Introduction
FLOVEL successfully accomplished the challenging design of Kaplan turbine for the NILWANDE Project.

Parameters
The NILWANDE HPP is equipped with two (2) units of Kaplan machines running at synchronous speed of 428.6 rpm, operating under rated net head of 38.5 m. Each unit can generate output of 3,500 kW + 20% continuous overload. The powerhouse is situated at an elevation of 593.50 m above mean sea level with maximum temperature of (+) 400C. The scheme of the project and is a run-off the river type.

Plant parameters:

• Number of Units: 02
• Type of turbine: Vertical Kaplan
• Maximum Head: 58.5 m
• Maximum Design pressure: 7.5 bar
• Rated turbine output: 3,780 kW + 20% COL
• Synchronous speed: 428.6 rpm
• Runaway speed: 1,065 rpm
• Speed rise: 50%
• Pressure rise: 25%

Turbine type definition
The consideration for turbine selection is mostly concentrated in the following aspects:
Environmental – Economic – Technical – Operational

Since the heads in this project are in the higher side when considering the Kaplan application range, the possibility of Francis machines could not be discarded in a first approach. However, in the comparison with Francis machines the Kaplan alternative has the following positive aspects to be considered:

• Smooth operation to low flow
• Higher efficiency over a wide range
• Higher specific speed and rotational speed

In the other side we have the Francis alternative well known as less expensive machine solution.

In NILWANDE Project the Customer definition was that only Kaplan with vertical shaft system would be acceptable since the viability of project was based on annual energy generation and hence the entire range to be explored.

The final selection to meet the requirements of operating flexibility in a wide range of heads (18.5 to 58.5 m) as well as high maximum head for a Kaplan turbine presents the following main characteristics:

• Vertical turbine with metallic spiral casing.
• Direct driven generator.
• Runner diameter of 1,600 mm.
• Runner with 8 no. blades.
• Double regulated turbine.

The rotation speed was also unique selection for a Kaplan turbine and 428.6 rpm was the selected.

Design challenges
To design a runner with 1,600 mm diameter in which a set of 8 blades are required was the most demanding. In fact, the design should accommodate inside of a relatively small runner hub, the set of blades trunnions, blades mechanism i.e., links and levers as well as the runner servomotor. More difficult the task is when the maximum head is of 58.5 m, being close to Kaplan limit of around 70 m.

Based on all the above design requirement and constraints it was expectable that we would find forces, pressures, stresses and strains close to admissible limits. During first steps of design and calculation it was confirmed and one of the measures taken, as an example, was the material of runner hub. Conventional material used in regular applications was not strong enough to withstand the loads. Special steel material was applied.

The use of 3D technology in this complex type of design is crucial in avoiding mismatches and mistakes that can require major rework at shop assembly, which also can lead to a loss of strength of parts. Each volume of part was defined within the limits, not allowing further reduction due to not expected interference.

Due to space restriction inside runner hub the servomotor sizing required special care as well. Reduced space implies in transmission torque arms also reduced. High pressure oil system was select. The servomotor capacity was determined based on a design torque that is know from experience to be higher than the real one. An appropriate margin to account for friction in the mechanism was considered with standard design of self-lubricating bushings for lower friction operation. Under this overview the runner servomotor capacity was defined.

Due to space restriction inside runner hub the servomotor sizing required special care as well. Reduced space implies in transmission torque arms also reduced. High pressure oil system was select. The servomotor capacity was determined based on a design torque that is know from experience to be higher than the real one. An appropriate margin to account for friction in the mechanism was considered with standard design of self-lubricating bushings for lower friction operation. Under this overview the runner servomotor capacity was defined.

Observations and Conclusions
After the commissioning of machines in the NILWANDE HPP, the design validation was done based on the observation that:

• The selected Kaplan type is proven to be the right choice of machine from the operational point of view and it is understood that viability based on annual energy generation was achieved by Customer.
• Selection of correct number of blades was also confirmed as most suitable since performance and operation in a wide range was verified.
• Rotation speed as well was confirmed to be most economical solution considering generator selection.
• The proper design of the complex mechanism and perfect assembly have been previously checked during factory testing.
• Runner servomotor capacity was attested during commissioning tests.
• All the environmentally friendly features applied in this project are helping in its sustainability.