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 the use of an impeller. They are the most common type of pump because of the simplicity and effectiveness of their design and operation. Because they are the most common, 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

Composition

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 or solid), stuffing box, and bearings.

Operation

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.

Types

Flow

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:

Impeller Design

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.

Closed propeller design. Image Credit: Mcnally Institute  

• Open and semi-open impellers are less likely to clog than closed designs, making them better suited for more viscous media.

Open propeller design. Image Credit: Mcnally Institute  

• 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.

Vortex impeller design. Image Credit: Egger Pumps

Performance Specifications 

Centrifugal pump selection is defined by a few key specifications. These are flow rate, head, power, and efficiency.

Flow Rate

Volume flow rate (Q), also referred to as capacity, is the volume of liquid that travels through the pump in a given time (measured in gallons per minute or gpm). It defines the rate at which a pump can push fluid through the system. In some cases, the mass flow rate () is also used, which describes the mass through the pump over time. The volume flow rate is related to mass flow rate by the fluid density (ρ) via the equation:

Pressure

Pressure is the force per unit area generated by the pump. It is usually given in bar or psi (pounds per square inch). Pressure is used to describe the performance of positive displacement pumps and some centrifugal pumps. Centrifugal pumps, however, typically use head (described below) instead of pressure to define the energy of the pump, since pressure in a centrifugal pump varies with the pumped fluid's specific gravity.

Head

Net head (H) or total dynamic head (TDH) defines the energy supplied to liquid (per unit weight) by the pump. It is expressed as a column height of liquid (either vertical lift or suction), given in feet of head (ft). In other words, if the liquid was pumped straight up, the head is equivalent to the height it reaches.

Pump head (H) can be converted to pressure (P) using the specific gravity (SG) of the fluid by the equation:

P = 0.434 · H · (SG)

or by the density of the fluid (ρ) and the acceleration due to gravity (g):

P = H · ρ · g

Selection Tip: Pump head in a centrifugal pump will be the same for all liquids if the shaft is spinning at the same speed. The only difference between fluids is the amount of power needed to get the shaft to the proper speed (rpm). The higher the fluid's specific gravity (SG), the more power is required.  

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 cavitation in a pump. Cavitation happens inside a pump when the local pressure falls below the vapor pressure of the liquid being pumped, causing the liquid to boil. The pressure inside the pump should be above the NPSH to avoid cavitation, which can result in noise, vibration, reduced efficiency, and damage to impeller blades.

Impeller blade damaged from pump cavitation. Image Credit: Pump Fundamentals

Power

Net head is proportional to the power actually delivered to the fluid, called water horsepower (WHP) even if the pumped fluid is not water (measured in horsepower or hp). This can be found by the equation:

WHP = gH = ρgQH

where g is the acceleration due to gravity and ρ is the density of the fluid.

The power supplied to the pump (specifically the impeller for centrifugal pumps) is called the brake horsepower (BHP). In all pumps there are losses due to friction, internal leakage, flow separation, etc. Because of these losses, the brake horsepower is always larger than the water horsepower.

Selection tip: When determining required power from a typical pump performance curve (discussed below), it is best to use the values at the end of the curve to ensure adequate supply at most operating conditions. For operations with little system variation (e.g. refineries), use the value at the operating point plus 10%.

Pumps can be driven by a number of different power sources. The most common are electric motors, but many other types exist.

• AC powered - pump operates on a form of alternating current (AC) voltage, typically from an AC motor.
• DC powered - pump operates on a form of direct current (DC) voltage, typically from a DC motor or battery.
• Air (pneumatic) - pump is powered using a compressed air source.
• Combustion engine (gasoline or diesel) - pump is powered using a gasoline or diesel engine.
• Hydraulic - pump is powered by a hydraulic system.
• Steam - pump is powered by steam.

Efficiency

The ratio between the water horsepower and brake horsepower (useful power vs. required power) describes the pump efficiency (η_pump):

 η_pump = WHP/BHP

Selection tip: A more efficient pump is not always the best choice when considering energy costs. For example, a pump that runs at 40% efficiency would be a better choice than one in the same family which is 60% efficient but consumes twice as much power.                                                                                                   

Performance Curves

Interpretation

Centrifugal pumps have a characteristic or performance curve that describes the flow rate produced at net or total head. Pump specifications relating head and flow rate correlate to those found on its characteristic curve. A simplified curve for a centrifugal pump will look something like this:

Original Image Credit: Pumpfundamentals.com

The pump curve illustrates the available total head at a given flow rate of the pump. Generally, more head is available in the pump as flow rate decreases. Manufacturers usually designate an optimum or best efficiency point (BEP) of the curve, which is indicated in this graph by the dotted line. Thus, this pump runs best when supplying a net head of 100 ft, which will provide a flow rate of 23 gpm.

Selection

When selecting a pump for incorporation into a system, users should map the system curve alongside the pump curve. A simplified incorporation of this curve will look something like this:

The system curve illustrates the required head for different flow rates in the system. It is constructed using a form of Bernoulli's equation for fluid mechanics, which is beyond the scope of this guide. Generally, more head is required as flow rate increases due to frictional forces and other losses in the system. The operating point of the pump in a system should be where the pump curve and system curve intersect. The best pump choice for a system is one in which the required operating point intersects at the pump's BEP.

Selection tip: Since every system is unique and has specific head requirements, the best choice mentioned above is not always commercially available. 

Materials

Types

The material(s) of a pump should be considered based on type of application. Some materials used are listed below.

• Cast iron
• Ceramic
• Brass
• Bronze
• Nickel alloy
• Plastic

• Steel and stainless steel alloy

Considerations

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. 

Application Considerations

• 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 size - The inlet (suction) and outlet (discharge) openings of the pump must be sized appropriately to the connections of the system. These sizes (typically diameters specified in inches) affect the performance characteristics (flow and pressure) of the pump during its design.
• 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 - Temperature specifications 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.

Resources

Engineering Toolbox - Centrifugal Pumps

Grundfos - The Centrifugal Pump (pdf)

Mcnally Institute - Learning About Centrifugal Pumps