A new general expression is developed for the hydrodynamic design of sieve plates, namely for the calculation of total pressure drop and its components. The division of total pressure drop into part pressure drops is theoretically based on the momentum theory and fundamental laws of fluid flow. Applying and developing the results of Haq3 and Davy and Haselden it was established: (1) The partial pressure drops, except that of wet plate pressure drop, are practically independent of surface tension, liquid density and viscosity. (2) The wet plate pressure drop is a parabolic function of gas velocity. It is practically independent of liquid density and viscosity in wide range, but strongly depends on surface tension. In conclusion a general expression is presented for total pressure drop of sieve plates, which is theoretically established, valid in all the three foaming regimes and expresses the influence of surface tension.