How to Select Centrifugal Pumps
Image Credit: Lutz-JESCO American Corp. | Price Pump Company | Grainger Industrial Supply
Centrifugal pumps are dynamic pumps which move fluids through a system using one or more impellers. They are the most common type of pump because of the simplicity and effectiveness of their design and operation. Because they are the most familiar, they also tend to cost less than other types of pumps. Compared to positive displacement pumps, they provide higher flow rates and lower pressures.
Understanding Centrifugal Pumps
Centrifugal pumps consist of a set of rotating vanes called an impeller. The rotary vanes are typically enclosed within a housing or casing, and are used to impart energy to a fluid through centrifugal force. The pump has two main parts: a rotating element which includes an impeller and a shaft, and a stationary element made up of a casing (volute), stuffing box, and bearings.
Image Credit: Pumpfundamentals.com
Centrifugal pumps operate using kinetic energy to move fluid, utilizing an impeller and a circular pump casing. A vacuum is created in the pump which draws fluid to the impeller by suction. The impeller produces liquid velocity and the casing forces the liquid to discharge from the pump, converting velocity to pressure. This is accomplished by offsetting the impeller in the casing and by maintaining a close clearance between the impeller and the casing at the cutwater. By forcing fluid through without cupping it, centrifugal pumps can achieve very high flow rates.
Centrifugal pumps generate flow by using one of three actions: radial flow, mixed flow, or axial flow.
Axial flow pumps are characterized by high flow and low pressure. They lift liquid in a direction parallel to the impeller shaft, operating essentially the same as a boat propeller. Pressure is developed wholly by the propelling action of the impeller vanes.
Axial flow impeller. Image Credit: Engineer's Edge
- Radial flow pumps are characterized by high pressure and low flow. They accelerate liquid through the center of the impeller and out along the impeller blades at right angles (radially) to the pump shaft. Pressure is developed wholly by centrifugal force.
Radial flow impeller. Image Credit: Engineer's Edge
- Mixed flow pumps incorporate characteristics from both axial and radial flow pumps, with typically medium flow and medium pressure. They push liquid out away from the pump shaft at an angle greater than 90°. Pressure is developed partly by centrifugal force and partly by the lifting action of the impeller.
Mixed flow impeller. Image Credit: Engineer's Edge
The image below provides visual example of how liquid might flow through these different types of pumps:
Centrifugal pump selection is defined by a few key specifications, including flow rate, head, power, and efficiency.
Flow rate describes the rate at which the pump can move fluid through the system, typically expressed in gallons per minute (gpm). The rated capacity of a pump must be matched to the flow rate required by the application or system.
Pressure is a measure of the force per unit area of resistance the pump can handle or overcome, expressed in bar or psi (pounds per square inch). As in all centrifugal pumps, the pressure in axial flow pumps varies based on the pumped fluid's specific gravity. For this reason, head is more commonly used to define pump energy in this way.
Head is the height above the suction inlet that a pump can lift a fluid. It is a shortcut measurement of system resistance (pressure) which is independent of the fluid's specific gravity, expressed as a column height of water given in feet (ft) or meters (m).
Net positive suction head (NPSH) is the difference between the pump's inlet stagnation pressure head and the vapor pressure head. The required NPSH is an important parameter in preventing pump cavitation.
Output power, also called water horsepower, is the power actually delivered to the fluid by the pump, measured in horsepower (hp).
Input power, also called brake horsepower, is the power that must be supplied to the pump, measured in horsepower (hp).
Efficiency is the ratio between the input power and output power. It accounts for energy losses in the pump (friction and slip) to describes how much of the input power does useful work.
For more information about these and other pump performance specifications, visit the Pump Flow information page on GlobalSpec.
A number of features and components are available and are important to consider for centrifugal pumps. For a more complete list of pump Features, visit GlobalSpec's Pump Features page.
The impeller design determines the type of flow and is the main variable in pump design affecting a pump's performance (namely its capacity and pressure).
Closed designs are best used for water pumps, as the vanes totally enclose the water for best performance.
Open and semi-open impellers are less likely to clog than closed designs, making them better suited for more viscous media.
Vortex impellers have a unique semi-open design which is the best solution for solid and "stringy" materials to prevent clogging, but are up to 50% less efficient than other designs.
Turbine impellers are special impellers designed for regenerative turbine pumps. These pumps generate higher head but have lower flow rates, similar to displacement pumps. For more information on regenerative turbine pumps, visit GlobalSpec's Turbine Pump Selection Guide page.
For a more in depth discussion of impeller design and other pump components, visit GlobalSpec's Pump Components page.
Pumps and their various components are made up of a number of different materials. Media type, system requirements, and the surrounding environment all are important factors in material selection.
Some of the most common materials used in pumps are described below:
- Cast iron provides high tensile strength, durability, and abrasion resistance corresponding to high pressure ratings.
- Plastics are inexpensive and provide extensive resistance to corrosion and chemical attack.
- Steel and stainless steel alloys provide protection against chemical and rust corrosion and have higher tensile strengths than plastics, corresponding to higher pressure ratings.
Other materials used in pump construction include:
When selecting the material type, there are a number of considerations that need to be taken into account.
Chemical compatibility - Pump parts in contact with the pumped media and addition additives (cleaners, thinning solutions) should be made of chemically compatible materials that will not result in excessive corrosion or contamination. Consult a metallurgist for proper metal selection when dealing with corrosive media.
Explosion proof - Non-sparking materials are required for operating environments or media with particular susceptibility to catching fire or explosion. See the Explosion Proof Pumps Selection Guide for more information on pumps designed specifically for these applications.
Sanitation- Pumps in the food and beverage industries require high density seals or sealless pumps that are easy to clean and sterilize.
Wear - Pumps which handle abrasives require materials with good wearing capabilities. Hard surfaces and chemically resistant materials are often incompatible. The base and housing materials should be of adequate strength and also be able to hold up against the conditions of its operating environment.
Pumps can be driven by a number of different power sources. The most common are electric motors, but many other types exist.
- AC powered pumps operate on a form of alternating current (AC) voltage, typically from an AC motor.
- DC powered pumps operate on a form of direct current (DC) voltage, typically from a DC motor or battery.
- Air (pneumatic) pumps are powered using a compressed air source.
- Combustion engine (gasoline or diesel) pumps are powered using a gasoline or diesel engine.
- Hydraulic pumps are powered by a hydraulic system.
- Steam pumps are powered by steam.
- Impeller design - While a pump manufacturer usually has the job of selecting or designing a pump's impeller, for certain applications (namely certain media types) the impeller must be specially selected. For example, a grinder type blade/impeller design may be required for handling thick slurries, abrasives, or other solids filled media (see the Grinder Pumps Selection Guide for more information).
Inlet and outlet connections - The inlet (suction) and outlet (discharge) openings of the pump must be sized appropriately to fit the requirements of the system. Proper pipe or tube fittings must also be considered to make the proper connections. Visit the Pipe Fittings Selection Guide and Tube Fittings Selection Guide for more information on selecting fittings.
Media viscosity - As mentioned before, the properties of the fluid being pumped and their implications need to be understood to select the right pump. Particularly with centrifugal pumps, the viscosity of the media can drastically affect the pump's performance. The chart below shows how changes in viscosity (measured in Saybolt Universal Seconds or SSU) can reduce flow and head and increase power requirements.
Image Credit: Pumpbiz.com
Pump stage - Stages describe the number of impeller sets in a centrifugal pump, which affects its performance. When a higher head pressure is required, a multi-stage pump is generally more economical to implement than a more complex single stage pump.
A two-stage pump system. Image Credit: Hydraulic Pump & Motor Troubleshooting
Temperature - Operating temperature defines the range or limit temperatures at which a pump can operate. Pumps which run at temperatures outside their operating range are susceptible to performance wear or failure. Running a pump dry or loading it with excessive torque can raise temperatures to higher levels and cause overheating.
Read user Insights about Centrifugal Pumps
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