HYDR9 TURBINE EFFICIENCY MEASUREMENT BY THERMODYNAMIC METHOD

Abstract

Efficiency is a percentage representing the actual rate at which work is done, divided by the theoretical rate at which work could be done if the machine converted all available energy to work. Usually the performance of large turbines is determined first in model tests and thereafter at field. Model efficiency curves are usually determined to high degree of accuracy. However prototype turbine installations always have some differences from their model, which alter their performance. These differences are caused by efficiency measurement set-up and scaling effects, in addition to the external influencing factors. Therefore efficiency testing of turbine is generally carried out in the field to check contractual guarantees, adjust blade/ gate mechanism for optimum efficiency, obtain information that can be used to evaluate the machine wear and cavitation degradation and to obtain accurate information on water use. Efficiency can be evaluated either by discharge-head measurement or by using thermodynamic method. Discharge-head measurement method is used for overall efficiency measurement of a hydro-electric generating unit and many assumptions are required to find out the hydraulic efficiency of the turbine. The thermodynamic method, on the other hand, gives directly the hydraulic efficiency of the turbine, and to find out the overall efficiency of hydroelectric generating unit many assumptions will be required to be made. The thermodynamic method is an absolute method for the hydraulic efficiency measurement of hydro machines, based on the principle of conservation of energy, i.e. first law of thermodynamics. It does not require discharge measurement, which otherwise is the most complicated and tedious task in the whole process of efficiency evaluation by discharge-head method. Hydraulic efficiency can be calculated directly from the specific hydraulic energy and specific mechanical energy equations. Friction losses in the machine are proportional to the temperature difference of water between high pressure and low pressure sides which occurs due to the heating of water when it comes in contact with runner / impeller of the machine. The temperature difference between the inlet and outlet of machine is very small and is typically of the order of a few mili-Kelvins. Hence it is a very difficult and critical task to measure the temperature rise accurately. For actual measurement of temperature difference, high-resolution and high accuracy data acquisition system along with precision type temperature sensors is required. Efficiency measurement using thermodynamic method has been carried out by the author in laboratory, both on a turbine and a pump, using stable temperature sensors, electronic pressure transmitters and high-precision, high-resolution data acquisition system. Thermo-wells are formed at inlet and outlet of turbine and pump for simultaneous temperature measurement at both the points. High-precision RTDs of PT-100 type (class 1/10 DIN, 0.03 °C) are used for the measurement of temperatures. The results of efficiency measurement of turbine and pump in laboratory using thermodynamic method have been found to be very satisfactory. The same instrumentation with suitable adaptation can be used for the efficiency measurement of turbine and pump on site.