Horizontal turbine arrangement
Francis turbines are suitable for application in the range of medium to high head, roughly in between 10 to 400 m. Francis turbines are the most used water turbine by number of existing installations.

When we analyse the type of shaft arrangement, horizontal and vertical units are possible to be layout. The advantages of choosing a horizontal Francis arrangement is the design made very compact, assembly and maintenance access facilitated by the position of equipment and, as main consequence, the cost of the unit can be offered very competitive when compared with vertical one.

However, this type of configuration has limited capacity and size, in high-powered machines the vertical layout is the only possible. As a guideline in FLOVEL design, the limit for a Francis unit with horizontal shaft shall be specially checked for 15+ MW output, which is considered as high output for this configuration. Globally very few turbine manufacturers have the operational references for horizontal Francis turbines with high power output.

Design challenges
The design of PA HU Francis units can be situated in a borderline between the two possible layouts, horizontal and vertical. The maximum power output of more than 15 MW combined with the runner diameter of 925 mm and 2,8 t, in overhang mounting on generator shaft, demanded a stiff shaft line system to be designed.

To maintain the guaranteed high efficiency performance, the runner clearances must be kept to a minimum. The consistency of this requirement with the movement of the shaft was an important object of study in this project.

During the analysis of the shaft movement, the following possibilities should be checked and their overlap in the different operating conditions should be considered: bending deflection of the shaft line, fluid film dynamics in the oil film of bearings, stiffness of the generator bearings in different directions, thermal expansion from the cold stand still condition to the hot steady state condition and other possible movements.

The characteristics of high head turbine, with unit speed of 36 rpm, determined the selection of a Francis runner with radial shape, high ratio outlet/inlet diameter. In this configuration, runner design is provided with double buckets (14+14 buckets) to reduce blade pitch at the inlet and stabilize the flow, avoiding recirculation of water. This type of design also leads to high efficiency levels. Due to high mechanical stresses, it was welded fabricated, from integral forged and CNC machined crown with buckets, and separate forged skirt.

The very high design pressure required a sturdy design of the static structures as well as distributor mechanism. The verification of main structures was performed using FE analysis software.

Observations and Conclusions
After the successful commissioning of machines in the PA HU HPP in October 2020, the design validation was done based on the observation that:

•  Regarding the mechanical structural analysis, the integrity of the components can be attested after the completion of the test program during the acceptance phase of the project.
• Runner clearances were appropriately designed, to keep efficiency values within guarantee and avoid mechanical interference in any operational condition.
• The smooth commissioning and excellent operational parameters are a testimonial of FLOVEL’s robust and reliable technology for high power output horizontal Francis units.