Liquid Pump Terminology

Liquid Pump Terminology

Cavitation—Process in which small bubbles are formed and implode violently; occurs when NPSH_a < NPSH_r.

Dead Head—The ability of a pump to continue running without damage when discharge is closed off. Only recommended for centrifugal pumps.

Density (specific weight of a fluid)—Weight per unit volume, often expressed as pounds per cubic foot or grams per cubic centimeter.

Flooded Suction—Liquid flows to pump inlet from an elevated source by means of gravity. Recommended for centrifugal pump installations.

Flow—A measure of the liquid volume capacity of a pump. Given in gallons per hour (GPH), gallons per minute (GPM), liters per minute (L/min), or milliliters per minute (mL/min).

Fluids—Include liquids, gases, and mixtures of liquids, solids, and gases. In this catalog, the terms fluid and liquid are both used to mean a pure liquid or a liquid mixed with gases or solids that acts essentially like a liquid in pumping applications.

Head—A measure of pressure, expressed in feet of head for centrifugal pumps. Indicates the height of a column of water being moved by the pump (without friction losses).

Pressure—The force exerted on the walls of a tank, pipe, etc., by a liquid. Normally measured in pounds per square inch (psi).

Prime—Charge of liquid required to begin pumping action when liquid source is lower than pump. Held in pump by a foot valve on the intake line or by a valve or chamber within the pump.

Seals—Devices mounted in the pump housing and/or on the pump shaft that prevent leakage of liquid from the pump.

Self-Priming—Pumps that draw liquid up from below pump inlet (suction lift), as opposed to pumps requiring flooded suction.

Specific Gravity—The ratio of the weight of a given volume of liquid to pure water. Pumping heavy liquids (specific gravity greater than 1.0) will require more drive horsepower.

Static Discharge Head—Maximum vertical distance (in feet) from pump to point of discharge with no flow.

Strainer—A device installed in the inlet of a pump to prevent foreign particles from damaging the internal parts.

Sump—A well or pit in which liquids collect below floor level; sometimes refers to an oil or water reservoir.

Total Head—Sum of discharge head, suction lift, and friction loss.

Valves:

• Bypass Valve—Internal to many pump heads that allow fluid to be recirculated if a given pressure limit is exceeded.
• Check Valve—Allows liquid to flow in one direction only. Generally used in discharge line to prevent reverse flow.
• Foot Valve—A type of check valve with a built-in strainer. Used at point of liquid intake to retain liquid in system, preventing loss of prime when liquid source is lower than pump.
• Relief Valve—Used at the discharge of a positive displacement pump. An adjustable, spring-loaded valve opens when a preset pressure is reached. Used to prevent excessive pressure buildup that could damage the pump or motor.

Viscosity—The "thickness" of a liquid or its ability to flow. Most liquids decrease in viscosity and flow more easily as they get warmer.