Abstract
Hydraulic systems continually experience dynamic transients or oscillations which threaten the hydroelectric plant from extreme water hammer pressures or resonance. In particular, the minimum pressure variations downstream of the turbine runner during the load rejection or other events may cause dangerous water column separation and subsequent rejoinder. Water column separation can be easily observed from the measurements of site transient tests, and has indeed caused serious historical damages to the machine and water conveyance system. Several technical issues regarding water column separation in draft tubes, including “S” instability of turbine characteristic curves, numerical instability and uncertainty of computer programs, are discussed here through case studies and available model and site test data. Catastrophic accidents experienced at a Kaplan turbine and in a long tailrace tunnel project, as well as other troubles detected in a more timely fashion, are revisited in order to demonstrate the severity of reverse water hammer. However, as there is no simple design solutions for such complex systems, this paper emphasizes that the design of hydraulic systems is always difficult, difficulties that are compounded when the phenomena in question are non-linear (water hammer), dynamic (involving wave interaction and complex devices of turbines, controls, and electrical systems), and non-monotonic (severity of response is seldom simply connected to severity of load as with vibrations and resonance, and the complexity of transient loads), and thus may lead to high economic and safety challenges and consequences.