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Centrifugal Pump Working Principle

Centrifugal Pumps: The Workhorses of Fluid Flow

An impeller, also known as a driven rotor, is used in centrifugal pumps to generate centrifugal force, which forces fluid through the pump and into the outlet pipe. Through the vane tips on the impeller, the fluid enters the impeller along its axis and is released by centrifugal force throughout its circle. Numerous sectors, such as food processing, mineral processing, coal and oil refineries, chemical plants, and oilfield solids control, use centrifugal pumps. 

Centrifugal pumps are the go-to machines for moving liquids using rotating magic. Here's the gist:

• The Basics: They take spinning energy, typically from an electric motor, and convert it into the movement of fluids.

• The Workhorse: An impeller with fan-like blades spins inside the pump. As the blades rotate, they push liquid outwards by centrifugal force (think spinning a bucket of water - the water wants to fly out!).

• The Flow: The liquid gains speed and pressure as it moves through the impeller and exits the pump.

• The Applications: Widespread! From watering your lawn (sprinkler systems) to powering giant cooling systems in factories, centrifugal pumps are everywhere.

Simple design, reliable operation, and versatility make centrifugal pumps essential tools for keeping fluids flowing!

The working principle of a centrifugal pump is centered around the use of rotational energy to move fluids. When the pump is activated, a motor drives a rotating component called the impeller. The impeller has curved blades that spin rapidly within the pump’s casing, pulling fluid into the pump through the suction nozzle. As the fluid enters the impeller, the blades accelerate it outward with high velocity, causing a powerful centrifugal force that pushes the fluid to the edges of the pump casing. This increase in fluid velocity is then converted into pressure as the fluid is directed out through the discharge nozzle.

The internal arrangement of the centrifugal pump is simple yet effective:

• Impeller: This rotating part is the heart of the pump, where the fluid gains speed and kinetic energy.

• Casing: Surrounding the impeller, the casing captures the high-velocity fluid and channels it into a more controlled flow, converting the speed into pressure.

• Shaft: The shaft connects the impeller to the motor and is crucial for transferring the rotational energy to the impeller.

• Mechanical Seal or Packing: Around the shaft, seals prevent fluid leakage, maintaining pressure inside the pump.

• Bearings: Bearings support the shaft, allowing smooth rotation and reducing friction within the pump.

• Suction and Discharge Nozzles: These inlet and outlet passages allow fluid to enter and exit the pump efficiently.

Overall, the centrifugal pump works by converting rotational energy from the motor into kinetic energy in the fluid, which then changes into pressure energy to enable fluid movement through the system. This design is both efficient and versatile, making centrifugal pumps ideal for many applications, from water supply to chemical processing.

How does it operate?

Fluid enters the pump axially through the impeller's eye (a low pressure area) in the volute, which rotates rapidly. The impeller and blades give the incoming fluid momentum as they rotate. A vacuum is produced at the impeller's eye as the fluid accelerates radially outward from the pump, drawing more fluid into the pump over time. The fluid's kinetic energy increases with increasing velocity. High-kinetic-energy fluid is driven into the volute from the impeller area. The fluid passes through a continuously growing cross-sectional area in the volute, where fluid pressure is created from the fluid's kinetic energy.In line with the Bernoulli principle.

While radial and forward-curved blade designs are also available, the impeller blades are typically backward-curved. Depending on the design employed, there is a little fluctuation in the output pressure. The blades might be closed or open. In order to assist in directing the flow toward the exit, the diffuser may also be equipped with fixed vanes. The velocity at the impeller's edge and the energy supplied to the liquid are equal. The velocity head increases with the size of the impeller or with its speed of rotation.

Euler's Turbomachine Equations in Pump Theory

Due to its ability to link the specific work Y with the impeller's geometry and velocities, Euler's turbomachine equation also known as Euler's pump equation plays a crucial role in the field of turbomachinery. The ideas of energy and angular momentum conservation serve as the foundation for the equation.

The turbomachine equations of Euler are:

Shaft torque: Tshaft = ρQ(r2Vt2 – r1Vt1)

Water horsepower: Pw = ω . Tshaft = ρQ(u2Vt2 – u1Vt1)

Pump head: H = Pw / ρgQ = (u2Vt2 – u1Vt1)/g

Centrifugal Pump Euler's Turbomachine Equations in Pump Theory

• where the impeller's diameters at the inlet and output are denoted by r1 and r2, respectively.

• The impeller's absolute velocities at the intake and outflow are denoted by u1 and u2, respectively (u1 = r1. ω).

• The tangential velocities of the flow at the intake and outflow are denoted by Vt1 and Vt2, respectively.

Using Euler's turbomachine equations, one can forecast how altering the impeller geometry will affect the head. Whether we are working with a turbine or a pump makes no difference. Work is being done on the fluid when the torque and angular velocity have the same sign (a pump or compressor). When a turbine's torque and angular velocity are in opposition to each other, work is being taken from the fluid. The Euler equations are therefore very helpful for the design of turbines and pumps.

Pump Performance Calculation, for instance

In this example, we will see, how to predict

• Design discharge.

• Water horsepower.

• Pump head.

of a centrifugal pump. This performance data will be derived from Euler's turbomachine equation:

Shaft torque: Tshaft = ρQ(r2Vt2 – r1Vt1)

Water horsepower: Pw = ω . Tshaft = ρQ(u2Vt2 – u1Vt1)

Pump head: H = Pw / ρgQ = (u2Vt2 – u1Vt1)/g

Centrifugal Pump Head Performance Calculation

Given are the following data for a centrifugal water Pump

• Diameters of the impeller at the inlet and outlet.

• r1 = 10 cm.

• r2 = 20 cm.

• Speed = 1500 rpm (revolutions per minute).

• the blade angle at inlet β1 = 30°.

• the blade angle at outlet β2 = 20°.

• Assume that the blade widths at inlet and outlet are: b1 = b2 = 4 cm.

• Solution: The radial velocity of the flow at the output must first be determined. The tangential component of the velocity is zero so, assuming the flow enters the impeller exactly normal, the radial velocity from the velocity diagram equals

Vr1 = u1 tan 30° = ω r1 tan 30° = 2π x (1500/60) x 0.1 x tan 30° = 9.1 m/s

The amount of volume flow rate entering the impeller is determined by the radial component of the flow velocity. Therefore, using the following equation, we can calculate the pump's discharge when we know Vr1 at the inlet. The impeller blade width at the inlet is denoted by b1 in this instance.

Q = 2π.r1.b1.Vr1 = 2π x 0.1 x 0.04 x 9.1 = 0.229 m3/s

Since it is assumed that the entrance tangential velocity Vt1 equals zero, we must find the output tangential flow velocity Vt2 in order to compute the necessary water horsepower (Pw).

The outlet radial flow velocity Q is conserved as follows:

Q = 2π.r2.b2.Vr2 ⇒ Vr2 = Q / 2π.r2.b2 = 0.229 / (2π x 0.2 x 0.04) = 4.56 m/s

The outlet blade angle, β2, can be simply stated as follows using the velocity triangle figure.

cot β2 = (u2 – Vt2) / Vr2

Thus, the velocity of tangential flow at the output (Vt2) is:

Vt2 = u2 – Vr2 . cot 20° = ω r2 – Vr2 . cot 20° = 2π x 1500/60 x 0.2 – 4.56 x 2.75 = 31.4 – 12.5 = 18.9 m/s.

Next, the necessary water horsepower is:

Pw = ρ Q u2 Vt2 = 1000 [kg/m3] x 0.229 [m3/s] x 31.4 [m/s] x 18.9 [m/s] = 135900 W = 135.6 kW

Additionally, the pump head is
H ≈ Pw / (ρ g Q) = 135900 / (1000 x 9.81 x 0.229) = 60.5 m

Centrifugal pumps may seem like simple machines, but they consist of several key parts working together to efficiently move liquids. These parts can be broadly categorized into two sections: the wet end and the mechanical end.

Cross Section of Centrifugal Pump

Simple Diagram with Parts Name of Centrifugal Pump

Wet End

• Impeller: The heart of the pump, the impeller is a rotating component with fan-like blades. As the impeller spins, it pushes liquid towards the outer edges of the casing due to centrifugal force.

• Casing: The stationary housing that surrounds the impeller. It channels the liquid from the suction inlet to the discharge outlet, and converts the velocity imparted by the impeller into pressure. Casings typically have a spiral-shaped volute design to efficiently collect and direct the pressurized liquid.

Mechanical End

• Shaft: Connects the impeller to the motor or other prime mover and transmits the rotational force to the impeller.

• Shaft Sleeve: A wear sleeve that protects the pump shaft from contact with the pumped liquid, reducing wear and tear.

• Bearings: Located around the shaft to support it and allow for smooth rotation. Bearings reduce friction between the rotating shaft and the stationary pump housing.

• Seals: Create a tight seal between the rotating shaft and the stationary casing to prevent leakage of the pumped liquid. Mechanical seals are the most common type used in centrifugal pumps.

Additional Components

• Suction and Discharge Ports: Openings in the casing that allow liquid to enter (suction) and exit (discharge) the pump.

• Support Frame: Provides a sturdy base for the pump and motor, and helps to dampen vibrations.

There are several types of centrifugal pumps, including:

• Radial flow pumps: These pumps have an impeller with vanes that are perpendicular to the shaft. The fluid enters the impeller near the center and is flung outwards by the spinning impeller, creating a radial flow pattern. These pumps are used for low-pressure, high-flow rate applications.

• Axial flow pumps: These pumps have an impeller with vanes that are parallel to the shaft. The fluid enters the impeller near the center and is pushed outwards along the axis of the shaft, creating an axial flow pattern. These pumps are used for high-pressure, low-flow rate applications.

• Mixed flow pumps: These pumps have an impeller with vanes that are at an angle to the shaft, creating both radial and axial flow patterns. These pumps are used for medium-pressure, medium-flow rate applications.

• Self-priming pumps: These pumps are designed to be able to start pumping fluid without being filled with fluid first. They are commonly used in applications where it is not possible to fill the pump with fluid before starting, such as irrigation, sewage treatment and construction.

• Submersible pumps: These pumps are designed to be operated underwater, they are commonly used in applications such as water wells, fountains, and sewage treatment.

• Multistage pumps: These pumps have multiple impellers that are stacked on the same shaft. They are used in applications requiring high head and low flow rate such as fire fighting, water supply and irrigation.

The impeller, the heart of a centrifugal pump, comes in various designs to suit different pumping needs.  These impeller types are primarily classified based on the presence or absence of shrouds around the vanes. Here's a breakdown of the most common impeller types:

1. Closed Impeller

• Design: Enclosed on both sides (shrouds encasing the vanes entirely), providing a high degree of efficiency and good pressure handling.

• Application: Ideal for clean liquids with minimal solids or abrasives, due to the tight clearances between the impeller and the casing. Commonly used in applications demanding high pressure, such as multistage pumps and boiler feed pumps.

2. Semi-Open Impeller

• Design: Features a shroud on the back of the impeller for structural support, but remains open on the front side. This design offers a balance between efficiency and solid handling capability.

• Application: Well-suited for liquids containing small to medium-sized solids or suspended particles, often found in applications like pulp and paper processing, wastewater treatment, and slurry pumps.

3. Open Impeller

• Design: Completely open on both sides, offering the least hydraulic efficiency but the most significant advantage of superior solids handling.

• Application: The go-to choice for handling large, stringy, or fibrous materials, such as raw sewage, unscreened liquids, and trash removal applications.

Additional Impeller Types

• Vortex Impeller: These bladeless impellers use a swirling, vortex motion to create flow, making them ideal for handling liquids with high gas content or entrained air.
• Mixed Flow Impeller: A hybrid design combining features of axial and centrifugal impellers, offering a good compromise between flow rate and pressure capabilities.

The selection of the right impeller type is crucial for optimal pump performance and efficiency.  Choosing the wrong impeller can lead to reduced flow rates, increased energy consumption, or even damage to the pump. By considering factors like the type of liquid being pumped, the desired flow rate and pressure, and the presence of solids, you can ensure you select the most suitable impeller for your centrifugal pump application.

Centrifugal pumps are one of the most versatile pumps around.  They use centrifugal force to move fluids.  Here are some of their applications:

• Water supply: Centrifugal pumps are used to deliver water to homes and businesses. They can also be used to transfer water from one location to another, such as from a reservoir to a treatment plant.

• Irrigation: Centrifugal pumps are widely used in agriculture to irrigate crops. They can move large volumes of water efficiently, which is essential for keeping crops healthy.

• Sewage treatment: Centrifugal pumps are used to move wastewater through treatment plants. They can also be used to remove solids from wastewater.

• Food and beverage processing: Centrifugal pumps are used to transfer a variety of liquids in food and beverage processing plants, such as milk, juice, and syrup.

• Chemical processing: Centrifugal pumps are used to transfer a variety of chemicals in chemical processing plants. They can also be used to circulate cooling water or other fluids.

• Petroleum industry: Centrifugal pumps are used to transfer oil and gas in the petroleum industry. They can also be used to inject water or other fluids into wells.

• Power plants: Centrifugal pumps are used to circulate cooling water in power plants. They can also be used to transfer other fluids, such as condensate and feedwater.

• Fire protection: Centrifugal pumps are used to supply water to fire sprinkler systems. They can also be used to transfer water from fire trucks to hoses.

• Heating and air conditioning: Centrifugal pumps are used to circulate water or other fluids in heating and air conditioning systems.

• Pulp and paper processing: Centrifugal pumps are used to transfer pulp and paper slurries in pulp and paper processing plants.

These are just a few of the many applications for centrifugal pumps.  They are truly versatile machines that are essential for a wide variety of industries.