AFFINITY LAWS: A set of relationships that tie together pump performance characteristics such as pressure, flow, and pump speed.

ASD: (Adjustable Speed Drive). Devices that allow control of a pump’s rotational speed. ASDs include mechanical devices such as hydraulic clutches and electronic devices, such as eddy current clutches and variable frequency drives.

BEP: (Best Efficiency Point). Commonly used to describe the point at which a centrifugal pump is operating at its highest efficiency, transferring energy from the prime mover to the system fluid with the least amount of losses.

DYNAMIC HEAD: The component of the total head that is attributable to fluid motion (also known as velocity head).

HEAT EXCHANGER: A device that transfers heat from one fluid to another.

MECHANICAL SEAL: A mechanical device for sealing the pump/shaft interface (as opposed to packing).

MFR: (Minimum Flow Requirement). A manufacturer specified limit that represents the lowest flowrate at which the pump can operate without risking damage from suction or discharge recirculation.

MOTOR CONTROLLER: An electric switchbox that energizes and de-energizes an electric motor.

PERFORMANCE CURVE: A curve that plots the relationship between flow and head for a centrifugal pump. The vertical axis contains the values of head, while the horizontal axis contains flowrates. Since flowrate varies with head in a centrifugal pump, performance curves are used to select pumps that meet the needs of a system.

PRIME MOVER: A machine, usually an electric motor, that provides the motive force driving a pump.

RECIRCULATION: A flow condition that occurs during periods of low flow, usually below the minimum flow requirement of a pump. This condition causes cavitation-like damage, usually to the pressure side of an impeller vane.

STUFFING BOX: The part of a pump where the shaft penetrates the pump casing.

TOTAL HEAD: A measure of the total energy imparted to the fluid by a centrifugal pump. This value includes static pressure increase and velocity head.

VISCOSITY: the resistance of a fluid subjected to shear stress.

The terms and definitions for this issue’s Word Search come from a glossary of terms included in the guidebook, “Improving Pumping System Performance,” which is a publication of the Industrial Technologies Program, a joint initiative by the U.S. Department of Energy’s Energy Efficiency and Renewable Energy program ( and the Hydraulic Institute (