This equation gives you an orifice diameter for all launders to compensate for any variation of flow due to the different lengths of the manifolds that go to the plant leveling tank. It is based on a desired flow ratio between the longest and shortest manifold.
The head loss through the orifice (minor loss) is large so you can neglect the major losses.
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$$
d_
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= \root 4 \of {{{\left( {1 - {{Q_s^2 } \over
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}} \right) \cdot D_
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^4 } \over {\left( {\left( {\sum K + f{{L_l } \over {D_
}}} \right) \cdot {{Q_L^2 } \over {Q_s^2 }} - \sum K - f{{L_s } \over {D_
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}}} \right) \cdot K_
^2 \cdot n_
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^2 }}}
$$