What type of pump uses small movable elements called vanes to maintain a seal against the pump casing?

The type of pump that utilizes small movable elements known as vanes to create a seal against the pump casing is the rotary vane pump. This design features blades or vanes that are mounted on a rotating shaft and slide into and out of a cylindrical casing. As the shaft rotates, the vanes create varying volume compartments that trap the liquid and move it through the pump. This mechanism allows for efficient fluid transfer and significant suction capabilities.

In the context of other options, while a positive-displacement pump does encompass designs that create sealed movement of fluid, it refers to a broader category that includes various types of pumps, not specifically the vane mechanism. A centrifugal pump relies on rotational energy to move fluid, creating a pressure difference that directs flow, but it does not utilize vanes for sealing. Lastly, a fire pump is a specific application of pumps designed for fire-fighting purposes but does not indicate a particular mechanism like rotary vanes. Thus, the uniqueness of the rotary vane pump’s design is what makes it the right answer in this context.