What are the advantages of centrifugal pumps over positive displacement pumps?
Pump selection is easier when you understand the pros and cons of the various types of pump technology. Narrowing it down to the best pump type to consider for your application is the first question, and will go along way to getting you on the right path in optimum pumping efficiency.
Nearly every pump can be classified as belonging to one of two types; those that move fluids on the principle of displacement, and those that create flow by exerting centrifugal force on the fluid.
Here we outline the many general advantages of centrifugal-type pumps, so it will soon become clear when this family will be your go-to, or if you are better off looking at a positive displacement (PD) pump.
The Centrifugal Pump Principle
In a centrifugal pump, the fluid flows into the central ‘eye’ of a circular chamber where a rapidly spinning impeller imparts energy to it. This creates centrifugal force, pushing the fluid out against the volute wall, where it builds pressure until the fluid ‘escapes’ out of the discharge port.
This creates a fast flow, which continues until the dynamic energy is dissipated through friction and gravitational forces, eventually reaching its maximum head height.
The overarching advantage of centrifugal pumps over PD pumps is high flow rates and greater head heights on any given power comparison.
Development of the Centrifugal Pump (15th – 20th century)
Early pump inventions were based on the positive displacement principle; bucket-type shadoofs (2000 – 3000 BC) to Ctesibius’ water organ and Archimedes’ Screw in 200 BC for example. The concept of the centrifugal pump appeared much later, with the earliest known evidence documented during the Renaissance. Denis Papin improved on this earlier design in the late 17th century, with John Appold making further efficiency improvements with curved vane impellers two hundred years later.
The Industrial Revolution’s “golden age of engineering” and the challenges of the 20th century led to rapid innovation in centrifugal pump development, with new manufacturers of pumps springing up: Apex, KSB, G & J Weir, Wilo, Warman, Durco, Ebara, LaBour, Graco, Lowara, Grundfos, and SlurryPro are some names you may recognise today.
During the 20th century, many step-by-step improvements have been made to improve the efficiency and capabilities of centrifugal pumps. These include lowering the NPSHr (Net Positive Suction Head required), introducing self-priming options, improving energy efficiency, improving wear life through rubber lining or hardened alloys, refining electrical controls, and introducing smart sensors and remote monitoring/cloud-based controls.
The Centrifugal Pump Today
Centrifugal pumps have been adapted to suit many applications, due to the numerous advantages of this type of pump, such as:
Flowrate
The dynamic energy imparted to the fluid enables a high flow rate, especially with low viscosity fluids such as water. The consistent rotational direction produces a smooth, non-pulsating flow.
Efficiency
The moving parts (impeller and shaft) of the centrifugal create low resistance and little friction as the stationary and moving surfaces are mounted on lubricated bearings and gland packing (or better still mechanical seals). Most large, industrial-sized clean water models show an efficiency of 70 – 90% when running at their BEP (Best efficiency point). Even with high-density slurries, the right centrifugal pump can achieve 55 – 60+% efficiency with the correct impeller selection and volute design.
One benefit of centrifugal pumps over positive displacement types is their ability to build sequential pressure and increased head when operated in series. This versatility makes them well-suited to booster pump applications.
Versatility
The hydrodynamic performance and wear life of a centrifugal pump can be optimised for the specified duty by the selection of materials, geometry, and speed/RPM. Whereas most PD pumps have only one or two variables (speed and sometimes material of wet-parts), most centrifugal models can be fitted with a variety of impellers. For example, ‘closed’ impellers are more efficient for clean water, while ‘open’ impellers are better for handling thick slurries and those with suspended solids. The same pump chassis is often compatible with many different designs of impeller, and can be lined with wear-resistant rubber or corrosion-proof alloy, or left as cast iron.
RPM can usually be altered by using a variable frequency drive (VFD), pulley sizing, or a combination of both.
This allows the pump specifier to match the required attributes to the application without incurring a significant and costly redesign.
Pressure building
Whereas PD pumps set their own pressure independently of the supply system, centrifugal pumps are affected by intake pressure. This makes them ideal for multi-stage pumping in booster applications as multiple units connected in series increase the pressure and head.
Reliability
The simplicity of centrifugal pumps and their single direction of rotation means less maintenance than more complex pumps such as reciprocating, diaphragm, or other piston-based pumps.
Tolerance
Generally, the drop-off in performance from wear is less pronounced on centrifugal pumps, compared with PD pumps which require exacting, undamaged surfaces to function properly.
Furthermore, some models allow the maintenance engineer to adjust the impeller as it wears, to achieve more life before a full replacement is needed.
Compact design
Often, Centrifugal pumps occupy less floor space than alternatives. Furthermore, their discharge port can usually be changed to an angle that facilitates easier installation in tight spaces – ideal for manufacturing plants and treatment facilities.
These pumps can also be built with the motor and pump connected to a single shaft, or offset via a pulley. Overhead mount is the most common type of centrifugal slurry pump mounting method, where the motor is situated above and parallel to the pump itself. This enables the user to benefit from the long wear life of a centrifugal slurry pump whilst maintaining a compact footprint.
Above: An overhead mount centrifugal pump with its drive pulley revealed
The other type of centrifugal pump is a vertically aligned one that is good for pumping high volumes out of a sump pit, as the motor sits above the fluid with the impeller under the surface. With their flooded suction end, they do not need priming.
With PD pumps such as progressive cavity types, the pressure capability relies on the rotor and stator length, meaning they often take up more space horizontally than centrifugal pumps.
Centrifugal pumps are used in many industrial applications for good reason: mine dewatering, water treatment plants, bilge dewatering, dredging, tunnel boring machines, steel mills, recycling centres, and more.
Our pump experts can confirm which pump technology will be best for your specific situation, and design the right centrifugal pump configuration for best lifetime value and performance. Talk to us via phone, book a site visit, or request further information via our online chat.
