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Guidelines for Use of Pumps and Siphons for Reservoir Drawdown

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Guidelines for Use of Pumps and Siphons for Reservoir Drawdown Guidelines for Use of Pumps and October 2012 Siphons for Reservoir Drawdown Presented by Molly Skorpik and Matt Barnes at the 2012 Montana Association of Dam and Canal Systems Conference Overview • Intro on Reservoir Drawdown • Overview of Pump Options • Overview of Siphon Options • Considerations for Selecting Pump or Siphon • Case Studies Overview • Intro on Reservoir Drawdown • Overview of Pump Options • Overview of Siphon Options • Considerations for Selecting Pump or Siphon • Case Studies Drawdown Scenarios Using Pump or Siphon • Emergency repair Risk of dam overtopping Risk of use of damaged spillway Downstream flow requirements • Standard maintenance Non-emergency • Source of water delivery This is how you usually delivery water Typical Dam pump siphon Reservoir Drawdown Pump versus Siphon Guidelines for the Use of Pumps and Siphons for Emergency Reservoir Drawdown Drawdown Considerations • Height of lift over dam crest • Depth of drawdown required • Discharge flowrate required • Access available at site • Power source available • Dam embankment stability • Inlet and outlet conditions • Operation and maintenance • Availability of equipment and cost Overview • Intro on Reservoir Drawdown • Overview of Pump Options • Overview of Siphon Options • Considerations for Selecting Pump or Siphon • Case Studies Overview of Pump Options • Submersible, diesel generator • Trailer or skid mounted, diesel-powered • Trailer powered by tractor Pumping System Components • Inlet/intake pipe • Pump • Power source • Outlet pipe • Air release valves, vacuum breaker valves and combination air valves Pumping System Components Inlet/intake pipe Size appropriately to avoid cavitation from high velocities Consult with professional engineer or pump supplier Pumping System Components Outlet pipe Energy dissipation Location Pumping System Components Air release valves, vacuum breaker valves and combination air valves Air release vents air during system operation Vacuum breaker provides vacuum and column separation protection by admitting large volumes of air Combination air valve does both Pumping System Advantages • Pumps handle relatively large lift • Relatively quick to set up Pumping System Disadvantages • Large component usually requiring good access to site • Rental and fuel costs can be high • Requires power source • Requires frequent and sometimes continuous monitoring Overview • Drawdown • Overview of Pump Options • Overview of Siphon Options • Considerations for Selecting Pump or Siphon • Case Studies Overview of Siphon Options Understanding Siphon Design Grades Maximum lift equation DCE – RWS <= 20’ – RWS/1000 Understanding Siphon Design Grades Example problem: Current reservoir elevation (full pool): 5,500 feet Desired drawdown reservoir elevation: 5,490 feet Crest elevation: 5,505 feet Understanding Siphon Design Grades Example problem: Current reservoir elevation (full pool): 5,500 feet Min. elevation is worst case Desired drawdown reservoir elevation: RWS = 5,490 feet Crest elevation: DCE = 5,505 feet Understanding Siphon Design Grades Maximum lift equation • DCE – RWS <= 20’ – RWS/1000 • Where DCE = 5,505’ and RWS = 5,490’ • 5,505’ - 5,490’ <= 20’ – 5,490’/1000 • 15’ <= 14.51’ Understanding Siphon Design Grades Maximum lift equation DCE – RWS <= 20’ – RWS/1000 Where DCE = 5,505’ and RWS = 5,490’ 5,505’ - 5,490’ <= 20’ – 5,490’/1000 15’ <= 14.51’ Siphon will not work at low RWS elevation! It will probably work at first and then fail or collapse. Yikes! Understanding Siphon Design Grades Siphon Components • Pipe materials • Inlet • Outlet • Priming appurtenances • Air vacuum breaker valves Siphon Components Pipe materials Siphon Components – Pipe Materials Pipe Material Considerations HDPE . Very flexible, which can eliminate the need for bend fittings. Can be fusion welded together to create a very air tight system, but welding can be expensive, time consuming and subject to the availability of appropriate equipment. May not be readily available in some locations. Pipe is buoyant and will float if measures are not taken to weigh the pipe down. Bend radius of pipe can be used to eliminate fittings. PVC . Can be bell/spigot jointed for good seal. Rigid pipe so bend fittings or the use of flexible couplers may be needed. Very common in MT, most pipe suppliers will have a quantity in stock. Must have adequate wall thickness (Schedule 40 or 80) to withstand vacuum pressures. Schedule 80 may be more appropriate at the crest of the dam to withstand greater vacuum pressures. Usually the least expensive option due to availability. Steel . Weld joints would be best for air tightness but time consuming and may be expensive. Pipe is capable of withstanding large pressures differentials. Relatively heavy creating difficulty in transport and setup. Relatively expensive. CMP . Typically not a good choice because of difficulty obtaining an air tight seal, hydraulic capacity, and more difficult to set up. Could be a good choice to transport outlet water to better downstream location to reduce erosion and effect of dam embankment. Aluminum Pipe . Often readily available. Pipe strength is minimal and should only be used if lift is minimal. Siphon Components Inlet Buoyancy Debris Vortices Siphon Components Outlet Submergence Energy dissipation Vandalism Siphon Components Priming the siphon This can be tricky… Requires a fitting at the high point to fill A. Valve upstream and downstream. Fill using small pump. Air must escape. B. Valve downstream only. Fill outlet side only. Use vacuum pump to pull water up the inlet side. Siphon Components Vacuum breaker valves Not always needed Use with caution Prevents pipe collapse on long downgradient run Siphon Advantages • Pipe materials for siphon usually available quickly and locally • Less oversight compared to pump • No fuel or rental costs (except for start-up) Siphon Disadvantages • Grade limitations • Maintaining air tight system • Pipe material selection Considerations for Selecting Pump vs. Siphon • Drawdown • Overview of Pump Options • Overview of Siphon Options • Considerations for Selecting Pump or Siphon • Case Studies Considerations for Selecting Pump vs. Siphon • Lift • Power • Priming • Lead time • Monitoring • Capacity • Access • Cost Drawdown System Considerations Consideration Pump Siphon Lift No limit, check pump curves Typically limited to less than 20’ lift, see Siphon Overview Power Required (electrical, generator, etc.) Only required for initial priming of siphon Priming Not required in most situations Required to start siphon Lead Time Varies, can be long for large pumps Varies, usually not as long as pumps Monitoring Required every 3-12 hours, sometimes Once per day or longer if site is isolated required continuously Capacity Varies with pump size and lift. Capacity is Varies with pipe size and lift related to water determined from pump curve and site surface elevation. Capacity is determined by conditions. Predicted Siphon Flowrate equation or energy equation. Access Must have good vehicle access for large May work best if access to site is limited pumps Cost Pipe and appurtances materials cost plus Pipe and appurtenances materials cost on-going costs associated with pump rental fees and fuel/power Overview • Drawdown • Overview of Pump Options • Overview of Siphon Options • Considerations for Selecting Pump or Siphon • Case Studies Case Study – Park Lake Dam Case Study – Park Lake Dam • Dam: Earthen • County, State: Jefferson County, Montana • Storage capacity: 423 acre feet • Elevation of reservoir water: 6,347 feet to 6,355 feet • Elevation of dam crest: 6,364 feet • Outlet elevation of siphon pipe: 6,307 feet • Emergency situation due to condition of dam • No low level outlet Case Study – Park Lake Dam Case Study – Park Lake Dam Case Study – Park Lake Dam Case Study – Park Lake Dam Case Study – Park Lake Dam Case Study – Park Lake Dam Conclusions Maximum lift equation does not work at low water surface elevation. This suggests significant vacuum pressures as reservoir level lowers Aluminum pipe was not strong enough to withstand vacuum pressures HDPE and ductile iron did work Case Study – Lower Dry Fork Dam Case Study – Lower Dry Fork Dam • Dam: Earthen • County, State: Sanders County, Montana • Storage capacity: 3,856 acre-feet • Elevation of reservoir water: 6,347 feet to 6,355 feet • Elevation of dam crest: 6,364 feet • Outlet elevation of siphon pipe: 6,307 feet • Blocked outlet works discovered at full pool • Irrigation needs and avoid emergency spillway Case Study – Lower Dry Fork Dam Case Study – Lower Dry Fork Dam Case Study – Lower Dry Fork Dam Case Study – Lower Dry Fork Dam Case Study – Lower Dry Fork Findings Maximum lift equation does not work at low water surface elevation Owner solution was to notch crest to reduce overall lift Bend radius of HDPE used to eliminate fittings Buried pipe to address vandalism concern Case Study – Crow Dam Case Study – Crow Dam • Dam: Earthen • County, State: Lake County, Montana • Storage capacity: 10,350 acre-feet (temporarily restricted to 3,500 acre-feet due to need for spillway repairs) • Elevation of reservoir water: 2,868 feet • Elevation of dam crest: 2,875 feet • Damaged outlet works • In-stream flow requirements Case Study – Crow Dam Case Study – Crow Dam Case Study – Crow Dam Case Study – Crow Dam Findings Grade limitation ruled out siphon Pumping system considered skid mounted pumps and submersible pumps with floatation Did not want to move suction lines with skid mounted and could not risk interruptions
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