Types of Pumps for Water Supply Systems: Centrifugal and Positive Displacement

Pumps are the backbone of any water supply system – whether it’s delivering water from a tube well to a dairy farm, supplying municipal water to a town, or irrigating hundreds of acres of farmland. Without pumps, water would only move through gravity, and that’s rarely enough to meet real-world demand. Understanding the different types of pumps helps you pick the right one for your setup, saving energy, money, and maintenance headaches in the long run.

Broadly, pumps fall into two main categories: kinetic (centrifugal) pumps and positive displacement pumps. Each works on a fundamentally different principle, and each has strengths that make it better suited for specific situations. Beyond these two families, specialized designs like vertical turbine pumps and submersible pumps address the unique challenge of extracting water from deep wells.

How pumps are classified 🔗

The simplest way to categorize pumps is by how they move water. Kinetic pumps use rotational energy – typically from an electric motor – to accelerate water continuously through an impeller. The velocity gained is then converted into pressure. Positive displacement pumps, on the other hand, trap a fixed volume of water and physically push it through the system using pistons, gears, diaphragms, or similar mechanisms.

This fundamental difference in mechanism leads to very different performance characteristics, especially regarding flow rate, pressure handling, and the types of fluids each can manage effectively.

Centrifugal pumps 🔗

Centrifugal pumps are the most widely used pump type across water supply, irrigation, and industrial systems. Their popularity comes down to a combination of simplicity, affordability, and the ability to move large volumes of water efficiently.

How centrifugal pumps work 🔗

A centrifugal pump has a rotating component called an impeller mounted inside a casing. When the motor spins the impeller at high speed, it creates a low-pressure zone at the centre. Water gets drawn into this zone through the inlet. The spinning blades then fling the water outward due to centrifugal force, increasing its velocity. As the water moves into the casing (often called a volute), this velocity energy gets converted into pressure energy, pushing the water through the outlet pipe.

The whole process is continuous – water flows in, gets accelerated, and flows out in a steady stream. This is why centrifugal pumps produce a smooth, non-pulsating flow, which is a significant advantage in water distribution networks.

Key characteristics of centrifugal pumps 🔗

Variable flow rate: One important thing to know about centrifugal pumps is that their flow rate changes with system pressure. As the pressure (or “head”) the pump works against increases, the flow rate decreases. This relationship is shown on a pump performance curve, which is an essential tool when selecting a centrifugal pump for any application.

Best for low-viscosity fluids: Centrifugal pumps work most efficiently with thin fluids like water, light fuels, and dilute chemicals. They are not well-suited for thick or viscous fluids because frictional losses increase significantly with fluid thickness.

Priming requirement: A centrifugal pump needs to be filled with liquid before it can start working – a process called priming. If air gets trapped inside the casing, the impeller simply spins without generating enough pressure to move water. This is an important operational consideration, especially in systems where the pump sits above the water level.

Simple design, lower maintenance: With fewer moving parts than positive displacement pumps, centrifugal pumps are generally easier and cheaper to maintain. This makes them suitable for applications where the pump runs continuously or for extended periods.

Common applications in water supply 🔗

Centrifugal pumps dominate water supply systems for good reason. They are used extensively in irrigation systems where large volumes of water must be delivered across fields, in municipal water distribution to supply homes and businesses, and in industrial processes where water needs to be circulated or transferred. In some chemical processing facilities, centrifugal pumps make up the overwhelming majority of all pumps installed.

Positive displacement pumps 🔗

Positive displacement (PD) pumps operate on a completely different principle. Instead of accelerating water with an impeller, they enclose a fixed volume of fluid and mechanically force it through the system. This action can be driven by pistons, gears, screws, lobes, or diaphragms.

How positive displacement pumps work 🔗

The basic cycle involves three steps: the pump creates a cavity that expands to draw fluid in (suction phase), the cavity then closes to trap the fluid, and finally the cavity contracts or moves to push the fluid out (discharge phase). This cycle repeats continuously, with each stroke or rotation moving a fixed, predictable volume of fluid.

There are two main sub-types:

Reciprocating PD pumps use a back-and-forth motion of a piston, plunger, or diaphragm. During the suction stroke, an inlet valve opens and fluid fills the chamber. On the return stroke, the inlet valve closes, the outlet valve opens, and fluid is pushed out. Piston pumps and diaphragm pumps fall into this category.

Rotary PD pumps use rotating elements – gears, lobes, screws, or vanes – to move fluid. As the elements rotate, they trap fluid between themselves and the pump housing, carrying it from the inlet to the outlet. Gear pumps and lobe pumps are common rotary types.

Key characteristics of positive displacement pumps 🔗

Constant flow rate: The defining feature of PD pumps is that they deliver a steady flow rate regardless of changes in system pressure. This makes them ideal for applications where precise metering or dosing is required.

High-pressure capability: PD pumps can generate very high pressures, making them suitable for applications like high-pressure cleaning or chemical injection. However, this also means they must never operate against a closed discharge valve – the pressure will keep building until something ruptures. A relief valve on the discharge side is therefore essential for safety.

Pulsating flow: Unlike the smooth output of centrifugal pumps, reciprocating PD pumps produce a pulsating discharge. The fluid accelerates during the compression phase and slows during suction. This pulsation can cause vibrations in the piping system. Using multiple cylinders or pulsation dampeners can help reduce this issue.

Self-priming ability: Most positive displacement pumps can start without being pre-filled with liquid, which is a practical advantage in many field installations.

Handles viscous fluids well: PD pumps are generally better at handling thick, viscous fluids – oils, slurries, and pastes – compared to centrifugal pumps. Their efficiency actually improves with thicker fluids in some designs.

Why PD pumps are less common in water supply 🔗

For general water supply and irrigation, centrifugal pumps are almost always the preferred choice. Water is a low-viscosity fluid with high flow rate requirements – exactly where centrifugal pumps excel. PD pumps are more commonly found in chemical dosing systems (where precise volumes matter), high-pressure cleaning, and food and beverage processing (where gentle handling and consistent flow are needed).

Centrifugal vs. positive displacement: a quick comparison 🔗

Flow behaviour: Centrifugal pumps provide a variable flow rate that depends on system pressure. PD pumps maintain constant flow regardless of pressure.

Pressure handling: Centrifugal pumps work best in moderate-pressure, high-flow scenarios. PD pumps handle high pressures at lower flow rates.

Fluid suitability: Centrifugal pumps are ideal for thin fluids like water. PD pumps handle viscous fluids more efficiently.

Efficiency pattern: Centrifugal pumps have a narrow peak efficiency range and lose performance if operated away from it. PD pumps maintain consistent efficiency across a wider range of operating conditions.

Maintenance: Centrifugal pumps have simpler designs and fewer parts, resulting in lower maintenance. PD pumps tend to require more upkeep due to their more complex mechanisms.

Vertical turbine pumps 🔗

When water must be lifted from deep underground sources – tube wells, borewells, or deep reservoirs – standard surface-mounted centrifugal pumps are not practical. This is where vertical turbine pumps come in. They are essentially a type of centrifugal pump designed with a vertical shaft configuration to move water from significant depths to the surface.

How vertical turbine pumps work 🔗

A vertical turbine pump consists of a motor mounted above ground, a long vertical shaft, and one or more impeller-and-diffuser assemblies (called stages or bowls) submerged deep in the well. The motor drives the shaft, which rotates the impellers below. Water enters at the bottom through a suction bell, gets accelerated by the first impeller, and then passes into a diffuser bowl where velocity energy converts into pressure. If more pressure is needed – as in very deep wells – additional stages are stacked vertically. Each stage adds more head (lifting capacity).

Deep-set vertical turbine pumps can be installed at depths ranging from about 30 metres to over 300 metres, with settings between 60 and 120 metres being common for agricultural and municipal use.

Applications and advantages 🔗

Vertical turbine pumps are widely used in agricultural irrigation, municipal water supply, and industrial cooling systems. Their vertical design saves space and eliminates the need for suction piping that surface pumps require.

A key advantage is that because the motor sits above ground, it remains accessible for maintenance and can be powered by electric motors, diesel engines, or other prime movers. This makes vertical turbine pumps practical for remote locations where electricity may not be readily available.

Submersible pumps 🔗

Submersible pumps take a different approach to the deep-well challenge. Instead of keeping the motor above ground, a submersible pump seals both the motor and pump together in a watertight housing, and the entire unit is lowered into the water source.

How submersible pumps work 🔗

The hermetically sealed motor drives the impeller directly, with no long shaft needed. Because the pump is already surrounded by water, it pushes fluid upward rather than pulling it from above. Most submersible pumps are multistage centrifugal units – the fluid passes through a series of impellers and diffusers, gaining pressure at each stage, until it exits through the discharge pipe at the surface.

Key advantages of submersible pumps 🔗

No priming needed: Since the pump is submerged, the intake is always filled with water. There is no risk of air entering the system, which eliminates the need for priming entirely.

No cavitation risk: Cavitation – the formation of vapour bubbles that can damage pump components – is largely avoided because the pump operates under positive fluid pressure at all times.

Energy efficiency: Submersible pumps use less energy because water pressure at the submerged intake assists the pump’s operation, reducing the work the motor must do.

Quiet operation: The surrounding water acts as a natural sound barrier, making these pumps significantly quieter than surface-mounted alternatives.

Compact and secure: Being entirely underground, submersible pumps take up no surface space and are protected from weather, tampering, and accidental damage.

Submersible pumps in agriculture 🔗

In farming, submersible irrigation pumps are among the most commonly used pump types, particularly for drawing water from borewells and tube wells. They deliver high flow rates from considerable depths, and can be paired with automation systems for precise irrigation scheduling. Their low maintenance needs and long operational life make them especially valuable in rural areas with limited access to repair services.

Vertical turbine vs. submersible: choosing for deep wells 🔗

Both vertical turbine and submersible pumps serve the same basic purpose – lifting water from deep underground – but the right choice depends on your specific conditions.

Depth and capacity: For very deep wells (especially beyond 30 metres) that require high flow rates and high pressure, vertical turbine pumps are often the better solution. For moderate depths with standard flow requirements, submersible pumps are more practical and cost-effective.

Maintenance access: Vertical turbine pumps keep the motor above ground, making routine inspection and servicing straightforward. Submersible pump motors, however, require the entire unit to be pulled out of the well for any service – a time-consuming and sometimes expensive process.

Installation constraints: Submersible pumps work well in wells that aren’t perfectly straight, where shaft alignment for a vertical turbine pump would be problematic. They are also preferred in locations where flooding could damage a surface-mounted motor.

Power flexibility: Vertical turbine pumps can run on diesel or gas engines in addition to electric motors, which is useful in areas without reliable electricity. Submersible pumps typically require electric power.

Factors to consider when selecting a pump 🔗

Choosing the right pump for a water supply system isn’t just about picking the most powerful option. Several practical factors come into play:

Required flow rate and pressure: Know how much water you need per hour and at what pressure. High-flow, moderate-pressure needs point toward centrifugal pumps. High-pressure, low-flow needs suit positive displacement types.

Water source depth: Surface or shallow sources can use standard centrifugal pumps. Deep wells require vertical turbine or submersible pumps.

Fluid characteristics: Clean water is straightforward – centrifugal pumps work fine. If the water contains significant sediment or if you’re also handling chemicals, the pump selection must account for this.

Energy availability: Electric, diesel, or solar – the available power source influences which pump types are feasible for your location.

Maintenance capacity: In remote farming areas, a pump that needs minimal and infrequent maintenance is worth its weight in gold. Submersible pumps and centrifugal pumps both score well here, though for different reasons.

Total cost of ownership: The purchase price is just the beginning. Factor in energy consumption, maintenance frequency, expected lifespan, and repair costs when comparing pump options.

What do you think? Given the specific water supply needs of your farm or project, which type of pump – centrifugal, positive displacement, vertical turbine, or submersible – would be the most practical choice, and why? How much weight do you give to energy efficiency versus ease of maintenance when making that decision?

References

1. https://en.wikipedia.org/wiki/Pump
2. https://www.ny-engineers.com/blog/main-types-of-pumps
3. https://www.gainesvilleindustrial.com/blog/centrifugal-vs-positive-displacement-pumps/
4. https://www.castlepumps.com/info-hub/positive-displacement-vs-centrifugal-pumps
5. https://www.michael-smith-engineers.co.uk/resources/useful-info/positive-displacement-pumps
6. https://cbeuptime.com/centrifugal-vs-positive-displacement-pumps/
7. https://www.pumpworks.com/what-is-a-vertical-turbine-pump/
8. https://www.introtopumps.com/pump-designs/vertical-turbine-pumps/
9. https://www.pentair.com/en-us/flow/commercial-products/water-supply-pumps/vertical-turbine-pumps.html
10. https://bbppump.com/what-is-a-submersible-pump-working-principle-explained/
11. https://pumpsolutions.com.au/how-submersible-pumps-work-advantages-and-disadvantages-of-submersible-pumps/
12. https://eddypump.com/education/what-to-know-about-submersible-pumps/