Cavitation Draft Tube Analysis of Francis Turbine Loads Variations: A Case Study from Cirata Hydropower Plant

Abstract

Hydropower plant (PLTA) is a power plant of EBT (Renewable Energy), which utilizes water as a primary energy source. PLTA has a very important role in balancing the stability of network systems. In improving network system stability, hydropower is operated in a wide load range, frequent start-stop, and varying loads. Other functions are also as a buffer of peak load and back up system in case of interference in the network system. Francis turbine is the most widely used turbine and has a maximum efficiency of 93-95%. Problems related to the operation of hydropower are operational instability, cavitation, vortex breakdown, pressure shocks, vibration and noise that sometimes cause turbine system failure. In relation to the above conditions then in this thesis will be studied how the occurrence of cavitation due to variations loading. Case study conducted in this research is at Cirata hydropower plant with capacity of 126 MW per unit, which has type Francis turbine vertical shaft (variation of load starts 40% and 100%). The research method is done numerically, with modeling and flow simulation using ANSYS CFX 18.2. The results of this study show that at 40% load-operating conditions there is a very low-pressure area (under vapor pressure) and the emergence of a vortex rope that causes cavitation in the draft tube area, especially in the elbow region. While at 100% load operating conditions only slightly visible low-pressure area and no vortex rope appears.