How to Select Industrial Liquid Handling Pumps

 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.

Understanding Pumps 

Selecting pumps requires an understanding of how pumps work and how they are described.

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:

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°.

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:

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:

 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, net head, power, and efficiency.

• 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). 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:

• Net head (H) or total dynamic head (TDH) is the pressure rise across the pump expressed as a head, or equivalent column height of water (measured in feet or ft).

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)

• 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.

• 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 brake horsepower (BHP), 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%.

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

 η_pump = WHP/BHP

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:

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:

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.

Image Credit: ITU.edu

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

Pump Selection

When selecting industrial liquid handling pumps, industrial buyers should consider the distinguishing features of different types of pumps. Once the appropriate type of pump is determined, performance specifications, media type, and system compatibility should be considered to make the best selection.

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.

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.

Table Credit: PDHengineer

Specifications

The essential parameters to consider when selecting pumps include flow rate, net head, and power. These are described in the above section under pump parameters.

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

Media Type

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. More viscous fluids like sludges require higher pump head to move through the system and generally are better suited for positive displacement pumps. Low viscosity liquids like water which generate low head are generally better suited for dynamic 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 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.

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.

Open propeller design. Image Credit: Mcnally Institute  

Vortex impellers are a unique semi-open design which are the best solution for solid and "stringy" materials, but are up to 50% less efficient than other designs.

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.

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.

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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