems are aluminum and ultraviolet radiation protected PVC (polyvinyl chloride plastic). Center pivot and lateral systems are the exception where it is common to use galvanized steel as the pipeline material. Above ground microirrigation laterals are usually made of polyethylene (PE) plastic. For pipelines that are buried below the ground, the most common material in agricultural applications is PVC, and in microirrigation systems it is PE. Sizing mainline pipelines is usually based on a maximum of 5 to 6 ft/s average velocity. Table 8.5 shows the typical flow ranges for selected aluminum and PVC pipe at various nominal sizes and 5 ft/s flow velocities. For example, the recommended maximum flow rate for an 8-inch pipeline is in the range of 700 to 800 gpm.

Table 8.5. Maximum flow rates in pipelines rounded to nearest 5 gpm at Vm = 5 ft/s (based on Table 8.2a and b)

Nominal size (in)	Aluminum Sprinkler		PVC IPS
	Inside diameter (in.)	Q (gpm)	Inside diameter (in.)	Q (gpm)
2	1.900	45	2.193	60
2.5	-	-	2.655	85
3	2.900	105	3.230	130
4	3.900	185	4.154	210
6	5.844	425	6.120	460
	Aluminum Gated		PVC PIP
6	5.898	425	5.776	405
8	7.898	765	7.658	720
10	9.898	1200	9.572	1120
12	-	-	11.486	1615

Example 8.5

A PVC PIP mainline will supply water to a drip irrigation system. The flow rate of the system is 700 gpm. The mainline is 600 feet long and drops (falls) 25 feet in its length. The pressure at the inlet to the pipe is 35 psi

Given: P₁ = 35 psi

Fall = 25 ft

L = 600 ft

Q = 700 gpm

Find: The appropriate size pipe for this system The pressure at the downstream end of the pipe, i.e., at the end of the mainline (ignore minor losses).

Solution

Referring to Table 8.5, an 8-in pipe should be selected to keep the mean velocity below 5 ft/s. From Table 8.2b, we find that the pressure loss due to friction is 0.39 psi/100 ft. Using the concepts from Figure 8.4, we can solve for the downstream pressure:

\(P_2=P_1-P_f-P_m+0.433 \times Fall \)

\(P_2=35 \text{ psi}-(0.39 \text{ psi}/100 \text{ ft}) \times 600 \text{ ft}-0+0.433 \times 25 \text{ ft}=43.5 \text{ psi} \)

In this example, pressure has increased with length in the pipeline because of the relatively steep fall.

Pipelines must be protected from excessive pressures and vacuums. It is also imperative that air is relieved from pipelines so that it is not compressed while filling the pipeline. At high points, it is important to relieve the air so that an air blockage to flow does not occur. Figure 8.5 shows the layout of valves which is required to adequately protect pipelines. To release air and relieve vacuums, a combination vacuum-air vent relief valve is used. These should be used at the entrance to the pipeline, at high points in the pipeline, and at the end of the pipeline. There should also be an air vent at 1,000-foot intervals along the pipeline. In addition to air and vacuum relief, pressure relief valves should be provided in case surges occur within the pipeline (Figures 8.5 and 8.6). These valves should be installed at the inlet and at dead ends of the pipeline. At the inlet to the pipeline, a check valve is suggested so that reverse flow will not occur when the pumping system stops. For pipelines connected to municipal water systems or when chemigation is used (Chapter 15), proper backflow prevention equipment must be installed. For pipelines that are buried shallower than the frost depth, drainage should be provided so that freezing water does not burst the pipeline. More information on pipeline hydraulics can be found in Colt Industries (1979) and Waller and Yitayew (2016).

Figure 8.5. Suggested location of valves for buried pipelines.

Figure 8.6. Irrigation pipeline protection valves.