The Centrifugal pump is one of the most critical components of the process plant. Proper selection from different types of centrifugal pumps is crucial for plant performance. It is used in all facilities such as refinery, oil production platform, petrochemical plant, power plant, etc. they also used in other industries such as agriculture, food processing, and even in residential buildings to supply water.
In the process plant, a centrifugal pump transport the fluid from one equipment to another. Pumps are also used to transport liquid at a long distance through cross country pipelines. It is also used to achieve the desired pressure for certain high-pressure process applications such as boiler feedwater pump.
Let’s learn about this critical piece of the plant in this article.
What is Centrifugal Pump?
Centrifugal pump is a piece of rotary equipment that converts kinetic energy into the pressure head of the liquid. External power from electric motor or diesel generator drives pump impeller. Fluid enters into the impeller, and by centrifugal force, it exits from the tip of the impeller. Through volute casing, it discharges.
In simple language, you can say that it is a machine, which used to transport fluid from one place to another. The image below shows a typical pump.
How Centrifugal Pump Works?
The working principle of the pump is the conversion of energy. Refer to the chart that makes it easier for this entire energy conversion process easy to understand.
Below schematic diagram of the centrifugal pump explains step by step working of the pump.
- Liquid enters the pump suction nozzle.
- It enters inside the rotating impeller vane through the eye. – Impeller gets it energy from the driver (Motor, Engine or Turbine)
- Rotating impeller forces liquid outward by centrifugal force. The liquid will get velocity and pressure during this process.
- Now, liquid enters insider the volute casing (sometimes with diffuser), which reduced its velocity and further increased the pressure. It also directs fluid towards the discharge nozzle.
- You can check the animation below for the fluid movement from suction to discharge of the pump.
- Impeller and casing are the main parts that do all the works of the conversion of energy. Impeller transfers the power to fluid, and casing helps to convert the same liquid head.
Factors That Impact Centrifugal Pump Working
Suction Pressure – If pressure at pump suction is less than required, it may lead to cavitation. Cavitation will severely impact pump performance and can damage the impeller permanently.
Flow at suction – Constant flow of fluid is required at pump suction; otherwise, it will not operate at the design condition. If flow reduces at suction, discharge flow will get reduced accordingly. If there is no flow at suction, it will seriously damage pump internals. You cannot dry run this type of pump. Furthermore, you have to prime the pump before you start if it is emptied after the last use.
Viscosity of the fluid – Centrifugal pumps are an excellent choice for a clean and low viscous fluid. Usually, liquid with less than 500 Centistoke is the right choice. However, you can design a pump to hand more viscous fluid, but that be more power hungry and inefficient.
Vapor Pressure of the Fluid – Process fluid will very low vapor pressure is not suitable as it may lead to cavitation. If you are working with such liquid, make sure you have enough pressure in your system all the time.
Density of the fluid – Liquid with high density will not work with the pump. As higher the density more, work has to be done by a pump to push the liquid. Again, this will create less than ideal situations for operation and maintenance due to increased power and maintenance requirements.
Centrifugal Pump Types
Before discussing the types of centrifugal pumps, I will show you how a pump, in general, is classified to understand the whole concept of these types of pumps easily. Refer to the chart below;
The chart above broadly classified pumps into two categories rotary and positive displacement. Under positive displacement, piston type and diaphragm type pumps are widely used.
Under rotary type, you can further classify pumps into a fixed volume pump such as gear and lobe and centrifugal pump.
There are multiple ways to classify the centrifugal pump. Refer to the chart given below;
Different Types of centrifugal pumps are used in industries based on the requirement. They are classified based on design code, impeller types & numbers, application, etc. Charts above cover the most common way to organize these pumps. The same pump can be a part of two or more groups.
Depending on flow type: Radial, Axial and Mixed Flow Types Centrifugal Pump
In the image below, you can see Radial, Axial, and Mixed flow types design.
Radial Flow Pump
In radial flow, fluid take a 90-degree turn from the suction. The most centrifugal pump comes under this category. Fluid enters through the horizontal suction flange and leaves through a vertical discharge flange. That means discharge is perpendicular to the pump shaft. This design is used when you want to increase discharge pressure with a limited flow of the fluid. You can simply say that it is high pressure – low flow rate pump. Most pumps used in oil and gas will fall into this category.
Axial Flow Pump
When the fluid flows parallel to the pump shaft, it is called an axial flow pump. In this type of pumping liquid moves parallel to the pump shaft. This action resembles working or propellant. This pump is useful when handling a large amount of fluid with very little pressure head. Dewatering pumps, water circulation pumps are everyday use cases of axial flow pumps.
Mixed Flow Pump
As the name suggests, when the fluid flow is mixed of both radial and axial, it is called a mixed flow pump. It is a tradeoff between radial and axial type pump. It can handle a high flow rate with a decent increase in pressure head.
Number of stages
Depending on the requirement, two or more impeller can be used in the pump. The image below shows the single-stage pump with a single impeller on the shaft and multiple stage pump with many impellers on the same shaft.
Single impeller designed is useful for large flowrate with relatively less increase in the pressure head. When you need, very high head at discharge, the multistage design is used. In this case, impellers are connected in the series so that each stage will boost the fluid pressure.
Use of Design Code
When it comes to using code for pump design, the following two are used in most cases.
- API 610 – This is most widely used in oil and gas, petrochemical, and similar process industries. It has its own pump classification.
- ISO 5199 / ISO 2858 / DIN 24256 – DIN 24256 is now merged with ISO 2858. These standards are used for all types of pump design.
Type of Volute Casing
Pump casing can be of the single volute type or double volute type, as shown in the image below. The most pump you will see inside the refinery will be of a single volute type. Double volute has an advantage over the single. It will minimize the shaft’s defection when the pump is operating outside the BEP (Best Efficiency Point).
Radial Split – Radially split pump casing opens perpendicular to the shaft axis and parallel to the impeller.
Axial Split – In this design, the pump casing is spliced into two halves that get separated horizontally, or you can say that parallel to the shaft axis.
It sounds difficult to understand but just look at the image, you will immediately know what I am trying to explain. It is the way you open a pump for any maintenance.
Vertical or Horizontal Shaft Position
In the case of a vertical pump, the pump’s shaft is in a vertical direction, and the pump is usually installed in the sump. This type of pumps is used in limited space. Pump in borewell and sump collection are some examples of this type.
Horizontal pumps are more widely used as it is easy to maintain. In this type, a shaft is a horizontal plane. The impeller can be overhung or between bearing type. The image shows both types.
Number of Suctions
The most centrifugal pump has a single suction design. However, when the flow rate is very high, single-suction will not be enough. In this case, double suction pumps are used. The impeller of these type of pump is designed in such a way that fluid enter from both sides as compared to single side in normal case.
Don’t get confused with the name, even in the double suction pump; you have single suction and discharge flange. It is the design of impeller and casing that are different.
The image below shows single and double suction centrifugal pumps and impeller.
Depending on Impeller Position
Overhung – In this type, the impeller is supported through a single bearing. The impeller is attached at the end of the shaft. This design facilitates both horizontal and vertical installation of the pump.
Between Bearing – In this type, the impeller is installed on the shaft and the shaft is supported by bearings at both ends. Horizontally installed multistage pumps come in this design.
Refer to the image given below for the example of these types.
Types of Centrifugal Pump based on API 610 Pump Types
API 610 is the standard for the centrifugal pump used in petroleum industries. It has categorized the pump based on the construction. The chart given below lists the classification of pump base on API 610.
OH1 Type Pump
The image shown here is OH1 (Overhung) type pump. This is a single-stage overhung with a foot-mounted base plate design. It is a horizontal pump with a flexible coupling. You can see in the image that the leg supports it at the opposite end of the suction.
All API Pump Image Sources – Ruhrpumpen
OH2 Type Pump
This type of pump is centerline supported. You can see that there is no leg support at the bearing end as compared to OH1 type in the image below. It is also single stage end suction pump with single bearing housing. During operation, this bearing housing helps to absorb the force imposed on the pump shaft. It also ensures the proper position of the impeller during the operation.
OH3 Type Pump
OH3 pump is flexibly coupled single-stage and in-line pump with overhung impeller. It has a separate bearing bracket. It comes with integral bearing housing, which helps in absorbing pump load. The motor is directly mounted on the pump. The image below shows the OH3 type of pump.
OH4 Type Pump
The only difference between OH3 and OH4 is the use of a rigid coupling method in OH4 compared to flexible coupling in OH3.
OH5 Type Pump
OH5 type pump is a closed coupled pump. The impeller of the pump is directly attached to a driver on the same shaft. It is a vertical, in-line, and single-stage pump.
OH6 Type Pump
OH6 is a high-speed pump as it uses speed increasing gearbox. The impeller is mounted directly on the gear shaft. The gearbox is connected with a driver by a flexible coupling. It is a single-stage overhung pump that can be installed both vertically and horizontally.
Image Source – Sundyne
BB1 Type Pump
BB1 (Between Bearing) is horizontally installed one or two-stage pump. The casing of this pump is an axially split type.
BB2 Type Pump
The difference between BB1 and BB2 is the way casing split. In BB2, the casing is radially split type.
BB3 Type Pump
A multistage between bearing pumps with Axially Split cashing design is designated as an API BB3 Type pump. This pump used flexible coupling to connect driver.
BB4 Type Pump
In this API pump design, each stage of the pump is like a ring and connected through tie rode around them. If you see the image, it will be clear why this pump is also called a ring section pump. Each stage is a radially split design and installed on the shaft supported by bearings at both ends.
BB5 Type Pump
BB5 type pump is known as Barrel Pump due to its barrel-like design. It is used in high-pressure services. It is a multistage, double casing, radially split design that uses flexible coupling.
VS1 Type Pump
VS1 type pump is a single casing vertically suspended type design. It is used as a wet pit or diffuser pump. The column of a pump is used as a discharge from the pump.
VS2 Type Pump
VS2 type pump uses volute casing design as compared to VS1. Other factors are similar to the VS1 type pump.
VS3 Type Pump
VS3 type pump is axial flow, and single casing vertically suspended type design. Pump column over which pump assembly is mounted, used as discharge from the pump.
VS4 Type Pump
This type of pump has a separate discharge column. It has a volute casing design. The vertically suspended shaft can be supported through intermediate bearings.
VS5 Type Pump
VS5 also has a separate discharge column. The difference is in the vertically suspended shaft. In the case of VS5, there will be no intermediate bearings. The pump shaft is a cantilever and supported by a top bearing.
VS6 Type Pump
VS6 pump has a double casing. It is also known as can pump due to this design. It is a vertically suspended pump with discharge through the pump column. This design is used to increase NPSH in some cases by installing a pump underground.
VS7 Type Pump
VS7 type pump uses volute casing design as compared to VS6. Other factors are similar to VS6.
Reference for Images
- Flowserve – https://www.flowserve.com/en/products/pumps
- Ruhrpumpen – https://www.ruhrpumpen.com/en/products
- Sundyne – https://www.delta-p.no/products/api-610-pumps/