Pressure fluctuations in pressure conduits of hydroelectric and pumped-storage stations

Abstract

The dynamic loads absorbed by pressure conduits of hydroelectric stations (HESs) and pumped-storage stations (PSSs) are composed of water:hammer, occurring during transient processes and caused by a change in the operating regime of the unit, and of fluctuations occurring both during steady and transient processes. Whereas the phenomenon of water hammer has been studied rather thoroughly and many scientific investigations and works have been devoted to this problem, the problem of fluctuations is still in the initial stage of study. Yet fluctuations can have a substantial effect on the reliability and life of pressure conduits and equipment and in connection with this their investigation and development of methods of taking them into account during design and operation is an urgent problem.Various types of pulsation loads are possible in pressure conduits. Sometimes selfexcited pressure fluctuations occur, which are created in a hydroelastic system having a device acting on the flow rate and characterized by the fact that with increase of head (pressure) the discharge being passed decreases. This system is hydraulically unstable and strong and even dangerous fluctuations with a pronounced resonance frequency can occur in it. Such fluctuations induced by vibrations of a spherical valve were noted, for example, at the Bersimis HES [6]. Stretches of unstable flow, for example, in forks, which occur in the presence of an unbalanced relationship of the discharges of the branches, when fluctuating stagnant zones are created, can be exciters of pressure fluctuations. But these are all special cases. The most common are forced pressure oscillations induced by disturbances created by the operation of the units. It is necessary to note that the nature of these disturbances has still notbeen completely revealed and on this account various hypotheses are expressed. It is considered that the cause can be vane disturbances (impacts) [I, 2], rotational separation of the flow in which the axial symmetry of flow past the circular rows of the blades of the unit is disturbed [3], and vortex filaments behind the runner and their movement (precession) in the draft tube [2, 4, 8, 9, i0]. Obviously, all the factors mentioned can act to a greater or less degree depending on the geometry and operating regime of the units.For calculations of pressure fluctuations in pressure cond@its the resultant action should be expressed in the change in the discharge being passed by the unit at a given head or in the change in the head being developed at a given discharge. So long as there is insufficient data on the actual characteristics of the disturbances it is ncessary to base oneself on some kind of working hypothesis. In the majority of works it is assumed that the disturbance can be represented in the form of the fluctuation of the discharge capacity or reduced discharge AQ'_.I for a given opening of the unit. Then the change in the discharge in dimensionless parameters Aq will be represented by the equa~lon Aq = Aq~ ~l + ~ + +~,where Aq = AQo...

Characteristics of the calculation of surge tanks of hydroelectric stations

“…In other limits the values calculated by Eqs. (7) and (8) are greater than those taken according to [5].…”

“…The coefficient k s is found from the expressions: The value of k s is usually assumed constant in calculations [5] regardless of the relationship of the discharges in the diversion and surge tank, which substantially simplifies calculation of fluctuations. Actually, the coefficient k s retains a value close to constant in the range Qs/Qd = 0.3-1.0 and Qs/Q t = 0.3-1.0.…”

“…Actually, the results of investigations of forks [2,5,6] show that the head losses in a branch can include a variable component of the velocity head; the structure of the losses in the transition changes as a function of the direction and relation of the water discharges in the branches.…”