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determine the size and number of siphon lines needed.
Siphon discharge depends on the difference in
elevation between the lowest desired reservoir
elevation (RWS) and the elevation of the outlet (OE).
Similar to the maximum lift, the discharge capacity is
reduced by friction and hydraulic losses. The following
variation of the Bernoulli Equation can be used: π=0.0438π·2.5 π»ππ’π‘0.5 (12ππΏ+πΎπ·+π·)β0.5
π=425 οΏ½π2 π·0.33οΏ½οΏ½
Q = flow in cubic feet per second
D = siphon diameter in inches
Hout = elevation difference in feet from the outlet to
the lowest desired reservoir water surface (RWS-OE)
K = sum of dimensionless coefficients of hydraulic
losses (entrance, bends, valves, exit, etc.). Typical
values can be found in most hydraulic reference
books
f= dimensionless friction factor
n = Manningβs n for the pipe
L = total length of pipe in feet
System Pressure
Vacuum (negative) pressures of the system must be
checked carefully to limit the risk of pipe collapse
during operation. The lowest pressure often occurs at
the apex of the siphon. However, the lowest pressure
point can occur downstream of the apex. This occurs
when friction and minor losses reduce the pressure in
the outlet leg more than the decrease in elevation
increases the pressure.
The equation to estimate pressure at the apex can be
given as: ππ΄=βπ»βπ22π(1 +πΎβ²+ππΏβ²/π·) Yπ΄ = Pressure Head (in feet) at the apex
H = Siphon Lift in feet (DCE β RWS)
Kβ = sum of minor loss coefficients between the RWS
and apex
Lβ = length of pipe in feet upstream of the apex
CAUTION: The designer needs to evaluate pressures
throughout the system to locate the lowest predicted
pressure.
If the vacuum pressures are found to be too great for
the preferred pipe material, a thicker-walled pipe
and/or an air-vacuum breaker valve within the outlet
leg will be required.
Design of Siphon Components
Siphon Layout and Valves: In order to help assure that
the siphon runs full and that air does not enter and
break the siphon through the discharge end of the
outlet leg, it is important that the discharge velocity
not exceed the inlet velocity. A practical way to help
prevent this from happening is to keep the length of
the outlet leg (distance from outlet to apex) greater
than the length of the intake leg (distance from intake
to apex). Another means is having the outlet leg be a
smaller diameter than the inlet leg; however, this
requires the use of a reducer which increases frictional
losses and may reduce the achievable lift height. If
needed, a vacuum-breaker valve can be designed and
placed along the outlet leg, at an elevation below the
lowest drawdown elevation of the reservoir.
It is often beneficial to place an air chamber at the
siphon apex, where air will gradually accumulate and
could be periodically released. This is particularly good
practice for siphons expected to operate for long
periods of time.
Intake: The intake needs to be submerged to avoid air
entering the system and breaking the siphon. It should
be placed a minimum of two feet below the lowest
desired reservoir surface elevation to limit air entering
through vortex action. Air entering will severely
decrease efficiency and could break the siphon. A
baffle (such as a piece of plywood or metal) can also be
used over the mouth of the intake to limit the vortex.
Outlet: The discharge end of the siphon must be at a
lower elevation than the lowest desired elevation of
the reservoir. Ideally, it should be submerged to
reduce the risk of air entrainment. Air entrainment
could break the vacuum and immediately stop the
flow. In many cases, submerging the outlet end is not
practical. If the outlet discharges to the atmosphere,
care should be taken to ensure the outlet pipe runs
full, and valves be installed to release air at the apex as
further described below. Additionally, if the discharge
end is not submerged, precautions should be taken to
ensure adequate erosion protection is provided or the
discharge end is kept as far from the toe of the
embankment as possible.
Pipe size and materials: The hose or pipe comprising
the siphon can be constructed of a variety of materials
(steel, PVC and HDPE are common), but must be free
of kinks or obstructions and must have water-tight