Sprinkle & Trickle Irrigation Lectures
Page 253
Merkley & Allen
Lectur
2
Manifold
e 3
Hydraulic Design
Introduction
•
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,
∆
H
l
•
Thi
I.
s simple relationship is given in Eq. 23.1:
(
)
m
s
a
H
H
l
H
∆
= ∆
− ∆
(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
l
s
∆
H
m
=
½
∆
H
s
, but this
half and half
proportion can be adjusted during the design
iterations
•
The lateral pressure variation,
∆
H
l
, is equal to the maximum pressure minus
the minimum pressure, which is true for single-direction laterals and
uphill+downhill pairs, if H
n
’ 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,
ec
is 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
I
wable Head Variation
b
designed
ause th
E
C
multiple outlets) on flat ground:
(no

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