Transient Analysis of Pelton Turbine Prototypes

Abstract

One of the most critical operating conditions in hydraulic turbines is the start-up transient. This may be characterized by the presence of several resonances, which increase the vibration and stress levels on the runners. At present, its study is of interest due to the more frequent starts and stops of the turbines in the last years. The present study is focused on analysing the start-up transient of Pelton turbines. To do so, an experimental investigation was carried out in a horizontal shaft prototype unit. The procedure started by determining the natural frequencies and mode shapes of the turbine by means of impact testing. Several accelerometers were placed on the runner, on the shaft and on the bearings. With experimental modal analysis the main vibration modes of the rotor and the runner were found. Once the structural response of the turbine was determined, the start-up transient was studied. For the tests, different types of sensors were installed on the machine. Accelerometers, proximity probes and acoustic emission sensors were placed on the bearings and on the turbine casing. Moreover, an accelerometer and a strain gauge were installed on the shaft with an on-board system. Different situations are identified during the start-up. At the very beginning of the transient, the initial collision of the water jet on the still runner produces large vibration levels due to the excitation of many bucket modes. Once the turbine starts rotating, the vibration levels increase every time the jet excitation at the bucket passing frequency meets a natural frequency of the structure. The effect of the transient regarding the change in the vibration levels and runner deformations is presented in this paper. The sensitivity of the different sensors to measure the runner vibrations is analysed in order to optimize the detection of the transient effects. The best locations to install the sensors are also identified.