Submergence Factors for Hydraulic Turbines

Abstract

The elevation of a hydroelectric turbine above or below tailwater is based on hydraulic model studies to determine the critical sigma, defined as the ratio of barometric pressure plus submergence depth at which cavitation commences divided by the rated head. Turbine model studies are undertaken by manufacturers to determine the sigma value for each model runner. This information is properietary and is never published. Consultants and utilities need to know the value of sigma during feasibility studies to establish unit setting and power plant layout. For this information, reference is usually made to published data relating sigma to other turbine parameters such as the specific speed. However, this method can result in erroneous settings, since there is a range of sigma values for each specific speed. This paper develops an alternative approach based on an analysis of over 80 recently installed turbines, wherein the turbine submergence is shown to be a function of the turbine throat velocity, the number of runner blades or buckets, the atmospheric pressure and the water temperature. The equation is applicable to Kaplan, propellor and Francis turbines with either horizontal or vertical axes. From this equation, another is derived to determine the turbine throat diameter as a function of the desired setting above or below tailwater.