Laboratory Method to Evaluate Fog Rejection Effectiveness of Hydrophobic Coatings for Steam Turbine Applications

Abstract

The application of hydrophobic coatings in steam turbines is expected to partially inhibit the formation of water films on blade surfaces, and consequently reduce the associated efficiency loss and mechanical damage. Since testing coatings in real steam turbines is impractical, a method is presented to assess, under laboratory conditions, the tendency of materials to reject rather than collect water droplets impinging on them. This behavior was demonstrated to poorly correlate with wetting angle measurements which proved the necessity to use a more complex approach. A test rig was therefore used which allows evaluating fog-rejection effectiveness, by placing coated specimens on a rotor and exposing them to a flow of saturated steam carrying micron size droplets. Steam condition, impact angle, droplet size and velocity are kept close to what is expected in low pressure steam turbines. While rotating, the formed water film is centrifuged and captured in tanks within the supports holding the coated specimens. The mass of their content is measured upon experiment completion. Having tanks in 4 different locations along a specimen’s edge gives an additional insight about variations in collection location and film flow direction. Repeatability and parameter variation tests have shown that the collection is reasonably insensitive to the changes in operation conditions, yet the scatter in the data remained high. Nevertheless, statistical analysis showed that the method is able to clearly detect differences in collection behavior, thus enabling the industrial partner to select coatings that provide enduring steam turbine efficiency increase.