Ngoi Phat Expansion Project – Vertical Pelton Unit with Generator directly mounted on the Turbine Housing
Introduction
This article aims at describing the challenging design of Pelton turbine for the NGOI PHAT Project in Vietnam.
Parameters
The turbine delivered by FLOVEL in NGOI PHAT is part of expansion project of the existing HPP. The supply is equipped with one unit of Pelton machine running at synchronous speed of 428.6 rpm, operating under rated net head of 289.4 m. This unit can generate output of 12.000 kW + 10% continuous overload. The powerhouse is situated at an elevation of 113.0 m above mean sea level.
Plant parameters
Number of units: 01
Type of turbine: Vertical Pelton
Maximum Head: 300.1 m
Rated Head: 289.4
Minimum Head: 282.5
Design pressure: 3,8 N/mm²
Rated turbine output: 12.000 kW + 10% COL
Synchronous speed: 428.6 rpm
Runaway speed: 770 rpm
Speed rise: 30%
Pressure rise: 15.5%
As the flow available in this project defines that number of jets is equal to four, the proper turbine arrangement is of vertical shaft.
A vertical configuration with four jet can be studied in two different solutions to support the generator loads:
- Generator supported on a concrete barrel with a turbine shaft to connect the Pelton runner and transmit the torque.
- Generator supported on the turbine housing with generator shaft extension direct connecting to Pelton runner.
The second solution can be a challenging solution when we are dealing with generators considerably big due to rotation speed and power output. The mass of generator interacting directly to the metallic housing and inducing forces of static and especially dynamic nature shall be verified accurately to guarantee the safety operation.
Design challenges
Housing in a Pelton turbine is of fundamental importance, as it houses the runner and nozzles, as well as the deflector servomotor. The specific design of housing in NGOI PHAT supports the complete generator structure of approximately 90 t. At the bottom of the housing, a structure for levelling and anchoring is designed. On the internal surface of the structure, the hydraulic profile of the tested model is observed and there is a proper provision to deflect the water jet and prevent interference of water deviated and splash with the rotation of the runner. The shape of the housing is polygonal.
Even though housing will be embedded in concrete, which will contribute to the dampening of vibrations, conservatively it is not considered its influence in both the structural verification and the dynamic analysis.
Structural analysis
Based on design guidelines and references of executed projects the main structure is defined including waterways envelopment as well as ribbing and reinforcement around. Special design for the generator supporting structure is done. After the basic design is finalized the structural analysis is carried out. In this analysis the process of design optimization takes place.
The loading cases include generator total weight plus the maximum torque transmitted to the structure, forces from the jets in different combination, forces coming from the connection with distributor pipe and loads from deflector mechanism.
The structural analysis was performed using FEA software and the safety for operation of the structure from the static point of view is granted by comparing the deformation and stress level against the allowable values with its corresponding safety factors.
Modal analysis
The modal analysis was performed also using FEA software. The dynamic behaviour of the housing is checked through calculation of the relevant natural modes of vibration and comparison with exciting frequencies.
As main exciting frequencies in this project it may be assumed:
- Nominal speed: 428.6 rpm = 7.14 Hz
- Grid frequency: 50 Hz
- Nozzle frequency: 4 x 7.14 Hz = 28.6 Hz
- Any other frequency is above 50 Hz and thus was not considered for evaluation.
Natural modes of vibration:
- Considering the complete assembly of housing and generator structures, the natural frequencies were determined:
Mode no.) Frequency in Hz
| (1) 16,32 | (2) 16,35 | (3) 20,32 | (4) 20,35 | (5) 22,64 | (6) 23,17 | (7) 23,63 | (8) 26,49 |
The modes 3, 4 and 8 were not considered as realistic results for the main structure since they have shown vibrations only of some minor structures, like thin walled cooler and power connection housings, which were simplified when modelling of the whole structure. The modes 6 and 7 reflects the modes of the rotating parts and this result are more precisely calculated with a separate dynamic shaft analysis.
The most important is the first frequency which is far enough from the nominal speed excitation thus no risk of not admissible vibrations.
Observations and Conclusions
After the commissioning of machines in the NGOI PHAT HPP in April 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.
- From the dynamic point of view, vibration levels verified during the commissioning tests and the beginning of the commercial operation of the unit can positively say in favour of a well-designed and analyzed project.
About FLOVEL:
FLOVEL name has been synonymous with the hydro power sector for over four decades. FLOVEL is a full-line supplier of electromechanical equipment and services for small and medium size hydro power plants manufacturer of hydraulic turbines, valves and turnkey supplier of electro mechanical packages for hydro power projects and ranks amongst the leading players globally. FLOVEL manufacturers complete range of Pelton, Francis, Kaplan, Axial Flow turbines and Valves for hydropower projects. The strength of FLOVEL is in its ability to offer all-inclusive solutions (Electro Mechanical-Turnkey Packages).
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