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<br /> <br /> <br /> <br />4 <br />joints. Also the pipe should be designed with a wall <br />thickness sufficient to prevent collapse due to negative <br />pressures. Thin-walled plastic pipe and aluminum <br />irrigation pipes are not recommended, because they <br />are typically not strong enough to accommodate <br />negative pressures in siphons. <br />Although there are no theoretical upper limits to the <br />diameter of a siphon, economics and practicality <br />usually dictate hose/pipe diameters in the range of ¾- <br />inch to 30-inches, but commonly in the range of 2 to 8 <br />inches. Additional siphoning capacity, if needed, is <br />usually provided by more siphon pipes rather than by <br />larger diameter pipes. Also, rather than having a single <br />siphon and valves for conveying and controlling all of <br />the water, it is often best to have several smaller- <br />diameter siphons, with or without throttling valves – <br />so that each can operate independently of the others. <br />Priming: The siphon must be primed (filled with water) <br />or pumped in order to start the flow of water. Both <br />ends of the siphon tube need to be closed to prime the <br />siphon. A gate or butterfly valve can be used to close <br />the ends of the pipe. This can then be used to control <br />the flow during and after priming. There are two <br />common methods for priming the siphon: <br />• Fill both the outlet leg and inlet leg of the siphon <br />pipe with water through a fitting at the apex. The <br />fitting must be large enough to allow air to escape as <br />water fills the pipe (i.e. greater than the pump <br />diameter). Place an airtight cap on the fitting when <br />the pipe is full of water and open the discharge valve <br />to begin the flow of water. <br />• Fill the longer outlet leg with water through a port at <br />the apex. A vacuum pump can be used to draw air <br />out and water in through the inlet side of the siphon. <br />The apex port is then plugged and the discharge <br />valve at the outlet opened to begin the flow of <br />water. <br />Sometimes an intake valve is used to assist with <br />priming, and, if needed, this valve needs to be opened <br />before the discharge valve to start the flow of water. <br />Other Siphon Applications <br />Although this article focuses on the use of siphons for <br />reservoir drawdown, siphons can also be used for <br />dewatering applications during construction, as a <br />means to pass additional inflows under major storm <br />events, or a permanent means for withdrawing water <br />on an on-demand need for reservoirs and canals. The <br />theory and design considerations are the same as <br />described above. <br />Conclusion <br />Siphons can, in the correct circumstances, provide a <br />low(er)-cost alternative to drawdown a reservoir. The <br />key operational parameters are: (1) the required <br />hydraulic lift cannot exceed the effective local <br />atmospheric pressure adjusted for vapor pressure and <br />frictional losses; (2) the discharge point of the siphon <br />must be lower in elevation than the body of water to <br />be siphoned; and (3) the pipe or hose used for the <br />siphon must be designed to withstand negative <br />pressures. <br />Common Pitfalls in Siphoning: <br />• Ensure required lift height is feasible for site location <br />and elevation [(Hmax<(DCE-RWS)] <br />• Check pipe strength against collapse. Lowest <br />pressure does not always occur at apex. <br />• Ensure outlet velocity does not exceed inlet leg <br />velocity (use a longer outlet pipe length and/or <br />smaller diameter pipe) <br />• Review need for vacuum break valves, throttling <br />valves, and air chambers as described. <br />References <br />• Morrison-Maierle, Inc. (2012), “Guidelines for Use of <br />Pumps and Siphons for Emergency Reservoir <br />Drawdown.” Technical Note #9, Montana Dept. of <br />Natural Resources and Conservation, December. <br />• Lombardi, Tina (1996), “Technical Review: Siphon – <br />An Economic Lake Drain Option.” ASDSO. <br />Other References and Resources <br />• Analysis of Siphon Lake Drain Performance for a Small Earthen <br />Dam (1996) by Bryant, Scott D; Jewell, C. Douglas <br />• Installation and Operation of a Siphon System to Provide Water <br />Supply Releases (2003) by Thomas, Craig; Horvath, Richard <br />• No Drain to Pull? Well That Sucks! Pennsylvania’s Emergency <br />Response Teams May Be Able to Help! (2006) by Reisinger, <br />Richard A. <br />• A Quick Guide to Siphons (2004) Watershed News. 10/2004 <br />• To Siphon or Not to Siphon: That is the Question (Among Others) <br />A Repair History of the Crossgate Dam, Raleigh, North Carolina <br />(1998) by Leumas, James K. <br />• Belle Fourche Irrigation District Siphon Project <br />• Reclamation - Central Arizona Project <br />• Lake Burnt Mills Dam Rehab