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Pumps
General Information

Liquid Pump Terminology
Cavitation—Process in which small bubbles are formed and implode violently; occurs when NPSHa < NPSHr. See page 1429 for more information. 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. Discharge Head The outlet pressure of a pump in operation. 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). See page 1429 for more information. 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. Shear Rate—The velocity of a liquid per unit distance between two surfaces. 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. See page 1429 for more information. 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 minus the suction head. 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. See below for details on pumping viscous liquids.

Viscosity Handling Characteristics
Pumping viscous liquids can present some difficult problems. When selecting a pump fluid viscosity must always be considered. The effects of viscosity can be better understood by looking at the behavior of viscous liquids when subjected to force. Newtonian Liquids—Viscosity remains constant regardless of changes in shear rate or agitation. As pump speed increases, flow increases proportionately. Liquids displaying Newtonian behavior include water, mineral oils, syrup, hydrocarbons, and resins. Pseudoplastic Liquids—Viscosity decreases as shear rate increases, but initial viscosity may be great enough to prevent flow from starting in a typical pumping system. Typical pseudoplastic liquids are gels, latex paints, and lotions. Dilatant Liquids—Viscosity increases as shear rate increases. Pumps can bog down and stall after initially pumping dilatant liquids. Some liquids showing dilatant behavior are slurries, clay, and candy compounds. Thixotropic Liquids—Like a pseudoplastic liquid, viscosity decreases as shear rate or agitation increases. When agitation is stopped or reduced, hysteresis occurs and viscosity will increase. Often the viscosity will not return to its initial value. Some examples of thixotropic liquids are soaps, tars, vegetable oils, shortening, glue, inks, peanut butter, and some slurries.

Viscosities for Typical Liquids
Typical liquid Water Milk No. 4 fuel oil Cream Vegetable oil SAE 10 oil Tomato juice SAE 30 oil Glycerine Honey Glue Mayonnaise Molasses B Sour cream Centipoise (cp) 1 3 12.6 20 40 88 180 352 800 1,500 3,000 5,000 8,640 15,000 Centistokes (cSt) 1 4 15.7 20.6 43.2 110 220 440 1,100 2,200 4,500 6,250 10,800 19,000 Saybolt Second Universal (SSU) 31 40 80 100 200 500 1,000 2,000 5,000 10,000 20,000 28,000 50,000 86,000

Pumping Viscous Liquids with Gear Pumps
Gear pumps are well suited for pumping viscous liquids if the following rules are observed: 1) Pump speed (rpm) must be reduced if viscosity is above 100 cp. When pumping viscosities above 100 cp call our Applications Department to determine optimal rpm and other operating parameters. 2) Suction and discharge lines must be increased by at least one, or better, two pipe sizes over the size of the pump ports. 3) Horsepower of the motor must be increased over the power required for pumping water under the same pressure and flow.

Viscous Liquid Behavior

Newtonian Pseudoplastic Dilatant

Pumping Viscous Liquids with Centrifugal Pumps
Centrifugal pumps are generally not suitable for pumping viscous liquids. They are best suited for pumping liquids with viscosities less than 200 cp. Volume and pressure capabilities of the pump are reduced with increasing viscosity. See page 1429 for information on horsepower requirements.

Flow

Thixotropic

More info
See pages R-17 to R-26 in the back of this catalog for “Chemical Resistance Charts”.

Force exerted by pump

1428

Call Cole-Parmer Instrument Company toll free: (800) 323-4340 Fax: (847) 247-2929 In Canada call Labcor: (800) 363-5900 In other countries fax direct: (847) 549-1700


				
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