According to the Hydraulic Institute (HI) pumps are classified in two families i.e, positive displacement and centrifugal (roto-dynamic), both having their best areas of application and uses. However, to identify and select a pump as per their working principle and type of application is also a professional job.
Centrifugal pumps are the most common type of pump used for transferring low viscous fluid at high flow rates, having a minimal design that contains one or more impellers that move fluid by rotation and draw fluid into the suction end. As liquid moves through the impeller fins outward, higher kinetic energy is imparted through centrifugal force. At the volute, this kinetic energy is converted into pressure energy. Therefore, centrifugal pumps are pressure generating machines, flow is the result.
Selection of Positive displacement pumps are done due to its handling capacity of high viscous fluids at high pressures and low flow rate as their efficiency isn’t affected by pressure. They draw a fixed volume of liquid into the pump through the suction valve, trapping it within the cavity and then pushing it out through the discharge valve. The way liquid is forced out depends on the type of PD pump. Therefore, positive displacement pumps are flow generating machines and pressure is the result of system restrictions such as pipe shear, liquid shear, or work required (hydraulic systems).
The two pumps operate very differently. Positive displacement pumps can handle variations in pressure, flow, and viscosity and remain efficient, unlike centrifugal pumps which lose its flow as viscosity and pressure changes. For accurate dosing, centrifugal pump cannot be used while Positive displacement pumps such as peristaltic, piston and diaphragm pumps are ideal solutions for dosing applications as it allows accurate metering to be carried out. Suction lift is not possible in standard designs of a centrifugal pump, on the other hand, PD pumps create a vacuum on suction thus can create suction lift. Centrifugal pumps are not ideal for shear sensitive fluids due to high-speed motor while PD pumps have a low internal velocity which means lower shear making it suitable for shear sensitive fluids.
