scroll

A scroll is a curved, spiral-shaped passage that surrounds a turbine runner and directs incoming fluid into the buckets or blades with minimal turbulence. In a centrifugal pump or turbine, the scroll converts the high-velocity discharge from the impeller into lower-velocity flow suitable for the next stage or outlet, reducing energy loss and vibration. The shape is not a simple spiral but a gradually expanding chamber that maintains roughly constant velocity as fluid travels around the circumference, distributing flow evenly across all inlet ports to the runner.

Scrolls appear in several contexts. In turbomachinery, a pump scroll collects fluid ejected radially from the impeller and channels it toward the discharge; a turbine scroll (also called a spiral case) distributes incoming water uniformly around the entire runner circumference before it enters the guide vanes. The scroll must be sized correctly for the design flow rate; undersized scrolls cause excessive pressure drop and heat generation, while oversized ones reduce efficiency and increase machine footprint.

Design and Performance

The scroll cross-section typically tapers as it wraps around the runner, following an involute or logarithmic curve in well-designed units. Metal scrolls are cast as part of the pump or turbine casing, often in ductile iron or bronze for corrosion resistance in water service. The volute angle, tongue radius, and exit geometry all affect how cleanly flow enters the impeller or turbine; poor design creates dead zones where cavitation can initiate or where swirling vortices waste energy. On centrifugal pumps, the gap between the impeller tip and the scroll inlet (the cutwater or tongue) is critical; too large and recirculation occurs, too small and blockage results.

The term scroll likely derives from its resemblance to a rolled parchment when viewed from above. In some European hydraulic traditions, the scroll is called a volute, particularly in pump nomenclature, though scroll is standard in turbine applications. The scroll's purpose is fundamentally to match the geometry of the rotating element to the fixed piping, preventing the sudden expansion or contraction that would generate noise, cavitation, and pressure surges.

Wear and erosion in scrolls typically occur at the tongue and in high-velocity zones where abrasive particles in the fluid scrape against the walls; this narrows the effective passage and degrades performance over years of operation. Cavitation damage appears as pitting on the low-pressure side of the scroll wall, a sign that local pressure has dropped below vapor pressure. In long-term service, scrolls can be inspected for erosion damage and, in some designs, bored out or relined to restore clearances.