Large Pump Basics and Glossary of Terms
Atmospheric Pressure — the force exerted by the atmosphere on the earth’s surface, which allows a centrifugal pump to operate. At sea level, the atmospheric pressure equals 14.7 psi.
Brake Horsepower (HP) — pump performance can be expressed in horsepower using the following formula:
Brake HP = GPM x Ft./Head ÷ 3940
Capacity — the water handling capability (volume) of a pump expressed as gallons per minute (GPM).
Cavitation — status in which the pump impeller is not receiving a full supply of material. This can be due to reduced flow or over rotation. Excessive pump rpm can cause a vortex in the eye of the impeller. Air bubbles attach to the metal surfaces and, under extreme pressure, implode, taking tiny bits of metal away with each implosion, pitting the impeller and volute surfaces.
Centrifugal Force — the action that causes something to move away from its center of rotation.
Centrifugal Pump — uses centrifugal force to move water or other liquids. Centrifugal pumps use an impeller and a volute to create the partial vacuum and discharge pressure necessary to move water through the casing. The impeller and volute form the heart of a pump—their design determines its flow, pressure and solid handling characteristics. As the impeller rotates and churns the water, it purges air from the casing, creating an area of low pressure, or partial vacuum, at the eye (center) of the impeller. The weight of the atmosphere on the external body of water pushes water rapidly through the hose and pump casing toward the eye of the impeller. Centrifugal force created by the rotating impeller pushes water away from the eye, where pressure is lowest, to the vane tips where pressure is the highest. The velocity of the rotating vanes pressurizes the water, forcing it through the volute and discharging it from the pump.
Check Valve — (swing check valve) a device used in a suction or discharge line that allows flow in only one direction, isolating the material being pumped.
Critical Lifts — suction lifts greater than 25'.
Dewatering Pump — designed for clear water applications (agricultural, industrial and residential). As a general rule, dewatering pumps are limited to a 10% solids concentration and a solids size of one-fourth the diameter of the suction inlet.
Diaphragm Pump — uses a positive displacement design rather than centrifugal force to move water through the casing, delivering a specific amount of flow per stroke, revolution or cycle. Diaphragm pumps are ideal for applications with slow seepage at the point of suction, due to their great air handling capabilities.
Duty Point — The point on a performance curve that plots flow (GPM) and head (feet).
Dynamic Discharge Head — the sum of the static discharge head and the discharge friction loss in the discharge line. Also referred to as Total Discharge Head.
Dynamic Suction Head — the sum of the static suction lift and the suction friction loss in the suction line. Also referred to as Total Suction Head.
Flow Rate — how many gallons per minute (GPM) of pump flow are required. Flow can also be expressed in gallons per hour (GPH) and in million gallons per day (MGD). 1 MGD = 700 GPM.
Float Switch — a device used to start and stop a pump based on preset water levels.
Fluid Type — whether the fluid being pumped is clean or dirty, contains any solids or abrasives or is a hazardous material.
Friction Loss — reductions in flow due to turbulence as water passes through hoses, pipes, valves and fittings. This includes both suction and discharge friction losses.
Head — gains or losses in pressure caused by gravity and friction as water moves through a system. It can be measured in lbs. per square inch (psi) or feet of water. A pump must produce 1 psi to push a column of water vertically 2.31 feet. Use the following formulas to convert:
Max pressure x 2.31 = Max Head Rating
Max Head Rating ÷ 2.31 = Max Pressure
High Head (high-pressure) Pump — capable of handling flows at significantly higher total dynamic head ratings (TDH). They utilize a closed design impeller and a compact volute called a diffuser to generate the high discharge pressure needed and cannot handle large solids.
Hose Length (or Pipe) — the suction and discharge hose or pipe lengths required for a given application. Longer hoses increase friction loss, reducing pump performance. Hose lengths should be kept as short as possible.
Impeller — a rotating disk with a set of vanes coupled to the engine or drive shaft that produces centrifugal force within the pump casing of a centrifugal pump.
Maximum Suction Lift — the height (approximately 25') that water can be lifted by a centrifugal pump in actual conditions, taking into consideration altitude, friction loss, temperature, suspended particles and the inability to create a perfect vacuum. The 25' suction lift is attainable for cold water (60ºF) at sea level.
Mechanical Seal — a spring-loaded pump component that forms a seal between the pump and the engine or motor. Pumps designed for working in harsh environments require a more abrasive resistant seal.
Net Positive Suction Head (NPSH) — the amount of energy in the liquid at the pump inlet. It must be defined to have meaning, as either available or required.
Performance Curve — a chart or graph that illustrates pump performance by plotting the total head and flow rate at various suction lifts. Performance curves for diesel-driven pumps also show pump performance at various engine rpms.
Prime — the creation of a partial vacuum inside the pump casing, which allows water to flow into the pump.
Seepage — the rate at which the fluid being pumped accumulates at the point of suction. Slow seepage allows air into the pump suction, which causes some types of pump to lose their prime.
Self-priming — the ability of a pump to purge air from its casing and suction hose, creating a partial vacuum and allowing water to flow freely into the pump.
Solids Concentration — ratio of solids to liquid in the overall volume of the material being pumped, which is helpful in determining the proper pump for the application.
Solids Size — average diameter of individual particles in the material being pumped, which is important to know when specifying a pump. Large solids can be filtered with strainers or rock guards.
Static Discharge Head — the vertical distance from the centerline of the pump impeller to the point of discharge. (see dynamic discharge head)
Static Suction Lift — the vertical distance from the lowest suction point to the centerline of the pump impeller. This distance should be kept to a minimum for maximum pump performance. (see theoretical and maximum suction lift)
Submersible Pump — a centrifugal pump designed to operate within the water source being pumped, thereby eliminating the suction lift limitations common to other types.
Theoretical Suction Lift — the maximum height (33.9') that water can be lifted inside a tube under perfect conditions (perfect vacuum) at sea level. At this point, the water inside exerts a pressure equal to the weight of the atmosphere pushing down on the ocean’s surface. Theoretical suction lift is calculated by dividing the atmospheric pressure at sea level (14.7 lbs. per square inch) by the weight of one cubic inch of water (.0361 lbs.). This equals 407.2" or 33.9'.
Total Dynamic Head (TDH) — the sum of the dynamic suction head and the dynamic discharge head. Also referred to as Total Head.
Trash Pump — designed to handle large amounts of debris, with a solid handling capability of 25% by volume. As a rule of thumb, trash pumps can handle spherical solids up to one-half the diameter of the suction inlet. Larger, diesel-driven trash pumps (4" to 12" diameter) are designed to handle 3" diameter solids.
Viscosity — the resistance to flow of a liquid at a given temperature. Highly viscous liquids are thick and tend to flow slower than liquids of low viscosity.
Volute — the casing surrounding the impeller in a centrifugal pump that collects the liquid discharged from the impeller.