Hydraulic Similarity of Headloss Predictions Derived Using Commonly Used Methods Versus Actual Results as It Relates to Wastewater Screen Elements

Experience has shown the predictive reliability of two commonly used equations for the prediction of headloss (known as the Bernoulli principle and the Kirschmer method) may be questionable under certain conditions, especially at higher flow velocities through fractional openings. To quantify this observed disparity, a series of tests were executed to compare predicted headloss—employing the results of both these equations—to actual results under controlled conditions. Test results indicate that the headloss predicted by the Bernoulli principle consistently overstates actual headloss through screens by a significant amount. The Kirschmer method correlated much closer to actual headloss, however it proved to be less than satisfactory for some openings or bar shapes. Study of the results of these tests, along with careful examination of the two equations, indicate that better correlations for headloss prediction versus actual measured values could be obtained by modifying elements of both the Bernoulli and Kirschmer equations, or by applying Computational Fluid Dynamics (CFD).