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WESTERN DAM ENGINEERING NEWSLETTER, VOLUME 1, ISSUE 1, MARCH 2013
SIPHONING, LOW LEVEL CONDUITS, FILTER DESIGN, CONSTRUCTION CONSIDERATIONS
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Research, Thesis, Technical Publications
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<br /> <br /> <br /> <br />3 <br />determine the size and number of siphon lines needed. <br />Siphon discharge depends on the difference in <br />elevation between the lowest desired reservoir <br />elevation (RWS) and the elevation of the outlet (OE). <br />Similar to the maximum lift, the discharge capacity is <br />reduced by friction and hydraulic losses. The following <br />variation of the Bernoulli Equation can be used: 𝑄=0.0438𝐷2.5 𝐻𝑜𝑢𝑡0.5 (12𝑓𝐿+𝐾𝐷+𝐷)−0.5 <br /> 𝑓=425 �𝑛2 𝐷0.33�� <br />Q = flow in cubic feet per second <br />D = siphon diameter in inches <br />Hout = elevation difference in feet from the outlet to <br />the lowest desired reservoir water surface (RWS-OE) <br />K = sum of dimensionless coefficients of hydraulic <br />losses (entrance, bends, valves, exit, etc.). Typical <br />values can be found in most hydraulic reference <br />books <br />f= dimensionless friction factor <br />n = Manning’s n for the pipe <br />L = total length of pipe in feet <br />System Pressure <br />Vacuum (negative) pressures of the system must be <br />checked carefully to limit the risk of pipe collapse <br />during operation. The lowest pressure often occurs at <br />the apex of the siphon. However, the lowest pressure <br />point can occur downstream of the apex. This occurs <br />when friction and minor losses reduce the pressure in <br />the outlet leg more than the decrease in elevation <br />increases the pressure. <br />The equation to estimate pressure at the apex can be <br />given as: 𝑌𝐴=−𝐻−𝑉22𝑔(1 +𝐾′+𝑓𝐿′/𝐷) Y𝐴 = Pressure Head (in feet) at the apex <br />H = Siphon Lift in feet (DCE – RWS) <br />K’ = sum of minor loss coefficients between the RWS <br />and apex <br />L’ = length of pipe in feet upstream of the apex <br />CAUTION: The designer needs to evaluate pressures <br />throughout the system to locate the lowest predicted <br />pressure. <br />If the vacuum pressures are found to be too great for <br />the preferred pipe material, a thicker-walled pipe <br />and/or an air-vacuum breaker valve within the outlet <br />leg will be required. <br />Design of Siphon Components <br />Siphon Layout and Valves: In order to help assure that <br />the siphon runs full and that air does not enter and <br />break the siphon through the discharge end of the <br />outlet leg, it is important that the discharge velocity <br />not exceed the inlet velocity. A practical way to help <br />prevent this from happening is to keep the length of <br />the outlet leg (distance from outlet to apex) greater <br />than the length of the intake leg (distance from intake <br />to apex). Another means is having the outlet leg be a <br />smaller diameter than the inlet leg; however, this <br />requires the use of a reducer which increases frictional <br />losses and may reduce the achievable lift height. If <br />needed, a vacuum-breaker valve can be designed and <br />placed along the outlet leg, at an elevation below the <br />lowest drawdown elevation of the reservoir. <br />It is often beneficial to place an air chamber at the <br />siphon apex, where air will gradually accumulate and <br />could be periodically released. This is particularly good <br />practice for siphons expected to operate for long <br />periods of time. <br />Intake: The intake needs to be submerged to avoid air <br />entering the system and breaking the siphon. It should <br />be placed a minimum of two feet below the lowest <br />desired reservoir surface elevation to limit air entering <br />through vortex action. Air entering will severely <br />decrease efficiency and could break the siphon. A <br />baffle (such as a piece of plywood or metal) can also be <br />used over the mouth of the intake to limit the vortex. <br />Outlet: The discharge end of the siphon must be at a <br />lower elevation than the lowest desired elevation of <br />the reservoir. Ideally, it should be submerged to <br />reduce the risk of air entrainment. Air entrainment <br />could break the vacuum and immediately stop the <br />flow. In many cases, submerging the outlet end is not <br />practical. If the outlet discharges to the atmosphere, <br />care should be taken to ensure the outlet pipe runs <br />full, and valves be installed to release air at the apex as <br />further described below. Additionally, if the discharge <br />end is not submerged, precautions should be taken to <br />ensure adequate erosion protection is provided or the <br />discharge end is kept as far from the toe of the <br />embankment as possible. <br />Pipe size and materials: The hose or pipe comprising <br />the siphon can be constructed of a variety of materials <br />(steel, PVC and HDPE are common), but must be free <br />of kinks or obstructions and must have water-tight
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