14.4.2: Mainline and Manifolds
- Page ID
- 44686
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The mainlines and manifolds for example microirrigation systems are illustrated in Figure 14.5. Actual systems may be different and far more complex than the illustration. The mainline carries water from the control station to manifolds which distribute the water to each lateral. Normally, there are no fixtures along the mainline other than elbows or tees. If the system is large and there are a number of manifolds, flow control valves and shutoff valves are located at the head end of each manifold. These fixtures assure the correct flow enters each manifold and accommodates manual or automatic control of irrigation water. The diameter of mainlines and manifolds are normally large, in the range of 1 to 6 inches. To determine the friction loss in plastic pipe, both polyvinyl chloride (PVC) and polyethylene (PE), refer to Chapter 8. The same procedures can be used to determine the appropriate pipe diameter for mainlines and manifolds for microirrigation as described in Chapter 8.
A mainline is required to convey 200 gpm, a distance of 500 ft from the control station to the microirrigation manifolds. What diameter of PVC pipe would you recommend if the pipeline velocity must be less than 5 ft/s?
Given: Q = 200 gpm
Pipe length = 500 ft
Pipe type = PVC
Velocity in pipe less than 5 ft/s
Find: Smallest pipe diameter recommended
Solution
From Table 8.5, a 4-inch diameter pipe has a large enough cross sectional area to keep the velocity less than 5 ft/s.
Determine the smallest diameter polyethylene pipe to be used for a manifold if the flow rate is to be 70 gpm. What will the friction loss be for the manifold if the length is 200 ft?
Given: Q = 70 gpm
Manifold length = 200 ft
Find: Smallest recommended pipe diameter
Friction loss for the manifold
Solution
From Table 8.5, a 2-inch diameter pipe with a flow of 70 gpm will require a water velocity in excess of 5 fps and is, therefore, not recommended. A 2.5-inch diameter pipe satisfies the velocity requirement of 5 fps and the friction loss would be 0.96 psi/100 ft or a total of 1.92 psi of pressure loss (Table 8.2a). This would probably be acceptable for friction loss in for a manifold.