Kaplan turbine construction and working

The Kaplan turbine is an axial-flow reaction turbine used for low heads (up to about 60 m) and very large discharge. Its runner looks like a ship's propeller, with blades that can be rotated about their own axes.

        Spiral casing
        +---------------+
        |  guide vanes  |  wicket gates
        |  ===========  |
        |      ||       |   water turns 90 deg
        |      \/       |   and flows axially
        |   [HUB]       |
        |  / | | | \    |  <- adjustable blades
        +--------+------+
                 |
            Draft tube

Construction

• Scroll casing and guide vanes like a Francis turbine; water enters radially and is turned through 90 degrees by the vanes before reaching the runner (a space without vanes, between guide vanes and runner, helps the whirl to become free-vortex).
• Runner: a hub (boss) carrying 3 to 8 blades. The hub is streamlined and is about 0.35 to 0.6 of runner diameter.
• Blade turning mechanism: inside the hub, a servo-motor operated by the governor turns the blades.
• Draft tube: usually elbow type; very important for recovering the exit kinetic energy.

Working

Water flows axially through the runner blades. The guide vane opening and the blade angle are adjusted together by the governor with the load, so the water always enters the blades without shock. The pressure drops across the runner, and the axial change of whirl momentum gives torque. Exit is axial and without whirl at design point.

Advantages

• High efficiency (about 90 %) over a wide range of load, since both guide vane and blade angles adjust. The efficiency curve is flat.
• High specific speed, so a small turbine and generator, and a high rotational speed, suit low head.
• Suitable for sites with variable head and discharge.

Kaplan versus propeller turbine