The use in the determination of the effective length of sand traps of hydroelectric plants of the procedure recommended in the norms indicates that a settlement probability W greater than 85-90 9o can be achieved only when they are very long. For this reason, in the TUiN norms it is recommended to limit the setdement probability to not over 80-9090, independently of the volume of sediments in the stream. This recommendation is basically unfounded, since for a large volume of sediments in a stream flow, the portion of the sediment which does not settle in the sand trap may exceed the quantity allowed according to the operating conditions of the turbines.The analytical graphs for the relations -:) recommended in the TUiN norms for determining the required length of sand traps, are constructed on the basis of relations obtained by integrating the settlement of sediments which are uniformly distributed over the entire depth of the flow. For this reason, the incorrectness of the analyses is clearly manifested only for high values of the settlement probability. However. for low values of the settlement probability, as wilt be seen below, the use of the procedure recommended in the TUiN norms also leads to the selection of an excessively long sand trap. The errors in this procedure can be seen by analyzing Eq. (38) in the TUiN norms, which is the initial expression used for constructing the analytical graphs in which S is the length of the settlement path of the sediment particles; w is the hydraulic size of these particles; v is the horizontal velocity in the sand trap; h is the depth of the position of die sediment particles in the initial section of the sand trap; and k is a parameter which characterizes the value of the probability for the settlement of the sediment particles. The relation between the values of k and W is presented in Table 2TUiN.Equation (1)
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