In brief
A centrifugal pump is a rotodynamic machine that accelerates liquids with a rotating impeller, thereby generating pressure and flow.
Contents
What is Centrifugal pump?
Centrifugal pumps are the most widely used type of pump in industry and building services. An impeller transfers energy to the liquid, which is converted into pressure in the casing, for example a volute. Unlike positive displacement pumps, the flow rate depends heavily on the back pressure of the system. Depending on impeller shape, a distinction is made between radial, mixed-flow and axial pumps; multistage pumps achieve higher heads.
Characteristic values are flow rate, head, power input and efficiency, which are summarised in the pump curve. The duty point results from the intersection of the pump curve and the system curve. If the pressure at the impeller inlet drops too far, vapour bubbles form and collapse abruptly; this cavitation destroys impellers. To avoid it, the NPSH available in the system must be sufficiently above the NPSH required by the pump.
Standardised pumps, for example to EN 733 or ISO 2858, have standardised main and connection dimensions, which makes replacement easier. Typical wear parts are the mechanical seal or gland packing, bearings, wear rings and the impeller.
In practice: maintenance and procurement
For spare parts or a replacement pump, the nameplate data, pumped medium, temperature, duty point, impeller diameter and the materials of casing, impeller and mechanical seal are required. Leaking mechanical seals should be replaced promptly, before escaping medium damages the bearings or motor.
Synonyms and abbreviations: rotodynamic pump · standardised pump · radial pump · volute pump · end-suction pump
Frequently asked questions
What is the difference between a centrifugal pump and a positive displacement pump?
A centrifugal pump transfers energy via a rotating impeller; its flow rate changes significantly with back pressure. A positive displacement pump, such as a piston, gear or screw pump, encloses volumes of liquid and pushes them onward; its flow rate remains largely independent of pressure. Centrifugal pumps are suited to large volumes of low-viscosity media, positive displacement pumps to high pressures or viscous media.
What is cavitation in centrifugal pumps?
Cavitation occurs when the pressure at the impeller inlet falls below the vapour pressure of the liquid. Vapour bubbles form, which collapse in areas of higher pressure and erode material from the impeller. Signs include crackling noises, vibration and loss of performance. Remedies include sufficient inlet pressure, shorter suction lines or an adjusted duty point.
What does NPSH mean?
NPSH stands for net positive suction head. The NPSHr value indicates the minimum suction head the pump needs at the inlet; it is taken from the pump curve. The NPSHa value describes what the system actually provides. To prevent cavitation, NPSHa must be above NPSHr with a sufficient safety margin.
What information is needed for a replacement centrifugal pump?
Required are the flow rate and head at the duty point, the pumped medium with temperature and density, connection nominal sizes, installation type and motor data. Materials and seal design are also needed. With standardised pumps, standardised dimensions make replacement easier; nevertheless, the pump curve and connection dimensions should be compared.