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<br />OOlG'j8 <br /> <br />this valve would provide no benefit to safety with respect to the valve itself <br /> <br />3. 72-inch ring-follower gates. - The 72-inch ring-follower gates are hydraulically operated and <br />are primarily intended for maintenance or long tenn sealing of pressure upstream of the 72-inch <br />hollow-jet valves. They are not intended for regulating the flow or more than occasional <br />emergency shutoff. They are operated by the same hydraulic operating system as the 72-inch <br />hollow-jet valves, and as such, have also used the same contaminated oil as the 72-inch hollow-jet <br />valves. <br /> <br />Due to maintenance considerations, the ring-follower gates have become the main emergency shut <br />off, although they are not designed as such. Emergency closure tests were successfully perfonned <br />on the ring-follower gates in 1993, when the gates were opened to 10 percent and then closed <br />under the full available differential head [21]. To continue operating with a margin of safety, the <br />gates should be tested according to the procedure provided in Appendix D, to verifY their <br />condition through their operating range. <br /> <br />In addition to the cylinder verification test, the head on ring-follower gate No. 1 should be <br />removed. The hollow piston should be cleaned out and any debris should be examined and its <br />source determined. If substantial wear metals or seal damage is present, further investigations or <br />repairs may be necessary. <br /> <br />I <br />~ <br /> <br />4. Wye branch tie rods. - There are two 8-inch-diameter tie rods within the main outlet works <br />pipe, one located at the wye branch to the power plant (shown on drawing No. 711-D-310), and <br />the other located at the bifurcation upstream of the hollow-jet valves (shown on drawing No. 711- <br />D-309). Over the years there have been a number of tie rod failures at Reclamation facilities. <br />When a tie rod fails, there are two possible consequences: 1) the tie rod breaks away and is <br />carried downstream, causing damage to the outlet valve, gate, or power unit, and 2) depending on <br />the amount of restraint provided by the embedment material around the failed tie rod area of the <br />pipe, the stiffening member(s) to which the tie rod was attached could fail. <br /> <br />Under certain flow conditions, "streets" of vortices can fonn behind a "bluff", or circular <br />member. The energy in the vortices gets fed back to the member, causing amplification of the <br />force on the member. Depending on the stiffuess and end restraints of the member, vibration of <br />the member can result. The member will vibrate at the frequency of the forcing source, or at the <br />vortice frequency. The vortice frequency is a function of the Strouhal number, velocity of the <br />fluid, and diameter of the member. If this forcing frequency corresponds to a natural frequency of <br />the member, the member will resonate and the energy being transferred from the water will be <br />magnified. The rate of magnification is a function of the damping of the member. <br /> <br />A tie rod is a classic example of a bluff member. It is circular and has a relatively long length, or <br />span, when compared to its diameter. The tie rod in water has a very low damping value, and <br />consequently there will be high magnification of the exciting forces if resonance occurs. <br /> <br />21 <br />