Definition of coefficients for the downstream valve of a Francis turbine

In a storage power plant the pump as well as the turbine are operated simultaneously on one shaft during hydraulic short-circuit operation mode as control features of storage pumps are only poor or non-existent. Whenever the total capacity of the pump is not needed the exceeding power is being operated by the turbine which is much easier to regulate. In case of parallel operation of pump and turbine, the interaction between plant and hydraulic machines differs from the interaction when operated separately, and in exceptional circumstances a reverse flow through the Francis turbine can occur. And thus, it can also happen that the shut-off valves of the turbine are passed in both directions. In order to ensure safe and correct functioning of the shut-off valves during all operational modes, knowledge of their hydraulic behaviour during all these operational modes possible is essential.

The Institute for Hydraulic Fluidmachinery received the order to define the valve coefficients (flow rate and torque) for the downstream valve of a Francis turbine in a storage power plant. By means of measurements the development of these parameters as function of the valve opening should be determined. Therefore, a model of the valve (scale of 1:13) as well as its implementation into the test cycle had to be designed and constructed and then installed and measured on the Institute’s 4-quadrant test rig. Additionally, the cavitation behaviour of the model valve has been examined. In order to enable the visualisation of the flow situation, especially in cavitation mode, nearly the whole valve housing was made of acrylic glass.

As of its construction, the valve exhibits high eccentricity and thus the differences regarding the valve parameters especially for the torque of the two flow directions differ significantly. The cavitation behaviour has not only be measured but also comprehensively documented by means of photographs.