This pump selection guide explains how to define the application, fluid and pump system before comparing pump technologies or individual pump curves.
Before choosing a pump, define:
- The required flow range
- The system head at each important operating condition
- Suction conditions
- Fluid viscosity, density and vapour pressure
- Solids size, concentration and behaviour
- Temperature and chemical composition
- Required materials and sealing arrangement
- Operating hours, control method and maintenance constraints
- Site safety, electrical and environmental requirements
Only then should you compare pump technologies and individual pump curves.
The correct pump is one that handles the fluid, meets the full operating range and remains within acceptable mechanical and hydraulic limits. A pump that reaches one nominated duty point can still be unsuitable for the application.
Start with the information the pump selector needs
Incomplete application data leads to assumptions. Those assumptions usually become extra pump capacity, extra motor power or a familiar pump technology that may not suit the service.
Record normal, minimum and maximum conditions. Include start-up, shutdown, cleaning and foreseeable upset conditions where they affect the pump.
| Selection input | Information to record | Why it affects the selection |
|---|---|---|
| Flow rate | Normal, minimum and maximum flow in L/s, L/min or m³/h | Establishes pump capacity and operating range |
| System head | Static head, pressure requirements and friction losses | Establishes the head the pump must generate at each flow |
| Suction conditions | Liquid level, vessel pressure, elevation, pipe losses and temperature | Determines priming requirements and Net Positive Suction Head Available |
| Fluid identity | Chemical name, mixture, concentration and contaminants | Affects materials, seals and containment |
| Viscosity | Dynamic or kinematic viscosity at operating and start-up temperatures | Changes hydraulic performance, suction losses, torque and pump technology suitability |
| Specific gravity | Minimum and maximum expected value | Changes pressure conversion and power requirements |
| Solids | Size, concentration, hardness, shape, fibres and settling behaviour | Affects blockage risk, wear rate, clearances and pump speed |
| Temperature | Normal, minimum, maximum and cleaning temperature | Affects viscosity, vapour pressure, clearances and material limits |
| Duty cycle | Continuous, intermittent, batch, standby or dosing duty | Affects sizing, controls, cooling and maintenance expectations |
| Site conditions | Hazardous area, washdown, dust, weather, altitude and available utilities | Affects drive, enclosure, materials and compliance requirements |
The application should also state whether leakage is acceptable, whether the fluid is toxic or valuable, and whether the product can tolerate shear, pulsation or contamination.
See What Information Do You Need Before Selecting a Pump? for a detailed data collection checklist.
Pump selection guidance from the US Department of Energy identifies fluid composition, temperature, solids, specific gravity, vapour pressure, viscosity, system flow and system head as core inputs.
Match the pump technology to the fluid and duty
The first technology decision is often between a rotodynamic pump and a positive displacement pump.
A centrifugal pump is the most common rotodynamic design. Its impeller transfers velocity to the fluid, and the casing converts part of that velocity into pressure. Its flow changes as system resistance changes.
A positive displacement pump traps and moves a defined volume during each operating cycle or shaft revolution. Its flow is generally more directly related to speed, although internal slip, compressibility and valve behaviour can affect the delivered volume.
Neither principle is universally better.
| Application condition | Centrifugal pump tendency | Positive displacement pump tendency |
|---|---|---|
| Clean, low-viscosity fluid at moderate or high flow | Often a strong candidate | May be suitable, but can add unnecessary complexity |
| High-viscosity fluid | Performance and efficiency may fall substantially | Often handles viscosity more predictably |
| Accurate low-flow dosing | Standard centrifugal pumps may be difficult to control accurately | Metering designs are commonly used |
| Variable system pressure | Flow changes along the pump curve | Flow can remain comparatively stable, subject to slip and drive capability |
| Large or abrasive solids | Requires a suitable slurry or solids-handling design | Suitability depends heavily on clearances, valves and pumping principle |
| Shear-sensitive fluid | High-speed designs may damage some products | Some low-speed designs provide gentler handling |
| Pulsation-sensitive process | Usually provides smooth flow | Some designs require pulsation dampening |
| Blocked discharge | Flow falls as shut-off is approached, subject to pump limits | Pressure can rise rapidly; correctly sized pressure protection is required |
Positive displacement pumps require a safe path for full pump flow if the discharge becomes blocked. The relief arrangement must protect the pump and every component exposed to the resulting pressure.
The technology shortlist may include:
- End-suction, inline, multistage, vertical or submersible centrifugal pumps
- Slurry pumps
- Air-operated double-diaphragm pumps
- Mechanical diaphragm or plunger metering pumps
- Progressive cavity pumps
- Gear, lobe, vane or screw pumps
- Peristaltic hose or tube pumps
Each design has its own limits. A broad technology family is only the beginning of the selection.
See Centrifugal vs Positive Displacement Pumps for the main operating differences. Use How to Choose the Correct Pump Technology when comparing individual pumping principles after the application data is complete.
Define the duty point and operating range
The duty point combines the required flow rate and total pump head.
Total pump head is the energy per unit weight that the pump must add to the fluid. It includes the effect of static level differences, vessel pressures, pipe friction, fittings, valves and process equipment.
For a centrifugal pump, the actual operating point occurs where the pump curve intersects the system curve. The pump does not independently choose the flow rate shown on the datasheet. The pump and system establish the operating point together. Hydraulic Institute guidance treats pump and system curve interaction as a core part of rotodynamic pump selection.
A single design point may be inadequate where:
- Tank levels change
- Filters or strainers gradually block
- Several discharge routes operate
- Control valves change position
- Multiple pumps start or stop
- Product viscosity changes with temperature
- Production demand varies
- Pipework may be extended later
Develop system curves for the important operating cases. Plot these against the proposed pump curve and check efficiency, power, Net Positive Suction Head Required, minimum flow and the manufacturer’s permitted operating region.
For a positive displacement pump, calculate the differential pressure for each operating case. Confirm that the pump, drive, coupling, relief system and pipework can tolerate the highest credible pressure and starting torque.
Maximum flow and maximum head are not one operating point
Maximum flow normally occurs near the low-head end of a centrifugal pump curve. Maximum head occurs at or near zero flow.
A listing that states “maximum flow 240 L/min” and “maximum head 50 m” does not mean the pump produces 240 L/min at 50 m. Those figures describe different ends of the curve.
Select against the complete performance curve and the required system curve. The duty should fall within the pump’s approved operating region, with enough margin for genuine uncertainty rather than an arbitrary percentage added to every value.
See Stop Choosing Pumps Based on Max Flow and Max Head for a focused explanation of this mistake.
Check the fluid properties before accepting the curve
Most published centrifugal pump curves are based on water or another defined test liquid. The real fluid can change performance, power, wear, suction behaviour and material life.
Viscosity
Viscosity describes a fluid’s resistance to flow.
As viscosity rises, pipe friction and suction losses usually increase. A centrifugal pump may produce less flow and head than its water curve indicates, while efficiency falls and required input power changes. Hydraulic Institute guidance provides correction procedures for estimating rotodynamic pump performance on viscous liquids.
Positive displacement pumps often handle viscous fluids more effectively because increasing viscosity can reduce internal slip. High viscosity can still create problems:
- The fluid may not enter the pump fast enough
- Suction pipe losses may become excessive
- Starting torque may exceed the drive capacity
- Pump speed may need to be reduced
- Valves or small internal passages may respond poorly
- Viscosity may rise sharply during a cold start
Use the viscosity at the coldest credible pumping temperature as well as the normal operating value.
See How Viscosity Affects Pump Selection for technology comparisons and correction requirements.
Specific gravity
Specific gravity is the ratio of a fluid’s density to the density of water under defined conditions.
For a centrifugal pump handling fluids with similar viscosity, a change in density does not directly change the head produced at a given speed. It does change the pressure represented by that head and the power required.
Hydraulic power can be estimated using:
Pₕ = ρ × g × Q × H
Where:
- Pₕ = hydraulic power in W
- ρ = fluid density in kg/m³
- g = gravitational acceleration, approximately 9.81 m/s²
- Q = flow rate in m³/s
- H = pump head in m
A denser fluid requires more power at the same flow and head. Motor selection, shaft loading and drive torque must account for the maximum expected density. The Department of Energy also identifies specific gravity as a required input when determining pump power.
See How Specific Gravity Affects Pump Selection for the distinction between head, pressure and power.
Solids
A solids description requires more than “contains slurry”.
Record:
- Maximum particle size
- Typical particle size distribution
- Solids concentration by mass or volume
- Particle hardness and abrasiveness
- Particle shape
- Fibre length and tendency to wrap
- Whether particles settle
- Whether solids are soft, fragile or shear-sensitive
- Whether the fluid can dry, crystallise or set inside the pump
These details affect pump passages, impeller design, valve type, speed, clearances, wear materials and suction arrangement.
A pump may pass one large particle but wear rapidly in a fine abrasive slurry. Another design may tolerate abrasive fines yet block when exposed to stringy fibres. Settling solids can also collect in suction lines, casings and idle pumps.
See How Solids Affect Pump Selection for the full solids assessment.
Temperature, vapour pressure and entrained gas
Temperature can alter viscosity, vapour pressure, corrosion rate, elastomer behaviour and component clearances.
Use the temperature at normal operation, start-up, shutdown and cleaning. A pump that handles a warm product during production may struggle to start after the product cools and thickens.
Vapour pressure affects the pressure margin available at the pump inlet. Entrained gas can reduce centrifugal pump performance and may interrupt prime. Gas-tolerant pump designs still have limits, particularly when gas content changes.
Check suction conditions and NPSH
A correctly selected pump can still cavitate or lose prime when the suction system is poor.
Net Positive Suction Head Available (NPSHa) is determined by the system. It depends on absolute pressure at the liquid source, liquid level, vapour pressure, suction pipe losses and pump elevation.
Net Positive Suction Head Required (NPSHr) is a pump characteristic stated for defined operating conditions. The Hydraulic Institute distinguishes system-specific NPSHa from pump-specific NPSHr and recommends applying an appropriate margin rather than treating equality as a reliable design condition.
Check:
- Minimum liquid level
- Maximum fluid temperature
- Lowest expected atmospheric pressure or site altitude
- Pressure or vacuum inside the supply vessel
- Suction pipe length and diameter
- Valves, strainers, elbows and reducers
- Fluid viscosity
- Pump speed and flow
- Whether the pump must lift the fluid
- Priming and venting arrangements
Highly viscous fluids and some positive displacement pumps also require enough inlet pressure to fill the pumping chambers. A favourable NPSH calculation does not correct an inlet that is too small, badly arranged or unable to feed the pump at the required speed.
Use the pump manufacturer’s NPSH and inlet-condition instructions for the proposed model.
Select materials, seals and containment as one system
Material selection covers every wetted component, not only the casing.
Review:
- Casing and covers
- Impeller, rotor or pumping elements
- Shaft and sleeves
- Mechanical seal faces and secondary seals
- Diaphragms, hoses and tubes
- Valve balls, seats and springs
- Bushes, wear plates and liners
- Gaskets and O-rings
- Fasteners exposed to the fluid
- Flush, barrier or quench-system materials
Chemical compatibility depends on the exact chemical, concentration, temperature, contaminants and exposure time. pH alone cannot confirm compatibility.
A material that resists corrosion may have poor abrasion resistance. A hard wear material may be brittle or unsuitable for the chemical environment. Elastomers can swell, harden, soften or lose strength without visible metal corrosion.
Containment requirements also influence the design. Hazardous, toxic, flammable, sterile or high-value fluids may require a specific sealing arrangement, leakage monitoring, double containment or a sealless design.
Use current compatibility information from the manufacturer of each wetted component. Confirm the final combination against the actual fluid composition and temperature. General compatibility tables are screening tools, not approval for service.
See How to Select Pump Materials for the full material-selection process.
Do not oversize the pump to cover missing information
An oversized pump does not provide free safety margin.
An oversized centrifugal pump may require constant throttling, bypass flow or repeated starting and stopping. It may operate away from its Best Efficiency Point (BEP), increase pipe velocity and add unnecessary load to seals and bearings. The US Department of Energy identifies throttled valves, heavy bypass use, frequent seal or bearing replacement and intermittent cycling as common signs of oversizing.
Oversizing can also make process control less stable. A control valve may spend most of its life near the closed position because the pump produces more head than the system needs.
The better approach is to:
- Separate confirmed requirements from estimates.
- Calculate realistic minimum, normal and maximum duties.
- Plot the full operating range.
- Apply margin only where there is a defined uncertainty.
- Check whether staging, parallel pumps or variable speed provide a better fit.
- Select a motor for the credible load range without masking a poor hydraulic selection.
Operating close to BEP does not mean every pump must run at one exact flow. Use the manufacturer’s preferred and allowable operating regions for the selected model. The expected operating points should remain within those limits.
See Why Oversizing Pumps Causes Problems and Common Pump Selection Mistakes for the reliability and control consequences.
Pump selection guide: a repeatable process
Use the same sequence for new installations, replacements and upgrades.
- Define the process requirement.
Establish what the system must deliver, transfer, dose, circulate or drain. - Collect the fluid data.
Record composition, concentration, viscosity, specific gravity, solids, temperature, vapour pressure and hazards. - Define the operating cases.
Confirm minimum, normal, maximum, start-up, shutdown and upset conditions. - Calculate the hydraulic duty.
Determine flow, static head, pressure differences and friction losses. Build the required system curves. - Assess the suction system.
Calculate NPSHa where applicable and review priming, inlet velocity, gas handling and chamber-filling requirements. - Shortlist suitable pump technologies.
Remove technologies that conflict with the fluid, operating range, containment or control requirements. - Compare individual pump selections.
Review curves, speed, efficiency, absorbed power, NPSHr, allowable operating region, solids passage and mechanical limits. - Select materials and sealing.
Confirm all wetted components against the complete chemical, temperature and wear conditions. - Review the drive and controls.
Check motor or drive torque, speed range, starting method, overload protection, pressure protection and control response. - Check installation and maintenance requirements.
Confirm pipe loads, access, lifting, drainage, flushing, alignment, foundation, spare parts and safe isolation. - Compare whole-of-life consequences.
Consider energy use, wear life, maintenance labour, downtime, cleaning, consumables and disposal. - Document the selection basis.
Keep the application data, calculations, pump curve, assumptions, exclusions and manufacturer confirmation with the equipment records.
The final review should use the real operating range rather than one catalogue point. Department of Energy guidance similarly recommends measuring or defining actual head and flow, developing a system curve and selecting for efficiency across the expected operating conditions.
Make the final selection against real operating conditions
Before issuing a purchase order, confirm that the selected pump:
- Delivers every required operating point
- Remains within the approved operating region
- Has adequate suction margin
- Handles the maximum viscosity and solids load
- Has sufficient motor power and starting torque
- Uses compatible wetted materials
- Includes suitable sealing and pressure protection
- Meets site safety and electrical requirements
- Can be installed, isolated and maintained safely
- Has documented limits for abnormal conditions
Do not accept a selection based only on pump type, connection size, motor rating, maximum flow or maximum head.
Ask for the completed pump curve, efficiency, absorbed power, NPSHr or inlet requirements, materials list, speed, impeller or displacement details, operating limits and selection assumptions.
Where the application involves hazardous chemicals, flammable fluids, pressure risk, sanitary service or uncertain compatibility, obtain product-specific confirmation and an appropriate engineering and safety review before installation.
Pump selection FAQs
What is the most important information needed to select a pump?
The required flow range, system head, suction conditions and complete fluid properties form the minimum technical basis. Fluid properties should include viscosity, specific gravity, temperature, vapour pressure, solids and chemical composition.
Can I select a pump from flow rate and pressure alone?
No. Flow and pressure define part of the hydraulic duty, but they do not establish technology suitability, suction performance, material compatibility, solids handling, operating range or maintenance requirements.
Should the duty point be exactly at the Best Efficiency Point?
The normal duty should generally sit in a suitable region around the Best Efficiency Point, but the acceptable range depends on the pump design and manufacturer’s limits. Check every expected operating point rather than forcing one point to match BEP exactly.
When should I choose a positive displacement pump?
Positive displacement pumps are often considered for viscous fluids, controlled dosing, stable flow against changing pressure, low-speed handling or duties where a centrifugal pump performs poorly. The correct subtype depends on solids, shear, pulsation, materials and maintenance requirements.
Does specific gravity change centrifugal pump head?
For fluids with similar viscosity, changing density does not directly change the head produced at the same speed. It changes the pressure corresponding to that head and the power required from the pump and motor.
Why is an oversized pump a problem?
Oversizing can force a centrifugal pump to operate with throttled valves, bypass flow, frequent cycling or a duty far from BEP. These conditions can waste energy, reduce control quality and increase mechanical wear.
Who is responsible for confirming the final pump selection?
Responsibilities vary by project, but the application owner must provide accurate process and system data. The pump supplier or manufacturer should confirm model-specific performance and limits. The system designer must confirm the pump’s interaction with the complete installation.
About The Pump Expert
The Pump Expert provides independent, practical education for pump users, engineers and maintenance teams. TPE explains how pumps and pump systems behave so readers can make better technical decisions without supplier bias.
Last Updated on July 27, 2026 by TPE