Water Injection In Draft tubes
DiffCon shoots water tangentially into the exit pipe of Francis-turbines to stabilize the flow field, reduce pressure pulsations and loads on the shaft. By installing DiffCon, power plants can increase the operating range of Francis-turbines and especially utilize partial load operation.
Together with Norwegian power companies, FDB has been developing DiffCon since 2004. This partnership has continued and DiffCon is now being developed by FDB in collaboration with Statkraft. The basic principle is that the system tries to control the total rotation within the hydro turbine draft tube. The total rotation is defined as the integral value of the product of tangential and axial velocity over the cross-sectional area. The flow becomes naturally unstable for given load settings and where large vortices and pressure pulsations dominate. By injecting water at high speed into the draft tube, this instability is mitigated and resulting in a calmer machine.
FDB offers design of complete injection system, including:
- Planning of high-pressure water abstraction
- Control valves
- Control- and instrumentation solutions
- Injection nozzles
FDB also provides support and supervision for the mechanical work in the station.
Håkon Francke in FDB investigated the physics behind draft tube injection and optimized design parameters for DiffCon in his PhD project at NTNU.
We have so far installed four DiffCon systems:
Schematic diagram of DiffCon on Francis straws
DiffCon User Interface (HMI)