Features of the Lake Powell Pipeline Preliminary Design to Minimize Hydraulic Transients

Abstract

The proposed Lake Powell Pipeline will convey up to 100,000 acre-ft per year from Lake Powell to the Sand Hollow Reservoir near Saint George, Utah. The pipeline covers a distance of 133 miles and has been defined as three separate subsystems; the Intake System, the Water Conveyance System and the Hydro System. The proposed intake system location is just upstream of Glen Canyon Dam on Lake Powell near Page, Arizona. The intake will include two vertical intake shafts, six horizontal intake tunnels and a pump station. The proposed Water Conveyance System includes four pump stations, two regulating tanks, one hydro station and a 69-inch diameter pipeline which will convey water from the Intake System to Regulating Tank-2 located approximately 50 miles northwest of Page, Arizona. The four pump stations will lift the water approximately 2,100 feet. The proposed Hydro System includes six hydropower stations and a 69-inch diameter penstock which will convey water from the High Point Regulating Tank-2 to Sand Hollow Reservoir located approximately 13 miles northwest of Saint George, Utah. Four of the hydro stations are twin jet impulse (Pelton) hydropower stations and are located along the penstock at strategic locations. The fifth hydro station is a 4 jet impulse (Pelton) station located at the base of the Hurricane Cliffs. The sixth hydro station is a twin jet impulse (Turgo) station, the Sand Hollow Hydro Station, and is located adjacent to the Sand Hollow Reservoir. The pipeline profile is divided into distinct hydraulic segments to limit the pumping lift and to limit the length of pipeline subjected to hydraulic transients caused by loss of power at a single pump station. A forebay tank is provided at each pump station as a free water surface to divide the pipeline into distinct hydraulic segments. Pumping station discharge pressures are limited to 300 psi to minimize equipment and piping cost. Pumped and most gravity portions of the pipeline are sized for maximum velocities of 5.6 ft per second to limit hydraulic losses and to minimize the potential for high hydraulic transients. Pumping stations are located to prevent pipeline profile high points from exceeding the drop in HGL during the worst case hydraulic transient event thereby avoiding negative pressure zones, vapor cavities and high pressure spikes from vapor cavity collapse. Hydropneumatic tanks are provided at each pump station as the primary hydraulic transient mitigation devices. Gravity segments are outlet controlled by an energy dissipating flow control valve or by a hydroelectric turbine. Flow control valves will be operated slowly to prevent large hydraulic transients. The hydroelectric turbines in the Hydro System will be the impulse type with deflectors at the spear valve nozzles. The deflectors will be used during load rejection to prevent turbine runaway while allowing the spear valves to be closed slowly to prevent large hydraulic transients in the pipeline.