The spiral casing spreads the water all around the distributor at constant velocity. Its inlet diameter is chosen from the material.

Overview: Hydro Turbine design

The spiral casing, or volute, brings the water from the penstock all around the distributor. It narrows as it gives water to the guide vanes, to keep a constant velocity. The Spiral casing tab sets whether it exists, its inlet diameter and the flow direction it gives to the distributor.

Contents:

• With or without casing: flow direction at the distributor inlet
• Casing inlet diameter and water velocity
• Recommended casing velocity by material

With or without casing: flow direction at the distributor inlet

• Inlet from casing: the water enters the distributor with a tangential component, that of the casing. Inlet direction = Atan(Vr inlet / Ve casing), Vr being the radial velocity at the distributor inlet and Ve the velocity in the casing.
• No casing: the water arrives radially, inlet direction = 90°.

The resulting angle feeds the "inlet angle of attack" loss of the distributor: guide vanes badly oriented with respect to this flow create a shock loss.

Example

Top view with casing and distributor inlet details: the flow direction at the guide vane inlet.

Casing inlet diameter and water velocity

• Use recommended: the diameter follows from the allowable velocity of the material chosen in the Recommended speed block.
• Input: you impose the inlet diameter (mm).
• Casing Ve: inlet velocity at maximum flow, derived from the diameter. It cannot exceed √(2·g·Hn).

The casing narrows along its path, in eight sections S1 to S8 shown in the table of the tab, as it gives water to the distributor: it thus keeps this velocity constant.

Examples

The table of the eight casing sections, S1 to S8: decreasing areas and diameters.

The casing seen from above in 3D: the water velocity stays roughly constant along the spiral.

Recommended casing velocity by material

Three rules, from the A. Tenot reference, are chosen by radio button:

• steel or cast iron casing: Ve ≤ 0.2·√(2·g·Hn), capped at 12 m/s;
• riveted sheets: Ve ≤ 0.15·√(2·g·Hn), capped at 12 m/s;
• concrete: Ve ≤ 0.14·√(2·g·Hn), not exceeding 3 m/s because of erosion.

The block shows the recommended velocity and the matching inlet diameter, which the Casing inlet D block applies when "Use recommended" is selected.

Example

The spiral casing in 3D: its inlet diameter follows from the velocity allowed by the material.

Going further

The Hydro Turbine design reference page presents the complete method and a worked case. The other help pages of the window:

• Control panel
• Layout
• Spiral casing
• Distributor
• Runner
• Draft tube
• Specific speed
• Statistics
• Views and piezometric graph
• Fabrication kit
• Results, method, errors