Sprinkle & Trickle Irrigation Lectures Page 253 Merkley & Allen Lectur2Manifolde 3Hydraulic DesignIntroduction •Manifolds in trickle irrigation systems often have multiple pipe sizes to: 1. reduce pipe costs 2. reduce pressure variations •In small irrigation systems the reduction in pipe cost may not be significant, not to mention that it is also easier to install a system with fewer pipe sizes •Manifold design is normally subsequent to lateral design, but it can be part of an iterative process (i.e. design the laterals, design the manifold, adjust the lateral design, etc.) •The allowable head variation in the manifold, for manifolds as subunits, is given by the allowable subunit head variation (Eq. 20.14) and the calculated lateral head variation, ∆Hl•ThiI.s simple relationship is given in Eq. 23.1: ()msaHHlH∆= ∆− ∆(437) •Eq. 23.1 simply says that the allowable subunit head variation is shared by •Recall that a starting design point can be to have ls∆Hm= ½∆Hs, but this half and halfproportion can be adjusted during the design iterations •The lateral pressure variation, ∆Hl, is equal to the maximum pressure minus the minimum pressure, which is true for single-direction laterals and uphill+downhill pairs, if Hn’ is the same both uphill and downhill I. Allo•Equation 20.14 (page 502 in the textbook) gives the allowable pressure head variation in a “subunit” •This equation is an approximation of the true allowable head variation, ecis equation is applied before the laterals and manifold are •After designing the laterals and manifold, the actual head variation and expected U can be recalculated •onsider a linear friction loss gradient In this case, the average head is the laterals and manifold ∆H = ½∆H , and Iwable Head Variation bdesigned ause thECmultiple outlets) on flat ground: (no
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