What is Impeller in Pump? Definition, Meaning, Types, Maintenance, Defects

When we started to learn about pumps, we repeatedly heard a word, i.e., “Impeller.” The impellers were introduced more than two hundred years ago, but they have been widely developed and used in the last seventy years. The first Centrifugal impeller was found in 1772 in an abandoned Portuguese copper mine in San Domingos. It was made of 10 wooden double-curved blades. Here we will discuss all the aspects and related information about impellers, their uses, advantages, etc.

What is Pump Impeller? Definition, Meaning

Impeller Definition

The impeller is among the foremost part of a centrifugal pump. It consists of a series of several vanes. The impeller is a rotating component made with vanes or blades. The impeller rotates and this rotational motion helps the fluid to accelerate and discharge through the impeller’s vanes into the pump casing.

It increases the pressure and flow of liquids, gases, and vapors. Impellers are mounted at the heart of the compressor. Impellers are the basic component for functionalities that are specified to meet the required performance. Impellers are basically chosen on the basis of two foremost qualities, aerodynamic performance, and mechanical integrity. There are three different ways by which impellers can be classified and these are,

• Vane’s position,
• Specific speed, and
• Design.

In general, impellers are designed either open, semi-open, or closed.

Uses of Impellers

• Impellers are used in water jets to generate high power for boats.
• Impellers are also used in turbochargers (automobiles) to generate high-pressurized air that transfers to the combustion chamber. By doing so, the engine performance is enhanced.
• It is also used to reduce the efficiency and pass solids.
• It is also used to handle suspended solid service.

Manufacturing process of impellers

The method of manufacturing impellers, compressing machine hub, shroud, and core portions.

How Does an Impeller Pump Work?

As we know, the vital component of a centrifugal pump is its impeller, as it transfers the energy from the pump motor to the fluid. An impeller pump basically depends upon the inertia. As we also know, the natural tendency of an object and fluid to move in a straight line when moving in a circular motion. Water hitting the impeller’s blades naturally moves outward tangent to the radius. This phenomenon creates velocity, converted into pressure due to the fluid being confined by the pump casing.

Eye of the impeller: The upper end of the suction pipe is connected to the inlet of the pump or center of the impeller, called the eye of the impeller.

Types of Pump Impellers

Impellers are designed three types they are:-

• Open impellers
• Semi-open impellers
• Closed impellers

Pump Open impellers

What is an Open Impeller?

Open impellers are named due to the vanes open on both sides. Its vanes do not support either side, and they are supposed to be weaker. The vanes of the impeller are connected to the central hub without any support or attachment to the sidewall.

These impellers are effectively used in low-cost and small pumps, in which impellers rotate between two side plates, between the stuffing box head and the suction head. These pumps are smaller and less capable of reducing the breaking vanes.

Advantage of Open impeller

• The advantages of open impellers are easy to install, faster to remove for clean and repair with the passage of slurries.
• It does not block easily, as they have no areas with small clearance.
• Vane alteration is possible and economical.

Disadvantage of Open impeller

• Open impellers are the least efficient as the flow is not guided between the impeller vanes.
• These impellers do not have walls, shrouds; hence structurally, these are weak.
• Adequate space between impeller and casing must be maintained to avoid damage to impeller or casing.
•  Open impeller vanes are quite thick to not break or bend the operation.
• Only clear liquid can pass through this type of impeller.
• The soft non-sparking materials are required in a few applications.

Would you like to know about Velocity Triangle Basics

Pump Semi-open impeller

What is a Semi Open Impeller?

Its name knows semi impellers because this type of impeller’s vanes is fixed on one part of the shrouds. Very few vanes are present in these impellers, but these lengths are greater than open impellers. These impellers pump liquid-like stool, mud, junk, etc. The construction design of the semi-open impeller is shown below:-

Advantage of the semi-open impeller

These kinds of impellers are widely used with fluids and fluids that are mixed with solids.

Disadvantage of the semi-open impeller

Semi-open has no front shroud; hence, the impeller’s discharge pressure comes from behind. To neutralize this, we need to pump out vanes or other mechanical means which have their maintenance disadvantage. After some time has passed in the field, the gap between the vane of the semi-open impeller and the casing will increase due to wear and tear. This phenomenon has limitations in the aspect that causes reduction of efficiency and discharge pressure.

Pump Closed impellers

What is a Closed Impeller?

As we read above about the open and semi-open impeller, the closed impeller’s name consists of the back and front shrouds, for maximum strength. They have low net positive suction heads required and provide more efficient flow. The design of the close impeller is very complicated. Expensive design due to their dependence on close clearance wears rings to reduce axial loads and help to maintain efficiency.

These impellers are prevalently used for those places where there clear liquid comes into play. They are least capable of the solids and are difficult to clean if they become clogged.

Advantage of closed impellers

• An axial trust may reduce which helps to increase the life of bearing.
• Increase efficiency
• Compensate for thermal growth.
• Suitable for volatile fluid.

Disadvantage of the closed impeller

• Impeller adjustment can’t be possible; they have to replace once the wear ring doubles.
• Initially, the impeller is efficient, but as the wear ring clearance increases, it loses its efficiency after a period.
• This impeller type is difficult to cast because internal parts are hidden and tough to inspect for flaws.
• The specific choice of speed is very limited.
• The impeller is very difficult to modify to improve its performance.
• Maintenance and inspection canon

Difference between open, semi-open & closed pump impellers

	Open impeller	Semi-open impeller	Closed impeller
Construction	Design is simple, and construction is very easy.	Design is simple, and construction is easier than a close impeller.	The construction and design of this impeller are complicated.
Speed choice	Wide range of specific speed choices.	Wide range of specific speed choices.	The choice of specific speed is limited.
Manufacturing cost	Least cost	Least expensive	Very expensive
Strength	Least strength	More strength than open impeller but least strength than close impeller.	Maximum strength.
Adjustment of impeller	Adjustment possible	Adjustment can be possible.	No adjustment is possible.
Efficiency	Efficiency can be maintained through  impeller clearance adjustment	Efficiency can be maintained.	Initially, the impeller is very efficient, but its efficiency will lose as the span passes.

Specific speed (Ns)

Specific speed (Ns) is when an impeller with a geometrical similarity to the model but with a diameter of 1 meter provides a flow of 1 kg/m³ and 1 meter of total dynamic head. This value designates a relationship between the impeller flow and the output head. Also Read: How to Read a Pump Curve?

Impeller Vanes

The impeller vanes or blade can be:

Backward-curved blade design

What is Backward-curved blade design?

With Backward curved impellers, the air exists in a radial direction. These impellers are dirt and dust resistant; hence these are widely used in industries. Its hydraulic efficiency is very high; hence, these vanes are ideal for cooling at high-pressure conditions.

Benefits of Backward Impeller Blade

Backward curved impeller blade operates in environments where high static efficiency is needed—this type of fan non-overloading non-overloading horsepower characteristics. The benefits are particularly important as static efficiency means energy savings in brake horsepower, which increases plants’ operating costs.

Radial Impeller Blade

What is Radial Impeller Blade?

A radial impeller Is known for its unique diverging vanes which are situated from its center point. These vanes are used to increase as well as decrease the pressure and flow of a liquid. These types of impellers operate mechanically to generate energy. The number of divergence vanes could be influenced to generate the desired pump flow energy. A radial blade impeller is an impeller at which the flow leaves the impeller in the radial direction, perpendicular to the pump shaft.

These blades are attached to the rear of the impeller backplate and run in the space between the volute side and the impeller.

Parts of Radial Impeller Blade.

A radial blade impeller mostly consists of this thing as follows:-

• Vanes housing
• Inlet and outlet duct
• Drive mechanism
• Driveshaft

Benefits of Radial impeller blade

The benefits of radial impeller blade are given below:-

• These blades are familiar for heavy-duty and tough applications.
• The maintenance cost of radial impeller blades is very low due to simple design and self-cleaning characteristics.
• It’s self-cleaning features also provides to increase in operation efficiency.
• Its construction and durability make them the perfect choice for handling air streams containing dust and large numbers of particles.
• Its construction makes them suitable for high temperature, high pressure, and corrosive and abusive environments.

Forward curved blade

What is Forward Curved Blade?

This type of impeller has cylindrical in shape and there are many small vanes are mounted over the peripheral to the impellers. These blades help impellers to generate a high airflow volume. Forward curve blade impellers are mostly used in high-temperature furnace applications and ventilation with a low chance of dust loading.

Hence, they cannot be used in applications with a chance of heavy dust loading. When dirt and dust settle on the vanes that affected the volume supply of air.

Benefits of forward-curved blade

Following are the benefits of a forward-curved blade:

• It is widely used as an external motor, a plastic forward-curved or metal forward curved impeller.
• The small and mannered structure avail this easy to install and due to small in size it have also light weight.
• It is effectively used in automatic control devices, forced ventilation of electronic equipment.
• It is also used in indoor duct dust removal, exhaust heat removal, environmental protection, air purification, and pollution control.

Impellers Balancing and Maintenance Procedure.

The process of minimizing vibration, noise, and bearing wear of the rotating body is known as impeller balancing. For the centrifugal pump, the direction of rotation of the impeller should be correct.

Need of impeller balancing

• Impeller balancing can prevent vibration, which helps to improve efficiency.
• Without providing balance to pumps, noise can’t be reduced, higher operating speeds can’t be achieved, and the bearing life of the pump shortens.
• To acquire the customer needs and maintain quality as per company standard. The impeller balancing perform vital role to fullfill all requirement like efficiency and equipment speed.

Impeller maintenance procedure

Impellers are mostly used in industries, and for some time, abrasive & corrosive environments tend to reduce the operational life of impellers. That’s why we need to maintain clearance checks, adjustments, and impeller balancing to save from replacing the impellers in industrial equipment. This helps in follows:-

• Maintaining the good performance of industrial equipment.
• Extends its life.

Impeller clearance Setting

• The proper impeller ensures that the pump runs at high performance.
• After impeller rear clearance has been set, the suction seal ring clearance must be set.
• The impeller clearance is also set by the measuring gap between the front of the impeller and the suction side liner. The above method also may perform for those assemble parts which do not have the adjustable suction seal rings.

Maintenance Schedule

The maintenance schedule includes this type of inspection:-

• Routine maintenance
• Continuous inspection
• Quarterly inspection
• yearly inspection

We can shorten the inspection intervals appropriately if the fluid is abrasive or corrosive or if the environment is classified as potentially explosive.

Routine maintenance

This performance was done by any individual when they perform routine maintenance; this involves

• Lubricate the bearing.
• Inspect the seal.

Continous inspection

This task would be performed whenever we do the routine inspection, and this involves:

• Check the level and condition of the oil on the bearing frame.
• Check the unusual noises and vibration and bearing temperature.
• Check the impeller and piping of the leak.
• Analyze the vibration.
• Inspect the discharge pressure.
• Inspect the temperature.
• Check the seal chamber and stuffing box leaks.
• It is mandatory to check that there are no leaks from the mechanical oil seal.
• Replace the packing in the filled box if you notice excessive leaks.

Quarterly inspection

This task can be performing in every three months, and this involves:

• Check that the function and the hold-down bolts are tight.
• Check the packing if the impeller has been left idle, and replace as required.
• Change the oil every quarter at a minimum.
• Check the shaft alignment and realign as required.

Yearly inspection

Perform this inspection one time each year:-

• Check the capacity.
• Check the pressure.
• Check the power.

Defects or Problems in Pump Impellers

The first and foremost function of a centrifugal pump is its impeller. Without this, centrifugal force can’t be generated upon the entering liquid; we can’t imagine that without an impeller centrifugal pump work. Hence proper care of the impeller is true. The impellers which are critically or minimally damaged can also cause flow-oriented defects in the pump. The top three causes of impeller failure are cavitation, erosion, and corrosion.

Cavitation

The process of formation of bubbles or cavities in moving liquid due to the pressure difference or due to low pressure developed around the Impeller. Cavitation is frequently developed in the pump impeller, and this is caused by pressure difference either on the suction pump body, discharge pump body or the impeller.

The pump cavitation is left untreated then it can cause

• The pump housing may cause failure.
• Demolition of impeller may occurs
• This cause the decrease in flow and pressure
• Very heavy vibration.

Due to cavitation, the mechanical problem raise in the pump that causes the maintenance cost may increase. Following are the most suitable location for cavitation:

• From the suction of the rotary pump, whenever it functions at a pressure quite dismal to the designed net positive suction head required.
• As a result of discharge of throttled regulator valve, when it is near close setting.
• In a system with a sudden expansion of pipe diameter,, it causes a sudden pressure reduction.
• At a pipe joint like tee joint, elbow, and bends.

Following are the ways which are cause to reduce the cavitation:

• By using suitable work-hardened material for the surface.
• Some simple modifications on the pump suction side can reduce the heat loss of the pump, such as by increasing the suction pipe’s internal diameter and reducing the suction pipe’s length.
• By reducing the operating temperature of a fluid the net positive suction head(NPSH) may increase for a pump
• Minimize the pipe fitting at the suction side and operate at the variable speed pump at a lower speed range
• The sped of the connected pump and the speed of motor may increases when the voltage increases. Hence we can use voltage limiting protection to avoid over voltage.
• Uses of design factor for net positive suction head required which depend on :
• design of impeller inlet
• Impeller shape
• Fluid flow rate
• Density of the pump.
• RPM of the pump.

Materials for Durable the Cavitation Resistance

• Ceramic coating.
• Epoxy coating
• Polyurethane coating
• Plasma spray and thermal spray
• Weld overlays

Weld overlays

It is the earliest method used, but they are costly and dependent on human skill. Now a day a process that is most familiar i.e Nano –coating and is also known as ceramic coating is applied to minimize the cost. This process is applied on the surface layer that repels dust particles, oil, and water. This may be found in both solid and liquid forms. The Nano-coating characteristics provide many favorable conditions for surface scratch resistance, it improves the surface hardness and also makes it free from bacteria. The advantages of weld overlays,

• The surface is very easy to clean
• The life of coating is sustainable
• The surface is highly resistance.

Thermal sprays

For cavitation resistance is also minimized by the thermal spray process. In this process metal, ceramics can be applied in the form of powder, wire, and rods to a torch. The electric sources can be used to melt or heat. That heated metal-ceramic fed into the specimen. This process may cause residual stress in the coated component.

Polymer-based coating

The acceptance of polymer is due to:-

• Lower labour cost and lower level of complexity of skills required
• Minimal residual stresses in the coated components

Epoxy coating 

Epoxy coating is a coating compound that consists of two elements one is epoxy resin and another is hardened ceramic particles. These two will be mixed with each other and that also engages in the chemical reaction then it creates cross-linking of the elements as it restores. When the epoxy coating is fully restored, that provides product is more durable. This process may take several minutes to several hours and turns the epoxy liquid into a strong and durable solid. This coating is popular because it provides a quick-drying, tough, and protective coating for metals and other materials.

Erosion

Erosion is the surface deterioration and damage due to the relative movements of an electrolyte fluid about a metallic and non-metallic substance and in combination with an electrochemical reaction. This type of defect occurs due to small and hard particles coming into contact with the impeller, causing gradual wearing down of the entire impeller. Factors that affect this damage include contaminants wear debris, fluid characteristics, and fluid flow velocity. There are the following ways to minimize erosion of impeller:-

• By design the pump so that the flow velocities through the close-running clearance are low.
• Using the smooth-coated surface with anti-erosion, anti-corrosion coatings.
• Corrosion coating via cathodic protection.
• Reducing gritty contaminants by providing effective filtration and different sections to evacuate the contaminant’s particles.

Corrosion

This a natural phenomenon that causes the transformation of pure metal into its oxides when they react with the substances like air and water. Due to this, we need to select appropriate materials for manufacturing impellers. For example, if we choose cast iron material for impellers and pump caustic soda fluids, the impeller will immediately deteriorate, likely resulting in other corrosive damage to the pumps.

The ways to minimize corrosion of impeller:-

• Took suitable design parameters during the design of the impeller.
• Select suitable materials for components like impellers, valves, and pipe fitting.
• Anti-corrosion coating reduces corrosion damage.

Impeller Removal and Installation

The basic we have to know how impellers are removed and installed. Here we can signify how we make it easier in terms of installation and removal. First, here we study how impellers are removed and which is as follows:-

• Remove the cover or case
• Insert the puller against the shaft
• For ejecting the impeller from the housing we need to screw the puller against the shaft.
• Now we need to twist or pull the impeller while ejecting from the pump.

How to Install Impeller?

• Apply non-petroleum-based lubrication to the impeller (e.g., Silicon or soapy water)
• Install the impellers with a twisting motion onto the shaft
• Use a puller tool to push the impeller to pump by losing and lowering the nut until tightness against the impeller insert, then turning for installation.
• For setting of “o – rings “we need some non-petroleum based lubrication.
• Now align the key and key path through the shaft.

Materials Used for Impellers

Let’s see the common materials used for impellers are as follows:

• Gray cast iron
• Ductile iron
• Alloy steel
• Aluminum
• Bronze
• Brass

Application of impellers

As we read above, all impellers consist of similar design elements, and they may perform slightly different operations depending upon their intended application.

• Impeller in pumps
• Impeller for mixing 
• In washing machine
• In medicine
• In air pump

Impeller in pump: Impellers are vital components of centrifugal pumps and vacuum pumps, or other pumping devices. Pump impellers are based on Bernoulli’s principle – which states that when there is an increase in fluid velocity is a decrease in pressure or potential energy. When fluid or gas media enters an impeller pump, it becomes captured between the impeller vanes and pump wall; thus, it increases in velocity as it moves from the impeller eye towards the out diameter of the impeller.

Impellers for mixing: Impellers are frequently used in agitation tanks to mix fluids and slurries. For mixing, impellers are generally taken as an open design that is fitted to the center of the shaft. At the time of fitting a tank with an impeller, selection between axial and radial flow types is necessary.

Radial impellers have rectangular vanes. They are used when the tank media is excessively viscous or consists of two immiscible materials. For homogenization and mixing an axial impellers are used. It carries the pump media in the direction along the revolving axis of the impeller. Axial flow impellers applications are mainly due to their high speeds that help in rapid dispersion and are used at low speed for keeping solids in suspension.

Impellers in Air pumps: Impellers are also used in blowers, exhaust systems, superchargers, or turbochargers for internal combustions engines. In these meshing, impellers are used to move air through a system.

In medicine: Impellers are an efficient component of axial flow pumps used in ventricular assist devices to augment or fully replace cardiac function.

In Washing machines: Top loading washing machines are generally equipped with impellers to agitate the laundry during the machine.

What are the Similarities between Impellers and Propellers?

Although there are differences, let us try to understand the similarities between impellers and propellers,

Description	Impeller	Propeller
Energy	Normally water or liquid is used to create energy	Normally water or liquid is used to create energy
Driver	Impeller cannot work without the driver.	Propeller cannot work without the driver.
Rotation	Driver rotates the impeller and moves the water or liquid.	Driver rotates the propeller and moves the water or liquid.
Lubrication	Required	Required
Noise	Very high speed can give noise	Very high speed can give noise
Maintenance	As it’s a rotating part, the impeller needs to have proper maintenance.	As it’s a rotating part, the propeller needs to have proper maintenance.
Thrust	It provides thrust	It provides thrust
Function	Without impeller, pump, or compressors, etc. will not work.	Without propeller, ship or boats, etc. will not work.

What is the basic Difference between Impeller and Propeller?

Let’s see the difference between impeller and Propeller, as follows:

Description	Impeller	Propeller
Basic	An impeller is a rotor that produces a negative pressure at impeller eye	The propeller can be defined as a fan which propels a fluid by pushing against it
Converts	Impeller converts rotational energy into pressure energy	Propeller converts rotational motion into linear motion.
Function	Impeller is used to create water or liquid pressure	Propeller is used to create water propulsion
Main part	Vanes are the main part of impeller	Blades are the main part of propeller
Hosing	Impeller is housed within pump or compressors	Propeller is open in water
Contaminant	It can be contaminated as it is enclosed within a casing	It open to water and no possibility of contamination
Motion	Forward rotation	Clockwise rotation
Open or close	Impeller may be open or closed or semi open	Propeller is always open
Application	Pump, Compressor, etc.	Ship, Boat, etc.

Conclusion

Here we read and acknowledge the impellers; by this article, one can easily choose a proper impeller for their respective pumps. This article also involves all the vital information in daily needs for an impeller. This article aims to give a clear concept about an impeller, where it may be used, where it may be applicable, and the causes for its damage and their rectification is also described elaborately, which makes it effective.

Based on this, we also discuss the different types of impellers and their respective problems, and their solution will help us overcome them effectively. Apart from the impellers, this article gave brief information about the areas applied frequently. This article also discusses the preventive method from carrion and the feasibility of coating the components with a polymeric coating.

One of the alternatives is thermal sprays that can be used for cavitation protection of the surfaces. Numerous other ways are yet to be explored for improved cavitation protection of the surface. However, there are lots to discuss impellers that have their importance which can be discussed further.