A centrifugal pump can be defined as a mechanical device designed to move fluid or liquid through the transfer of rotational energy from single or multiple impellers, also referred to as rotors. In this, the liquid enters the impeller, which rotates rapidly along the axis and is excluded by centrifugal force along the circumference with the vane tips of the impeller.
Because of the impeller, the fluid’s velocity and pressure increase and are directed towards the pump’s outlet. Therefore, the casting of the pump has been designed to compress the liquid from the pump inlet and direct it towards the impeller. Thereafter it is slowed down to control the liquid before discharging it.
In a centrifugal pump, the key component happens to be the impeller. The impeller consists of various curved vanes. These veins are usually sandwiched between two discs that are enclosed impellers. Therefore, a semi-open or open impeller is best for fluids with solid particles.
The fluid enters the impeller at the eye of the axis and goes out through the circumference between the vanes. On the other hand, the impeller on the opposite side of the eye is connected to a motor through a drive shaft. When the motor rotates at high speed, almost 500 – 5000 RPM (rotation per minute), the impulse rotational motion accelerates the liquid through the impeller and into the pump casing.
When we talk of pump casing, there are two basic designs – diffuser and volute. The purpose of both designs is to translate the flow of the liquid at pressure into a controlled discharge. In the case of a volute casing, the impeller is offset, thereby creating an effective curved funnel. As a result, it has an increasing cross-sectional area toward the pump outlet. Because of the design, the pressure of the liquid increases when it reaches the outlet.
The same principle applies to the diffusion design as well. Here the pressure of the liquid increases as it is expelled between a pair of stationary vanes around the impeller. It is worth noting that a diffusion design can be customized depending on the nature of the application. Hence it is more efficient.
A volute case is better suited for entrained solids or high viscosity liquids when it is best to avoid the added constriction of the diffuser vanes. In addition, because of the asymmetry in the volute design, there can be a lot of wear on the impeller on the drive shaft.
When it comes to pumps, there are two main families. The first is a positive displacement pump, while the other is a centrifugal pump. Compared to a positive one, a centrifugal pump is way more effective for higher flows and for pumping low viscosity fluids (down to 0.1 cP).
In most plants, one would come across a centrifugal pump, almost 90 per cent. However, there are a few applications where a positive displacement pump performs better.
For an effective outcome from a centrifugal pump, the impeller’s high-speed rotation must be constant. A centrifugal pump can be increasingly inefficient if there is high viscosity feeds. The resistance will be more, and high pressure will be required to maintain a specific flow rate.
Therefore centrifugal pumps are best for low-pressure fluids but high capacity pumping applications within a viscosity range of 0.1 and 200 cP. Slurries like high viscosity oil and mud can lead to excessive heating and wear. As a result, it can lead to early damage and premature failures. On the other hand, a positive displacement pump operates at considerably lower speeds and is less prone to such problems.
Any pumped medium sensitive to the separation of slurries, emulsions, or biological liquids (shearing) can get damaged by the high speed of the impeller of a centrifugal pump. In such instances, the low speed of a positive displacement pump is more viable. Another limitation, unlike a positive displacement pump, is that a centrifugal pump cannot function well when dry. This is because it has to be initially primed with the liquid that it will pump. Hence centrifugal pumps are not so suitable for any application where the liquid supply is not constant.
On top, if the feed pressure varies too much, a centrifugal pump may produce inconsistent flow, unlike a positive displacement pump that is insensitive to pressure variations and can provide a constant output.
A centrifugal pump works best when it is used to pump oils, acids, bases, organics, solvents, water, or liquid. It does not matter whether it is a domestic, agricultural, or industrial application. In fact, a design for a centrifugal pump is suitable for virtually any kind of application related to low viscosity fluid.
A canned motor pump is used for chemicals, hydrocarbons, and other materials where there cannot be any leakage. The features of this pump include sealless, an impeller directly attached to the pump’s rotor, and a weighted part inside a can.
A chopper or grinder pump is used for industrial purposes like wastewater, sewage and food processing, and chemicals. The features of this pump include an impeller having grinding teeth for shopping solid materials.
A circulator pump is mainly used for ventilation, heating and air conditioning. The most striking feature of the span is an inline compact design.
A multistage pump works best that involves high-pressure applications. It comprises multiple impellers for achieving increased discharge pressure.
A cryogenic pump is most suitable for coolant and sand liquid natural gas. These farmers are made up of special construction materials so that they can be operated at low temperatures.
A trash pump is mainly used at pits, construction sites, and drain mines. This kind of pump is specially designed to pump water that has solid debris.
A slurry pump is best suited for industrial slurries, mineral processing, and mining. These pumps are specifically designed to withstand and handle highly abrasive slurries.
In case you are familiar with the operation, maintenance or specifications of a centrifugal pump, you must have heard of the pump curve. In simple terms, it gives an idea of the rate of flow or the output that can be expected from the pump at a specific pressure.
As the pressure of a centrifugal pump increases, the flow rate decreases to the point when it stops and does not give any output. On the other hand, if there is no pressure in the system, the pump can eject the maximum possible flow. On a graph, it can be seen that these points generate what is called the pump performance curve.
For a better understanding, let us take the example of a car. A car has multiple gears. The first gear can indeed provide acceleration from 0 to 40 km per hour; it would not be good for the engine to keep running at 40 km per hour in the first year. Likewise, it will not be good for the car’s engine if it is driven at 10 km per hour in third gear, even though the engine has been designed to operate at such speed.
Using the same logic, although a pump curve shows the various pressures and flow rates a pump can operate at, it does not indicate that the pump can be operated at all points of its curve. So the important question is, at what point should the pump run?
As the term indicates, BEP, or the best efficiency point of a pump, refers to the conditions specified by the manufacturer at which the pump runs most efficiently, extending the longest possible lifespan and least maintenance.
When the pump is operated at a lower capacity than the BEP, it is referred to as the operating ‘left of the curve’. Similarly, operating the pump at a higher capacity is called the ‘right of the curve’.
Although it is not practically possible to achieve the precise BEP on all occasions, it is recommended that the pump is within 10 per cent of the value. All operators and pump specifiers have found that the best efficiency point is in terms of cost and energy consumption. However, it must be noted that going too far right or left for a prolonged period can lead to cost issues and breakdown beyond wasted energy.
If the pump is operated low for a prolonged period, it leads to the fluid not flowing through the pump properly, leading to recirculation at the suction and discharge points.
When the flow of a centrifugal pump increases beyond its BEP, it leads to a phenomenon called cavitation. The pumped fluid goes below the vapour pressure, and bubbles have formed that burst once things migrate back to a high-pressure region.
A centrifugal pump operates when rotational energy is transferred from one or multiple impellers or rotors. The impeller increases the velocity and pressure of the fluid, directing it toward the pump outlet. The design is simple, and it is easy to operate and maintain as well.
