Centrifugal vs Positive Displacement Pumps: Choosing the Right Pump for Water Plants

Every water treatment plant depends on pumps. They lift raw water from borewells, push it through filters, feed RO membranes, dose chemicals, transfer sludge and deliver treated water. Choosing the wrong pump leads to high energy bills, frequent breakdowns or poor process control. The first and most fundamental choice is between two families: centrifugal pumps and positive displacement pumps.

How Centrifugal Pumps Work

A centrifugal pump uses a rotating impeller to throw water outward, converting rotational energy into flow and pressure. The flow rate depends on the system resistance: as pressure in the system rises, flow decreases. This relationship is shown on the pump curve.

Common Types

• End suction pumps for general transfer duties
• Vertical multistage pumps for higher pressures, such as RO feed
• Submersible pumps for borewells and sumps
• Split case pumps for large flows

Strengths

• Smooth, continuous flow
• Simple construction with few wearing parts
• Efficient for clean water at moderate to high flows
• Wide range of sizes and materials

Limitations

• Flow varies with system pressure
• Struggles with viscous fluids or high solids
• Not suited for precise metering

How Positive Displacement Pumps Work

A positive displacement pump traps a fixed volume of fluid and pushes it forward with each cycle. The flow rate stays almost constant regardless of system pressure, as long as the pump’s pressure rating is not exceeded.

Common Types

• Diaphragm metering pumps for chemical dosing
• Progressive cavity pumps for sludge and viscous fluids
• Piston and plunger pumps for very high pressure
• Lobe and gear pumps for specific applications

Strengths

• Accurate, predictable flow
• Handles viscous fluids and slurries well
• Delivers high pressure where needed

Limitations

• Pulsating flow in some designs
• More wearing parts in certain types
• Must never run against a closed valve without protection, as pressure can rise dangerously

Matching Pumps to Duties

Raw Water and Transfer

Centrifugal pumps are the natural choice for moving clean or lightly turbid water.

Filter Feed and Backwash

Centrifugal pumps sized for the required flow and filter pressure drop work well. Backwash pumps need adequate flow to expand the media bed.

RO High-Pressure Feed

Vertical multistage centrifugal pumps are widely used for brackish water RO. For seawater RO at very high pressures, specialised high-pressure pumps, sometimes positive displacement types, are used.

Chemical Dosing

Positive displacement metering pumps, typically diaphragm types, are preferred because they deliver accurate, adjustable volumes.

Sludge Transfer and Dewatering

Progressive cavity and other positive displacement pumps handle thick sludge reliably, especially for filter press feed.

Efficiency and Energy

Pumps run for many hours a day, so energy cost over the life of a pump often exceeds its purchase price many times. Selecting a pump that operates near its best efficiency point is essential. Oversized pumps throttled with valves waste energy. Variable frequency drives allow pump speed to match demand, saving power especially in systems with variable flow.

Materials of Construction

Material selection depends on the water and chemicals involved. Stainless steel suits most treated water applications. Highly corrosive chemicals need appropriate plastics or specialised alloys. Seawater requires duplex or super duplex steels in many cases.

Reliability Features

• Standby pumps for critical duties
• Dry-run protection for submersible and transfer pumps
• Pressure relief for positive displacement pumps
• Proper suction design to avoid cavitation

Conclusion

The right pump is not the cheapest one or the biggest one, but the one matched to the specific duty. Understanding the strengths of centrifugal and positive displacement designs helps plant owners and engineers make choices that save energy, improve control and reduce downtime for years to come.

Need help with your water or wastewater project? Aguapuro Equipments designs, supplies and services water treatment, effluent treatment and bottling solutions. Send us an enquiry or contact our team to discuss your requirements.