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<br />0'01659 <br /> <br />The cavitation index is an extremely useful parameter to indicate the state of cavitation in a <br />hydraulic structure. In general, cavitation will not occur if the cavitation index is greater than <br />about 1.8. As the cavitation index decreases below 1.8, cavitation bubbles begin to form within <br />the flow, and cavitation damage may occur at flow surface irregularities, such as offsets into the <br />flow, offsets away from the flow, abrupt slopes away from the flow, voids or transverse slots, <br />roughened surfaces, and protruding joints, when the bubbles collapse and produce large impact <br />pressures against the structure surfaces. Major damage can occur as the cavitation index <br />decreases below about 0.3, when large supercavitating vapor bubbles can form. Cavitation <br />damage will always occur downstream from the source of the cavitation. The rate of cavitation <br />damage will increase as the roughened surface produces more sources of vapor bubbles. Loud <br />noise and vibration may be detected during cavitation [4]. <br /> <br />Based on the results shown in table 1, the areas of the main outlet works most susceptible to <br />cavitation damage during large discharges will be at the 30-inch and 1 IO-inch wye branch <br />locations, at the bifurcation to the two 72-inch-diameter pipes, and at the hollow-jet valves. The <br />wye branches and bifurcations include abrupt slopes away from the flow, and vertical tie rods <br />within the flow, which may be particularly wlnerable to cavitation damage or damaging <br />vibrations. Minor cavitation damage has been reported previously for the hollow-jet valves, <br />particularly where the splitter joins the outside shell of the valve body [5]. Some cavitation may <br />also develop at the 20-inch-diameter manhole locations, which provide an abrupt offset away <br />from the flow. The upstream elbow at the intake structure may be subject to some potential <br />cavitation damage, but cannot be inspected without installation of the upstream circular bulkhead <br />at the bellmouth entrance. <br /> <br />Operation of the 30-inch hollow-jet valve concurrently with the two 72-inch valves would only <br />increase the total outlet release capacity by about 100 ft:ls, or less than 3 percent. Estimated <br />releases would be approximately 3,900 fi3/s from the 72-inch valves, and 200 ft:/s from the 30- <br />inch valve, for a combined release of 4,100 fi3/s at reservoir elevation 6065. This would have a <br />negligible effect on the computed cavitation indices for the main outlet works, and would produce <br />an average flow velocity in the 30-inch-diameter pipe of about 40 ftIs, with a minimum cavitation <br />index of about 3.5. However, concurrent operation of the 30-inch and 72-inch hollow-jet valves <br />in the past has produced turbulence and vibration in the operation of the 72-inch valves, and rapid <br />cavitation noises from the upstream 1 IO-inch-diameter pipe [5]. The March 1980 Review of <br />Operation and Maintenance (RO&M) Examination Report [6] recommended that both the <br />Designers' Operating Criteria (DOC) [7] and the Standing Operating Procedures (SOP) [8] be <br />amended to prohibit concurrent operation of these valves (Recommendation No. 79-2-1). <br /> <br />The preceding discussion assumed no power plant releases. Operation of the two 72-inch hollow- <br />jet valves concurrently with the power plant would significantly increase upstream head losses, <br />resulting in a significant reduction in the net operating head on the power plant units. This <br />operation is evaluated further in part D below. <br /> <br />lfthe existing 72-inch hollow-jet valves could be replaced with more efficient gates ofa similar <br /> <br />4 <br />