A centrifugal pump is a machine that’s made to move fluid from one or more driven rotors (impellors), with the help of rotational energy. The fluid gets into the fast rotating motor along its axis, and it is cast out via centrifugal force through the impeller’s vane tips along its circumference. The driven rotor’s action increases the pressure and velocity of the fluid and ideally directs it towards the outlet. The machine’s casing is designed to constrict fluid from the inlet, direct it to the rotor and gradually control the fluid before being discharged.
How Do Centrifugal Pumps Operate?
As you may have already deduced, the impellor or driven rotor is the primary component of the machine. It is made up of a series of curved vanes, which are usually sandwiched between two discs (referred to as an enclosed impeller). For fluids containing entrained solids, a semi-open or fully open impellor is preferred.
Centrifugal pumps casings usually have two basic designs, the diffuser, and volute. The role of both designs is to transform the fluid flow into a controlled state where it can be discharged at pressure. In the latter, the rotor is offset, thus creating a curved funnel with a cross-sectional area that increases towards the outlet. This leads to an increase in fluid pressure toward the outlet.
As for the diffuser, the same principle applies. The pressure of the fluid increases as it is being expelled. Volute casings tend to be more ideal for applications that involve high viscosity fluids or entrained solids when it is best to avoid the extra constriction of diffuser vanes. Diffuser designs, on the other hand, can be tailored for specific applications, thus making them more efficient.
Features of a Centrifugal Pump
Well, there are two primary families of pumps, namely positive displacement and centrifugal pumps. The latter tend to be specified for higher fluid flow and pumping low viscosity fluids, down to .1 cP. Most of the pumps in chemical plants are of the centrifugal type, but you will come across some applications that call for positive displacement plants.
The Downsides of Centrifugal Pumps
The efficiency of a centrifugal pump largely depends on the constant, high-speed rotation of the rotor. When it comes to high viscosity fluids, they become very inefficient as there is higher resistance, and more pressure is required to maintain a certain flow rate. As such, centrifugal pumps are best suited for high capacity, low-pressure applications with viscosities not going beyond 200 cP.
High viscosity fluids and slurries like mud can lead to wear and overheating of the components. Positive displacement pumps tend to run at lower speeds and are rarely prone to such issues.
Any fluid that’s sensitive to the separation of emulsions, which is known as shearing, can ideally be damage by the centrifugal pump’s high speed. In such situations, the lower speed of positive displacement pumps is often preferred.
As you can see, every pump, whether a centrifugal or diaphragm pump, has its benefits, limitations, and a role to play.