TURBINE EFFICIENCY MEASUREMENT BY THERMODYNAMIC METHOD

Abstract

Efficiency is the ratio of the effective or useful output of any system to the total input to the system.Efficiency calculation of hydro-electric unit very essential as it can give the optimized operating condition of the machine. The performance of large turbines is determinedfirst bymodel tests and thereafter byfield tests. Model efficiency curves are usually determined to high degree of accuracy. However the conditions at the field may be differ from the model tests. The differences may be due to the construction of the civil parts or some error in the installations.Hence it is necessary to carry out field efficiency test.It also provides a check to 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 in the field 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 finding turbine efficiency in field. It is 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 low pressure and high pressure sides that occurs due to the heating of water when it comes in contact with runner 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 mK.Hence it is a very difficult and critical task to measure the temperature rise accurately.It requires sophisticated instruments and high-order of expertise for this purpose. 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 as well as in field, using stable temperature sensors, electronic pressure transmitters and high-precision,high-resolution data acquisition system. Overall efficiency has also been measured with the help of electronic pressure transmitter, ultrasonic transit-time flowmeter and precision class wattmeter. The results of both the methods are compared to the extent it is possible. The results of efficiency measurement of turbine using thermodynamic method have been found to be very satisfactory. iv