Spiral Case

in a hydroturbine, a housing designed to provide uniform water intake around the entire circumference of the distributor, that is, to permit all the guide vanes to be held in an axially symmetrical position. The cross section of a spiral case narrows at a constant rate downstream. Hydroelectric power plants operating under heads greater than 50–60 m use steel spiral cases with circular cross sections that shroud the stator almost completely. Spiral cases used with lower heads are made of reinforced concrete; they shroud approximately 225° of the stator’s circumference, and their cross sections are T-shaped. Unlike other hydroturbine settings, for example, open settings, spiral cases make it possible to locate a major part of the turbine mechanism outside of the water, which improves the turbine’s operating conditions.