Cavitation: The vacuum created when the discharge capacity of the pump exceeds the replacement in the suction line. Cavitation occurs when the pump is oversized, when the diameter of the influent line is too small or when there is an excessively long influent line. Cavitation causes bubbling and vibrations that can damage the pump if they occur over an extended period.

Centrifugal pump: The only type of pump used in a pool or spa circulation system. Uses centrifugal force created by the rotation of the impeller to power water through the circulation system.

Diffuser: A hosing or vaned part that fits around the impeller. The diffuser strips water from the rotating impeller and forces it out the discharge line of the pump, reducing the velocity of water through the pump while increasing the static pressure. In some pump models the diffuser and the volute are designed as a single part.

Flow Rate: The volume of water flowing past a given point during a specified time; measured in gallons per minute (gpm) or gallons per hour (gph).

Flow Meter: A gauge installed in the circulation line that measures the flow rate in gallons per minute (gpm).

Head: A measure of the amount of pressure or resistance in a hydraulic system. Resistance may include both the vertical distance and the amount of friction in a system and is measured in pounds per square inch (psi) or inches of mercury.

Impeller: A circular pump part with canes that rotate to provide centrifugal force for propelling water in the circulation system. The impeller spins on the end of the electric motor shaft and is the only moving part of a centrifugal pump. Closed , semi-open or open impellers are used.

• Closed impeller: A pump impeller with the vanes contained on both sides and an opening in the center (the eye). Water enters the eye and is centrifugally forced out the end of the closed impeller.
• Semi- open impeller: A pump impeller with the vanes exposed on one side; the open vanes face the volute wall, forming chambers through which the water flows.
• Open impeller: A pump impeller with exposed vanes like a water wheel. (Rarely used in pumps manufactured for use in the pool and spa industry).
• Pressure Differential: The difference in water pressure between two points in a hydraulic system.

Pressure Gauge: A gauge that measures the amount of pressure built up in a closed system.

Prime (priming the pump): Initiating or restarting the water flow in a circulating system; usually accomplished by filing the pump volute by hand, turning the pump on and allowing the pump to disperse all air from the influent line.

Pump Capacity: The volume of water a pump can circulate through a given resistance (total dynamic head) during a period of time; expressed as gallons per minute (gpm) or gallons per hour (gph).

Pump Curve: The curve on a bar graph that shows the performance characteristics of a pump. Variables include size, flow rate and resistance.

Self-Priming Centrifugal pump: A centrifugal pump that automatically removes air from the suction line and replenishes itself with water from within the pump, permitting uninterrupted water flow. In comparison, a flooded centrifugal pump would require manual priming, if air interrupted the flow of water through the pump.

Strainer Basket: A plastic mesh basket positioned in the strainer pot that traps debris in the water before it reaches the impeller.

Strainer Pot: A housing on the influent side of the pump that serves as a priming chamber and a receptacle for the strainer basket.

Total Dynamic Head: The sum of all the resistance in a pool and spa circulation system during operation.

Turnover Rate: The time required to circulate a volume of a given pool or spa.

Two-Speed Centrifugal Pump: A centrifugal pump that has a two-speed motor that turns the shaft and impeller at two rpm levels, providing an optional pump capacity.

Velocity: The speed of water that flows between two points; expressed in feet per second.

Velocity Head: The amount of energy required to accelerate water to a given velocity; expressed in feet per head.

Volute: A housing that contains the impeller and diffuser. (The diffuser is sometimes designed within the volute, instead of being a separate part.) Water is propelled through the volute's continuously enlarging spiral chamber, increasing the effect of the centrifugal force imitated by the spinning impeller.