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A Centrifugal pump is a piece of rotary equipment that converts kinetic energy into the pressure head of the liquid. External power from an electric motor or diesel generator drives the pump impeller. Fluid enters 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 used to transport fluid from one place to another.
The working principle of the pump is the conversion of energy.
Liquid enters the pump suction nozzle. It enters inside the rotating impeller vane through the eye. – Impeller gets its energy from the driver (Motor, Engine, or Turbine) The rotating impeller forces liquid outward by centrifugal force. The liquid will get velocity and pressure during this process. Now, liquid enters inside the volute casing (sometimes with a diffuser), which reduces its velocity and further increases the pressure. It also directs fluid towards the discharge nozzle. The impeller and casing are the main parts that do all the work of the conversion of energy. The impeller transfers the power to fluid, and the casing helps to convert the same liquid head.
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 must prime the pump before you start if it empties 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. With higher density, a pump must do more work to push the liquid. Again, this will create less than ideal situations for operation and maintenance due to increased power and maintenance requirements.
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 fixed volume pumps such as gear and lobe and centrifugal pumps.
There are multiple ways to classify the centrifugal pump.
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. The charts above cover the most common way to organize these pumps. The same pump can be a part of two or more groups.
In radial flow, fluid take a 90-degree turn from the suction. 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.
When the fluid flows parallel to the pump shaft, it is called an axial flow pump. In this type of pump, 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 and water circulation pumps are everyday use cases of axial flow pumps.
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 pumps. It can handle a high flow rate with a decent increase in pressure head.
Depending on the requirement, two or more impellers can be used in the pump.
A single impeller design is useful for a large flow rate with relatively less increase in the pressure head. The multistage design is used when you need a very high head at discharge. In this case, impellers are connected in the series so that each stage will boost the fluid pressure.
When it comes to using code for pump design, the following two are used in most cases.
Pump casing can be of the single volute type or double volute type. 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 defection when the pump operates 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 is the way you open a pump for any maintenance.
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 pump is used in a 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 types.
Most centrifugal pump has a single suction design. However, single-suction will not be enough when the flow rate is very high. In this case, double suction pumps are used. The impeller of this type of pump is designed in such a way that fluid enters from both sides as compared to a single side in a 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 the impeller and casing that are different.
Overhung – In this type of pump, 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.
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