Pumps Selection GuidePumps Selection Guide

 Image Credit: U.S. Plastic Corporation | Parker Hannifin / Hydraulics / Hydraulic Pump Division | Wilden Pump & Engineering, LLC.

 

Pumps, in their simplest form, are machines for moving liquids. Industrial liquid handling pumps include all pumps designed to handle industrial liquid media such as water, wastewater, chemical slurries, oil, coolant fluid, or sewage.

 

For a brief introduction to the world of pumps, check out this video below:

 

Video Credit: iecSimulations

 

Pump Operation

In terms of operation, all pumps are ultimately classified as either positive displacement or dynamic (kinetic). However, since most dynamic pumps in industry are centrifugal pumps, the distinction is often between positive displacement and centrifugal.

 

Dynamic (Kinetic)

Dynamic pumps, also called kinetic pumps, include all pumps which use fluid velocity to build momentum and produce pressure to move the fluid through the system. These pumps are classified as either centrifugal or specialized based on the method used to induce this velocity.

 

Centrifugal

Centrifugal pumps, which are the most common, use an impeller attached to a shaft which rotates to provide the energy to generate fluid velocity. The impeller is mounted in a casing which provides a pressure boundary and channels the fluid through a volute (funnel). The image below shows a simplified centrifugal pump layout:

 


 

Pumps Selection Guide

Image Credit: Engineers Edge

 

Centrifugal pumps can be further differentiated based on how they direct flow.

  • Axial flow pumps lift liquid in a direction parallel to the pump shaft. They operate essentially the same as a boat propeller.
  • Radial flow pumps accelerate liquid through the center of the impeller and out along the impeller blades at right angles (radially) to the pump shaft.
  • Mixed flow pumps incorporate characteristics from both axial and radial flow pumps. They push liquid out away from the pump shaft at an angle greater than 90°.

Pumps Selection Guide

 

Specialized

Some unique dynamic pumps induce velocity in a fluid through specialized means other than an impeller. These include jet ejector, reversible centrifugal, gas lift, hydraulic ram, and electromagnetic pumps.

 

For more information on centrifugal pumps, visit the How to Select Centrifugal Pumps page on GlobalSpec.

 

Positive Displacement

Positive displacement pumps provide pump pressure through fixed volumes which expand and contract to push fluid through a system. This direct application means that the flow rate generated by these pumps is relatively constant, and varies only based on the speed at which the pump runs. The moving parts in these pumps operate in either a rotary or reciprocating manner. 

 

Rotary

Rotary pumps use a rotor to move fluid, where parts (gears, ridges, vanes, etc.) of the rotor act as dividers between chambers. As the rotor rotates, liquid is forced through and out the pump. The image below shows a simplified vane-type rotary pump layout:

 

Pumps Selection Guide

 

Vane rotary pump. Image Credit: FAO Corporate Document Repository

 

Reciprocating

Reciprocating pumps move fluid using linear rather than rotary motion. They operate by moving a piston or diaphragm back and forth through a cylinder. Fluid moves in at the upstroke (suction) and out through a check valve on the down-stroke (discharge). The image below shows a simplified hand-operated reciprocating pump:

 

Pumps Selection Guide

 

 Hand-operated reciprocating pump. Image Credit: Maritime.org

 

For more information on positive displacement pumps, visit the How to Select Positive Displacement Pumps page on GlobalSpec.

 

Pump Parameters 

Pump operation and performance can best be described by a few fundamental parameters; flow rate, pressure, 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:

 

 

When selecting pumps, the flow rate or rated capacity of the pump must be matched to the flow rate required by the application or system.

 

Pressure

Pressure is a measure of resistance: the force per unit area of resistance in the system. A pressure rating in a pump defines how much resistance it can handle or overcome.  It is usually given in bar or psi (pounds per square inch). Pressure, in conjunction with flow rate and power, is used to describe pump performance. Centrifugal pumps, however, typically use head (described below) instead of pressure to define the energy or resistance of the pump, since pressure in a centrifugal pump varies with the pumped fluid's specific gravity.

 

When selecting pumps, the rated operating or discharge pressure of the pump must be equal to or more than the required pressure for the system at the desired flow rate.

 

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. It is defined as the mechanical energy of the flow per unit weight. It is expressed as a column height of water given in feet (ft) or meters (m). In other words, if water was pumped straight up, the pump 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

 

When selecting centrifugal pumps, the rated pump head must be equal to or greater than the total head of the system (total dynamic head or TDH) at the desired flow rate.

 

Selection Tip: Pump head in a centrifugal pump will be he 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.

 

Another specification to consider is net positive suction head (NPSH) - 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.

Selection Tip: 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.

 

 

Power

Net head is proportional to the power actually delivered to the fluid, called output power (Pout) or the water horsepower (measured in horsepower or hp). This is the horsepower rating which describes the useful work the pump will do to the fluid. It can be calculated by the equation:

 

Pout = gH = ρgQH

 

where:

ρ is fluid density

g is the acceleration due to gravity

Q is the volumetric flow rate

H is the pump head

 is the mass flow rate 

 

In all pumps there are losses due to friction, internal leakage, flow separation, etc. Because of these losses, the external power supplied to the pump, called the input power (Pin) or brake horsepower, is always larger than the water horsepower. This specification is typically provided by the pump manufacturer as a rating or in the pump's performance curve and is used to select the proper motor or power source for the pump.

 

Selection Tip: When determining the 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%.

 

Efficiency

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

 ηpump = Pout/Pin

 

Keep in mind that any efficiency rating of the pump given by the manufacturer assumes certain system conditions such as the type of fluid transported: water is a typical standard. The efficiency may not be accurate if these assumptions differ from the consumer's intended application.

 

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 requires twice the power.                                                                                                                                                                                                                                                                     

 

Pump Performance Curves

All 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:

 

Pumps Selection Guide

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.

 

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:

 

Pumps Selection Guide

 

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 pumps BEP.

 

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

 

Positive displacement pumps do not utilize fluid momentum, meaning that flow rate is relatively independent of pump head. Thus, (unlike dynamic pumps), positive displacement pumps have a definitive capacity across a wide range of head pressures (as shown in the characteristic curve below). Slippage is the result of high discharge pressures causing some liquid to leak back to the pump suction, reducing capacity.

 

Pumps Selection Guide

Image Credit: ITU.edu

 

Failure results when the total head of the system exceeds the maximum head of the pump.

 

Types of Pumps

Any pump type which can handle an industrial liquid can be considered an industrial liquid handling pump. However, variation in design makes different pumps suitable for particular applications. 

 

The diagram below provides an overview of pump classification by type.

 

Pumps Selection Guide

Image Credit: Pdhengineer

 

The number of different pump types can be overwhelming to even an experienced engineer. The following table provides and overview of the basic categories and their general properties.

 

Parameter

Centrifugal Pumps

Reciprocating Pumps

Rotary Pumps

Capacity

Medium/High

Low

Low/Medium

Pressure (Head)

Low/Medium

High

Low/Medium

Maximum Flow Rate

100,000+ GPM

10,000+ GPM

10,000+ GPM

Maximum Pressure

6,000 PSI

100,000+ PSI

4,000 PSI

Requires Relief Valve

No

Yes

Yes

Flow Type

Variable

Constant

Constant

Flow Characteristic

Smooth

Pulsating

Smooth

Space Considerations

Require Less Space

Requires More Space

Requires Less Space

Initial Costs

Lower

Higher

Lower

Maintenance Costs

Lower

Higher

Lower

Energy Costs

Higher

Lower

Lower

Liquids Recommended

Water and low viscosity (thin) liquids. Can pump solutions with solids given proper impeller. Liquid should not contain gas pockets.

Viscous liquids, dirty chemicals, tacky glue and adhesives, oil, and lubricating fluids. Specialty fitted pumps can handle abrasives.

Optimum for viscous fluids. Requires clean, clear, non-abrasive fluid due to close tolerances.

Table Credit: PDHengineer

 

This next table further breaks down pump classification into specific types, and provides a summary of the features, advantages, and recommended liquids associated with each. To learn more about selecting a certain type of pump, click on its associated link under the pump type column.

 

Pump Type

Parent Type 

Features 

Liquids Recommended 

Advantages 

Axial

 

Dynamic, centrifugal

Single stage (generally), high specific speed impeller for high capacity and low head.

Water and low viscosity (thin) liquids. Can pump solutions with solids given proper impeller.

Characteristically very high flow rate with very low head, a requirement for flood dewatering and many cooling applications. 

 

Mixed 

 

Dynamic, centrifugal

Single or multistage, medium specific speed impeller for medium head and medium flow. Generally mounted vertically.

Water and low viscosity (thin) liquids. Can pump solutions with solids given proper impeller.

Combines characteristics of radial and axial flow for medium flow and medium head.

Radial

 

Dynamic, centrifugal

Single or multistage, low specific speed impeller for high head and low capacity.

Water and low viscosity (thin) liquids. Can pump solutions with solids given proper impeller.

Lowest flow rates and highest head of centrifugal pump types.

Electromagnetic

Dynamic, special effect

Provides torque to impeller via inner and outer magnets. Isolated inner-can with no shaft penetration.

Chemicals, hydrocarbons, and other liquids which are difficult to seal or pose serious consequences with leakage; high temperature fluids or liquids prone to costly evaporative losses.

Eliminates mechanical seal (a large component of pump maintenance costs); leak-free.

Jet

Dynamic, special effect

Horizontal end suction pump with ejector on pump or located in well. Ejects liquid via high pressure fluid through venturi nozzle.

Domestic water wells and liquid/gas mixtures.

Rugged and simple construction; less maintenance requirements; simple operation. Good for variable well conditions.

Gear

Positive displacement, rotary

Liquid pumped between two gears and surrounding casing. There are internal and external gear types.

Oils and other high viscosity liquids. Usually only suited for clean liquids (no solids).

Most widely used for clean oil services; few moving parts; simple construction.

Peristaltic

Positive displacement, rotary

Roller or shoe that squeezes a tube or hose as it rotates. The squeezing action moves the liquid along the tube.

Wide range of liquids including liquids containing solids and corrosive liquids.

Requires no seal and keeps the liquid inside the tube, meaning zero leakage. Good for handling of chemicals or disinfectants and for precise metering or dosing.

Rotary Vane

Positive displacement, rotary

Rotor with vanes located in slots rotates in an eccentrically shaped casing. As rotor turns, vanes move in and out of the slots.

Oils and other high viscosity liquids. Usually only suited for clean liquids (no solids). Also good for thin liquids like gasoline and water.

Good for both thick and thin liquids; often chosen for terminals and truck unloading where many types of liquids are handled.

Screw

Positive displacement, rotary

Two-screw pumps make use of timing gears. Triple-screw types use one screw to drive the others and don't include timing gears.

Oils, fuels, and other high viscosity liquids. Also handles two-phase liquid/gas mixtures.

Provides highest flow rate of positive displacement pump types.

Diaphragm

Positive displacement, reciprocating

Reciprocating diaphragm driven by a solenoid, mechanical drive, or fluid drive. Contains inlet and outlet check valves.

Wide range of liquids including liquids containing solids and corrosive liquids.

Handles a wide range of liquids, including liquids containing solids; pump is sealless, and can run dry without damaging the pump.

Piston/Plunger

Positive displacement, reciprocating

One or more double acting pistons or single acting plungers, sealed with o-rings against cylinder walls.

Water and other thin liquids. Piston pumps specifically recommended for liquids containing abrasives.

Plunger pumps provide best means of achieving high pressures. Piston pumps are better for abrasive liquids. Slow speeds may mean less maintenance.

Table includes content from Pump Scout -Pump Types Guide

  

Materials

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.

 

Types

Some materials used 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:

  • Aluminum
  • Brass
  • Bronze
  • Ceramics
  • Nickel-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.

Media Properties

Industrial liquid handling pumps are distinguished as those pumps which deal with moving industrial liquids. However, there is a broad range of media under the scope of industrial fluids. Selecting the right pump thus requires an understanding of the properties of the liquid in the addressed system. These properties include viscosity and consistency.

  • Viscosity is a measure of the thickness of a liquid. Viscous fluids like sludges generate higher systems pressures and require more pumping power to move through the system. In many cases, positive displacement pumps are better suited for handling higher viscosity fluids. Low viscosity liquids like water and oil which generate low head are generally better suited for dynamic (centrifugal) pump types.
  • Consistency is the material makeup of the liquid solution in terms of chemicals and undissolved solids. Positive displacement pumps are generally better suited for handling these solids, but dynamic pumps which are designed correctly (i.e. with certain impeller blades) can handle them as well. Solutions with corrosive chemicals should be handled by pumps with materials and parts designed to withstand corrosion.

Physical Design

When selecting the right pump, the buyer may have to consider the design of the pump beyond its type and specifications.

 

Impeller Design

Impeller design is important for proper centrifugal pump performance.

 

  • Closed designs are best used for water pumps, as the vanes totally enclose the water for best performance.

Pumps Selection Guide

Closed propeller design. Image Credit: Mcnally Institute  

 

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

Pumps Selection Guide

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, but are up to 50% less efficient than other designs.

Pumps Selection Guide

Vortex impeller design. Image Credit: Egger Pumps

  • Single stage and multi-stage describe the number of impeller stages in a centrifugal pump, which affects the achievable head of the pump. When a higher head pressure is required, a multi-stage pump is generally more economical to implement than a more complex single stage pump.

Pumps Selection Guide

A two-stage pump system. Image Credit: Hydraulic Pump & Motor Troubleshooting

  • Simplex and multiplex describe the number of cylinders in a reciprocating pump, which determines its overall capacity. Simplex reciprocating pumps have only one cylinder while multiplex pumps have more than one. Most reciprocating pumps use two or three cylinders.

Power Source

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) - pumpis poweredusing 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.

References and Resources

 

Chemical Engineers Resource Page - Centrifugal Pumps: Basic Concepts of Operation, Maintenance, and Troubleshooting

 

Engineering Toolbox - Classifications of Pumps

 

IT University - Positive Displacement Pumps

 

Mcnally Institute - Open vs. Closed Impeller Design

 

PumpScout - Pump Types Guide

 

Pdhengineer - Pumps - Centrifugal vs. Positive Displacement

 

Rain For Rent - Pump Training

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