Laserfiche WebLink
<br />001674 <br /> <br />The hollow-jet valves were releasing 3,700 ft3/S at reservoir level 6057.07. Shortly after <br />discovering the surging, operators reduced flows to 3,500 tt:/s and the surging quit. On June 5, <br />1979, engineers from Denver visited the site to examine the outlet works for evidence of <br />cavitation and to investigate the reported surging [16]. Tests were conducted over the full range <br />of valve openings. No surging could be detected at any operation. Leaks in the fixed-wheel gate <br />bonnet seal were noted and repairs were recommended. A subsequent analysis of possible <br />forcing functions that may have induced the surging suggested low axial damping of the fixed- <br />wheel gate stem and leakage through the bonnet seal were possible factors. Replacing the bonnet <br />seal and, should surging occur again, increasing the damping of the stem were recommended. To <br />our knowledge, no further occurrences of surging in the 72-inch-diameter pipes have been <br />reported. <br /> <br />B. Auxiliary Outlet Works <br /> <br />1. Air slot modification. - Between 1967 and 1972, an air slot modification was made to the <br />auxiliary outlet works discharge tunnel downstream of the 4- by 4-foot regulating gate. The slots <br />were field tested in June 1972 [17]. Tests were run with gate openings from 10 percent to 100 <br />percent (48-1/2 inches). Air was supplied to the air slot through a 1,270-foot-long air supply pipe <br />and via the downstream tunnel above the flowing water. The test data resulted in a <br />recommendation to limit the maximum gate opening to 47-1/2 inches (3.95 feet), or about 98 <br />percent of full gate travel. The test results and the reasoning for the gate opening limitation are <br />described in the travel report as follows: <br /> <br />Test results revealed that the water jet was adequately aerated at gate openings up to and <br />including 47-3/4 inches. At this gate opening, the most severe subatmospheric pressure <br />was 8.6 feet of water on the vertical face of the bottom flow surface deflector underneath <br />the water jet. When the gate opening was increased to 48 inches, the subatmospheric <br />pressure at this location was 13.8 feet of water. With maximum gate opening of 48-1/2 <br />inches and a barometric pressure of 27.6 feet of water, the pressure on the vertical face <br />of the deflector was 24.5 feet of water subatmospheric. At this time there was no airflow <br />in the gate air supply pipe. The water jet evidently impinged on the air inlet ports in the <br />top flow surface downstream of the gate leaf and choked off the air supply. <br /> <br />The velocity of the water jet at the bottom flow surface deflector was about 100 feet per <br />secont/with gate openings of 47-3/4 inches to 48-1/2 inches. This created a velocity <br />head of 155 feet of water which would resist deflection of the jet by adjacent <br />subatmospheric pressures. However, the larger subatmospheric heads could conceivably <br />draw enough water onto the bottom flow surface to create a pool of water at the deflector <br />spring point and result in interference of the water jet trajectory. Also, vapor pressure <br />could exist in the water jet under full open gate conditions and cavitation erosion could <br />result where the ~ater jet impinges on the downstream flow surfaces. <br /> <br />Conclusions: The auxiliary outlet water jet was adequately aerated at gate openings up <br /> <br />17 <br />