FDB System – DiffCon – Waterinjection in Draft Tube

DiffCon shoots water tangentially into the draft tube, or turbine exit pipe, in order to stabilize the flow field, reduced pressure fluctuations and loads on the shaft dynamic system. By applying DiffCon operators can increase the safe operating range for Francis type turbines, and especially exploit the low load regions.

In 2004 FDB started developing the DiffCon- system together with Norwegian hydropower utilities. The partnership continues, and FDB now develops the system jointly with Statkraft. The basic principle behind the system aims at controlling the total swirl, defined as the integral value of the product of tangential and axial velocity over the cross-section are. For a certain swirl range, the flow becomes naturally unstable and large-scale vortex structures associated pressure pulsations appear. By injecting high-speed water, the DiffCon system moves the swirl outside the range resulting in such instabilities thus providing a calmer turbine unit.

FDB provides design of the complete system including:

  • Planning of high-pressure water intakes
  • Pipe layout
  • Control valves
  • Control and instrumentation solutions
  • Injection nozzles

FDB also engineer and supervises mechanical work at the power plant.


As of now, four installations of the DiffCon system exist:


Dr. Francke at FDB completed his PhD study at NTNU by investigating the physics behind, and optimizing design parameter combinations for the DiffCon system.

Principal sketch of DiffCon on a Francis Draft tube

Figure1: Principal sketch of DiffCon on a Francis turbine draft tube


Figure2: DiffCon userinterface (HMI)