Which of the following is NOT a type of impeller casing?

• A. Single volute
• B. Double volute
• C. Diffuser
• D. Open volute

Answer: (D)

Understanding impeller casings means recognizing how the casing guides flow and builds pressure around the rotating impeller. In centrifugal pumps, the casing is typically a volute or a diffuser. The volute is the spiral-shaped passage that collects the fluid as it leaves the impeller; you can have a single volute, where flow from one side is collected by one spiral, or a double volute, which uses two spiral paths to accommodate a double-suction impeller and balance the flow. A diffuser is a different arrangement that uses stationary vanes around the impeller to gradually slow the fluid and convert kinetic energy into static pressure; it’s a distinct approach to shaping the flow, not simply a variation of a spiral volute. Open volute, however, is not a standard category in typical pump casing classifications; it’s not commonly listed as a formal type alongside single volute, double volute, or diffuser. Therefore it isn’t considered a conventional type of impeller casing.