Centrifugal Pumps

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Introduction

This article will take an in-depth look at centrifugal pumps.

The article will bring more understanding on topics such as:

• Principle of Centrifugal Pumps
• Types of Centrifugal Pumps
• Applications and Benefits of Centrifugal Pumps
• And Much More…

Chapter 1: Principle of Centrifugal Pumps

This chapter will discuss what centrifugal pumps are, their construction, and how they function.

What is a Centrifugal Pump?

A centrifugal pump is defined as a hydraulic machine that changes mechanical energy into hydraulic energy by the use of centrifugal force acting on the fluid. This is a machine that uses rotation to enforce velocity on a liquid, the velocity will then be converted into a flow. Every centrifugal pump is made up of mechanical components that make the operation of the pump possible.

This mechanical assembly involves the pump’s shaft mounted on bearings, the sealing mechanism that prevents the pump from leaking uncontrollably, structural components that are designed to handle the loads and stresses imposed on the pump during use, and also wear surfaces that permit the pump to be returned and returned to its original specifications.

Components of Centrifugal Pumps

Every pump should have the following components:

• Shaft
• Impeller
• Casing
• Suction Pipe
• Delivery Pipe

Centrifugal Pump Shaft

It is the central part of the pump which rotates together with the impeller when connected. The shaft is linked to the prime mover in order to get the power. The shaft fits perfectly with the ball bearing.

Centrifugal Pump Impeller

It comprises an arrangement of backward curved vanes. It is mounted to an electric motor’s shaft. This is known as the rotating part of the centrifugal pump enclosed in a casing that is watertight. The impeller rotates and imparts velocity to a liquid.

Centrifugal Pump Casing

This is a passage surrounding the impeller, which will be airtight. It is made in such a way that the water’s kinetic energy discharged at the outlet is changed to pressure energy before the water leaves the casing and is delivered into the delivery pipe. It works as a cover so that it protects the system. The casing transforms the velocity developed by the impeller into a stable flow. There are basically three types of casings in centrifugal pumps namely volute casing, vortex casing and casing with guide blades.

Volute Casing or Spiral Casing

The impeller surrounds this type of casing. Such a casing provides a successive increase in the area of flow and hence decreases the velocity of water and increases the pressure.

Vortex Casing

This casing is a circular chamber that is introduced between the casing and impeller. The fluid from the impeller has to pass through the vortex chamber first and then through the volute casing. Velocity energy has been changed to pressure and has good efficiency compared to the volute casing.

Casing With Guide Blades

In this type of casing, there are blades surrounding the impeller. These blades are arranged and designed in such a way that the water from the impeller passes through the guide vanes without shock and forms a passage of increasing area, through which the water passes through and reaches the delivery to leave with pressure.

Suction Pipe With a Strainer and Foot Valve

The suction pipe has two ends. The first end is connected to the pump’s inlet and one end is dipped into the water in a sump. At the suction pipes’ lower end, a foot valve is fitted. The valve only opens in an upward direction as it will be a one-way type. To prevent the entry of unknown and unwanted bodies into the suction pipe, a strainer is fitted at the end of the pipe.

Delivery Valve

The delivery valve also has two ends. One end is connected to the pump’s outlet and the other end delivers the water at a required height.

Manufacture of Centrifugal Pumps

When selecting materials for centrifugal pumps, there are factors that need to be considered. These are strength, resistance to abrasive wear, corrosion resistance, casting and machining performance, repair and welding performance, and costs.

Materials Used in Centrifugal Pumps

Cast iron, cast steel, stainless steel, bronze, brass, carbon structural steel composite materials, alloy steel, and non-metallic materials are some of the materials used to make centrifugal pumps.

• Cast iron – This is the most common material used to make centrifugal pumps. It provides high tensile strength and abrasion resistance correlated to high-pressure ratings. It is also durable.
• Stainless steel - Austenitic stainless steel is the most common stainless steel that is used to make pumps. Stainless steel is usually used for chemical pumps as it is corrosion resistant. Its tensile strength is remarkably high.
• Cast steel – This material is suitable for high-pressure working conditions and has good mechanical properties. Though its corrosion resistance is not as good compared to other types of stainless steel used in corrosive and other chemical applications.
• Carbon structural steel – This material is widely used as pump shaft material where no corrosion is required.
• Alloy steel - It is usually used as a material in pump shafts for high-strength.
• Non-metallic materials – This material in pumps is mainly used for sealing purposes for example polytetrafluoroethylene, rubber, nitrile rubber, and fluorine. Polytetrafluoroethylene has excellent high temperature resistance and corrosion resistance. Is used for static seals of mechanical seals and chemical pump gaskets. It is advisable to use almost all chemical media within 250°C.
• Bronze – Can be used for the body of the pump. It helps the sealing of the pump body. For larger centrifugal pumps, tin bronze is used as a material for the body. Although nickel aluminum bronze is corrosion resistant and has the best mechanical properties it is expensive and incompetent.
• Composite materials - to improve the chemical resistance of the pump, a lining can be installed in the volute. The materials used for the lining can be rubber. Graphite monolithic ceramic and pumps are used in particular corrosive liquids, like hydrofluoric acid which is used in the pulp and paper industry and metal finishing industry. Composite materials are also used to make pump bodies.

Criteria for Choosing Material

Considerations when choosing the centrifugal pump material include:

• Chemical compatibility - Pump parts that will be in contact with the pumped media can be made from chemically compatible materials that will not be contaminated or result in excessive corrosion or contamination. Consulting a metallurgist for proper metal selection is advised when dealing with corrosive media.
• Explosion proof - Non-sparking materials are needed for operating on media or environments with the ability to catch fire or can explode.
• Sanitation- Pumps in the food and beverage industries require high density seals or unsealed pumps that are easy to sterilize and clean.
• Wear - Pumps that handle abrasives generally need materials with good wearing capabilities. Chemically resistant and hard surface materials are often incompatible. The housing and base materials should be of the right strength and also should be able to hold up against the conditions and environment being operated from.

Chapter 2: Types of Centrifugal Pumps

The different types of centrifugal pumps include:

12V Pumps

These pumps operate 12 V DC, with a maximum flow of 330 GPH. It consists of alligator clips, a battery cable, and an on/off switch for ease when operating. The lowest height of water being pumped out can be 1/8 inches with the suction strainer, and the largest height of water being lifted is 40 feet. 3/4 inches garden hose adapter at both outlet and inlets, 6 feet suction hose, gasket for replacement, and an extra impeller. It can be portable due to its lightweight. In that case, a carrying handle can be attached to the body for convenient use.

Chemical Pump

Chemical pumps are designed specifically for use in pumping chemicals that are resistant to corrosive materials, which makes it suitable for handling corrosive and abrasive industrial liquids such as paint, fuel, solvent, bleach, and many others. A pump that contradicts the chemical can result in brittleness or dissolving, swelling, and ultimately it will leak and fail. Special consideration needs to be given to the materials of a chemical transfer pump, along with the concentration and temperature of the fluid being handled. Whenever there is required a chemical dosing pump, one for tank to tank transfer or a barrel emptying pump, the pump can be tested.

Trash Pumps

These pumps are portable pumps and are generally for dewatering applications. They are planned to pump large amounts of water that contains soft and hard solids such as mud, twigs, leaves, sand, and sludge. Most trash pumps are heavy-duty, portable centrifugal pumps that have larger discharge openings and deeper impeller vanes than other pumps. Trash pumps are capable of processing materials with some suspended particulates that can clog other centrifugal pumps and can move hundreds better yet thousands of gallons per minute. The materials that enter the pump are not grounded in the trash pump. They are designed to have a large discharge opening, pump housing, and deep impellers veins. The pumps can be made from cast iron, steel, aluminum, and stainless steel. Most of these pumps have roll cages.

Semi-trash pumps are types of trash pumps that have a smaller opening. They are not conceived to handle large solids or high solid concentrations. As a result, regular trash pumps are better suited for uses that need rigorous pumping of solid-laden slurries or water. There are four types of trash pumps: syringe pumps, progressive cavity pumps, sanitary pumps, and positive displacement pumps.

• Syringe pumps are used for dealing with materials that require exact flow amounts at exact time intervals. Infusion pumps process fluid at highly controlled pressures and withdrawal pumps remove the fluid, these are the two types of syringe pumps.
• Sanitary trash pumps are pumps used in applications progressive cavity trash pumps that need high levels of sanitation such as in breweries, food, and biotech companies. This type of pump is also able to move meters and slurry solutions.
• Progressive cavity pumps are used for moving fluids and slurries with suspended solids. The fluids are suctioned from one side of the pump discharged from the other and then to a storage tank or through a pipeline. Cavity pumps can suitably transfer slower-moving viscous fluids and materials from these pumps and can be moved in a continuous flow.
• Positive displacement pumps use pistons, diaphragms, gears, and other devices to pump fluids through. They can also be moved by a vacuum created when the fluid is pumped into a fixed cavity and then pumped out again, creating a vacuum that sucks in other fluids. Displacement pumps are the best when it comes to viscous liquids that are subjected to great pressure.

Vertical Pumps

They are also known as deep well turbine pumps. These are vertical axis or mixed flow centrifugal pumps which include stages of stationary bowls and rotating impellers to process the guide vanes. Vertical pumps are used whenever the level of water pumping is below the volute centrifugal pump limits. Vertical pumps are costly and are more complicated to refurbish and fit. The design of the pressure head mainly depends on the impeller’s length as well as on the speed of its rotation. The pressure head designed with just one impeller is not that suitable. The different types of vertical pumps are:

• In-line pumps
• Barrel pumps
• Vertical column pumps
• Submersible pumps
• Deep well pumps
• Can Pumps

Some more or less similar types of pumps are vertical sum, vertical turbine, process, and industrial pumps.

Well Pump

The well pump is the heart of the water well system. They pump water upward and into the household or designated water system. Jet pumps and submersible pumps are the most popular pumps used nowadays. Both pumps depend on the centrifugal force for them to force water upward. Spinning rotors, also referred to as impellers, create a vacuum that forces the water upward through the well casing and into the distribution system. The type of suitable well pump required for a well system should depend on the amount of water required for each household and on the depth of the well.

Jet pumps are placed on top of the ground and lift water from the ground through a suction pipe, creating a vacuum with an impeller. The impeller drives water through a small nozzle. There is a need to first prime the jet pumps with flowing water as it pumps water. Shallow well jet pumps are utilized in wells with a depth of 25 feet while on the other hand, deep well jet pumps typically cover a depth of 150 feet.

Submersible well pumps have a much wider range in-depth and can be used in wells as shallow as 25 feet and as deep as 400 feet. As the name suggests, submersible well pumps are submerged deep in the well just beneath the water level. The majority of its energy is dedicated to pushing water upwards unlike sucking water from above as with the jet well pumps.

Submersibles are cylindrical in shape, and they consist of a pump motor and several impellers for driving water up the pump and into the drop pipe. Because of their durability, efficiency, and versatility in well depth, the majority of modern well systems use submersible pumps over some other pumps.

Radial Pumps

In radial centrifugal pumps, the fluid comes out of the impeller after rotating for 90 degrees relative to the suction. Many centrifugal pumps are found in this category. Fluid enters through the horizontal suction flange and leaves through a vertical outflow flange. The discharge will be perpendicular to the pump’s shaft. This design is commonly used when there is a flow limitation and a need to raise the discharge pressure. Thus, radial design is a low flow rate and a high-pressure pump. Most pumps that are used in the gas and oil industries fall into this category.

Axial Pumps

In an axial flow centrifugal pump, the fluid can move parallel to the shaft. This procedure is the same as the working of a propellant. The most important application of this pump is when there is a huge flow rate and very little pressure head. For instance, they are common in water circulation pumps and dewatering pumps.

Mixed Pumps

As the name states, in a mixed flow centrifugal pump, the fluid flow mixes both axial and radial properties. That is a trade-off between radial and axial pumps. Mixed pumps operate at a much larger flow rate with a decent increase in the head.

Single Suction Pump

In a single-suction centrifugal pump, the flow of the fluid is directed into the inlet, and the rest of the liquid immediately flows into the impeller eye, which is the inlet of the impeller. By the time when the water leaves the impeller, pressure is produced by the centrifugal force.

Double Suction Pump

Single-suction will be inadequate when the flow rate is too high. In such a case, double suction centrifugal pumps are utilized. The pump’s impeller is engineered so that the fluid enters from all sides as compared to the single side in a normal case. Nevertheless, the name “double suction” should not be confusing. Even in the double suction pump design, there is only a single flange discharge and suction. The difference is in the design of the casing and impeller.

Single Volute Pump

In centrifugal pumps that have a single volute casing, the flow is discharged from the impeller and goes into one volute, which winds completely around the impeller. This single volute casing has one cut-water that transports the fluid flow towards the pump’s outlet. Most of the centrifugal pumps in the refinery are of single volute types.

Difference Between Reciprocating Pump and Centrifugal Pump

• The centrifugal pump is generally constructed more simply than the reciprocating pump. The centrifugal pump has fewer parts.
• A centrifugal pump has extra weight for a given discharge. The reciprocating pump has less weight than the pump for a certain stated discharge..
• The centrifugal pumps are appropriate for large discharges but with a small head. Whereas reciprocating pumps do the opposite for less discharge with a high head..
• Centrifugal pumps need a heavy foundation and more floor space whilst the reciprocating pump requires less floor space with a light and simple foundation.
• A centrifugal pump has less tear and wear as compared to reciprocating pumps and can handle dirty water..
• The centrifugal pump’s delivery is continuous and requires priming. The reciprocating pump’s delivery is pulsating and does not require priming..
• Centrifugal pumps have low efficiency and can run higher. A reciprocating pump has high efficiency and is unable to run at higher speeds..
• Centrifugal pumps require less maintenance costs as compared to reciprocating pumps..

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Chapter 3: Applications and Benefits of Centrifugal Pumps

This chapter will discuss the applications and benefits of centrifugal pumps. It will also discuss the maintenance of centrifugal pumps.

Applications of Centrifugal Pumps

The applications of centrifugal pumps include:

• Centrifugal pumps are utilized in many buildings to pump the water supply. .
• They are used as a booster and for domestic water supplies..
• Centrifugal pumps are designed in such a way that makes them useful for pumping slurries and sewage..
• These pumps are also used in fire protection systems and for cooling and heating applications..
• In the beverage industry they are used to transfer bottled water, juice, and other beverages..
• In the dairy industry centrifugal pumps are used to transfer dairy products for example buttermilk, milk, and flavored milk..
• Many industries (manufacturing, industrial, chemicals, food production, pharmaceutical, and aerospace) – use the pumps for the purposes of refrigerants and cryogenics..
• In oil energy, they pump crude oil, mud, and slurry used by power generation plants and refineries..
• They can be used as metering pumps that can pump precise volumes of liquid for treating water for example wastewater, drinking water, swimming pool water, and boiler water..
• They are also used in process applications where metering of fluids is needed, where extreme high pressures are required, or where the sealless nature of the pump type is beneficial..
• In the pharma and cosmetics industry, they are used to transfer lactose, glucose, and some other drugs and personal care products of medium and viscosity..
• Circulator pumps are used for ventilation heating and air conditioning..
• Magnetic drive pumps are used where there are chemicals and hydrocarbons, no leakage is permitted..
• Cryogenic centrifugal pumps are used for liquid natural gas and coolants..

Benefits of Centrifugal Pumps

The benefits of centrifugal pumps include:

• There is reduced friction in the pump.
• Magnetic coupling breaking the pump will not overload and will also not get damaged.
• Corrosion Resistance – the pumps allow processors and manufacturers to transfer different types of fluids, even those that can quickly corrode the other pumps. The pumps even when used extremely well can offer a long service life. The pumps are able to withstand corrosive materials.
• Energy Efficiency - Centrifugal chemical pumps rank high in energy efficiency in comparison to all the other pumping technologies. Their efficiency reduces costs both over the life span or in the short term of each unit.
• Smooth Flow - Centrifugal chemical pumps avoid pulsing when some other pumps can produce a pulsing flow.
• Proven Reliability - Centrifugal chemical pumps are the best choice when reliability is important. The pump should be evaluated on the construction and design and features in order to ensure the specified pump will be durable enough to operate in extreme conditions.
• Low Maintenance -Due to the long life spans, some pumps may need to be frequently routine maintained, which can make them costly to operate. However, centrifugal chemical pumps have low routine maintenance requirements.
• Size Versatility - Centrifugal chemical pumps are available in a wide variety of sizes.
• Application Versatility- The same pump configuration is unsuitable for every application. With centrifugal chemical pumps, different configurations are there to provide solutions for multiple uses.
• There is no heat transfer from the motor – An air gap separates the pump chamber from the motor thus providing a thermal barrier.
• The absence of drive seals eliminates the risk of a leak. This means that hazardous liquids can be pumped efficiently without any spillages. Eliminating the drive seals is a way of getting rid of leaks, wear, friction loss, and noise and provides separation of fluid from the pump drive.

Drawbacks of Centrifugal Pumps

Some of the drawbacks of centrifugal pumps are:

• Cavitation -this can happen when the net positive suction head of the system for the selected pump is too low.
• Excessive wear of the impeller — this problem can sometimes be worsened by suspended solids.
• Corrosion may occur inside the pump being caused by the fluid properties.
• Surge or back surge in the line.
• Overheating of the pump may occur due to low flow.
• Lack of prime— In order to operate properly, centrifugal pumps must be filled with the fluid that needs to be pumped. If the pump casing is filled with gasses or vapor, the pump impeller becomes gas-bound and can stop pumping at all.
• Liquids that contain ferrous particles can be problematic when a centrifugal magnetic drive pump is used. This is a result of the particles collecting on the impeller magnet, and as time passes it can cause the pump to stop working. Some of the energy is lost in the coupling. This is basically due to some magnetic resistance. The coupling may slip if unexpectedly heavy loads occur.
• Centrifugal pumps use rotation to move water instead of suction and therefore have little or no suction power. This proves that a centrifugal pump must be primed or put underwater before it can move water or other liquids.
• The centrifugal pump may consume too much power when in use.

Maintenance of Centrifugal Pumps

The maintenance of centrifugal pumps involves:

• Determine the maintenance frequency with which the pumps should be checked and repaired. A certain time frame should be set to check the pump and verify if it is still working properly or as required.
• Inspection and replacement of mechanical parts should be done regularly. The inspection can be a quarterly inspection, routine inspection or annual inspection. This inspection involves steps such as checking pipe lines for leakages, checking bearing temperature, increased vibration, unnatural and uneven noise, stuffing box, mechanical seal, discharge pressure and operating current. For annual inspection, check if all mount points are secure, clean filter, inspect pump flange for leaks, replace the mechanical seal, inspect coupling, replace lubricating oil and check shaft alignment.
• Changing the pump's lube oil is part and parcel of the annual inspection routine or the scheduled maintenance. It is essential to prevent bearing damage. One should remember to follow the manufacturer’s guidelines when it comes time to lubricate the motor. Be careful not to over or under lubricate the system. The damage to the pump may be greater in case of over greasing than in under greasing, to prevent this, the manufacturer’s instructions must be followed. There is a need to frequently lube if the pump is used frequently on a daily basis.
• Inspecting the electric motors as a way of preventive maintenance is very important. Checking the insulation resistance of connections, ensuring all connections are properly and tightly secured, inspecting the motor for any signs of blocked or overheating the vents.

Conclusion

A centrifugal pump is a machine that changes kinetic energy into the fluid’s pressure head. The external power from a diesel generator or electric motor then turns the pump impeller. Under the influence of the centrifugal force, the fluid enters the impeller reaching its tip and leaving the volute casing. There are many types of centrifugal pumps for example chemical pumps, vertical pumps, and trash pumps to mention only a few. They can be classified according to flow type), based on the number of stages (single stage and multi-stage pumps), and also on the type of volute (single and double volute).

Maintaining the pump is easy; most of the measures are given in the manufacturer’s guide. If used and maintained properly the centrifugal pumps can last long and are less costly. Choose a pump that is compatible with what needs to be used for example when pumping hazardous chemicals, a chemical pump is advised.

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