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BASICS OF CENTRIFUGAL PUMPS History of Pumps Hydraulic energy (i.e. the idea of using water) was first converted into mechanical energy in India about 2000 years back, by passing the water through water wheels. Since olden times, man has been trying to find out convenient ways of lifting water to higher levels for purposes of water supply and distribution, supply of water at high pressure and for filtration. It is believed that the idea of lifting water by centrifugal force was first given by an Italian scientist in the end of 16th century. Since then, continuous advancement of this pump has brought it to a high degree of perfection, and today the technology is used all over the world. Definition of a pump A pump, in general, may be defined as a machine, when driven with an external source, lifts the water or some other liquid from a lower level to high level. A pump which converts kinetic energy into pressure energy by the principle of centrifugal force is called a centrifugal pump. Working principle of a Centrifugal pump Details of a centrifugal pump 18 / February 2017 / Indian Plumbing Today CHANDRASHEKHAR SONAWANE l Impeller that rotates exerts centrifugal force on the liquid l Kinetic energy is created- i.e. the liquid is made to move at a very high velocit l Centrifugal force throws the liquid out l Creates low pressure at the suction eye l This forces new liquid into the impeller inlet l Liquid thrown out of the impeller moves in the volute towards the outlet l Liquid slows down due to increasing sectional area converting velocity (kinetic energy) into pressure (pressure energy) In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. Bernoulli's principle can be applied to various types of fluids flow. The simple form of Bernoulli's equation is valid for incompressible flows (e.g. most liquid flows and gases moving at low speeds). Bernoulli's principle can be derived from the principle of conservation of energy. This states that, in a steady flow, the sum of all forms of energy in a fluid along a streamline is the same at all points on that streamline. This requires that the sum of kinetic energy, potential energy and internal energy remains constant. Bernoulli's principle can also be derived directly from Isaac Newton's Second Law of Motion. The impeller in a pump rotates at a very high speed. The fluid that leaves the vane at the edge of the impeller has a very high velocity (i.e. high kinetic energy). The volute of the pump is designed so that the cross sectional area increases along the flow of the fluid as it travels from the impeller edge out of the pump. The increase in area reduces the velocity and simultaneously increases the fluid pressure which allows it to be pumped to the required height. c) Long coupled pumps have a separate pump and separate motor, connected to each other with the help of a flexible coupling. These are generally used for flow up to 1200 m3/hr. Types of Centrifugal pumps Pumps can be classified as: 1) End suction pumps 2) Axially split case pumps 3) Vertical inline pumps 4) Submersible pumps 1)End suction pumps: They are also called as Horizontal pumps, Back pull out pumps, and Radial split casing pumps. End suction pumps are further classified as a) Monoblock pumps b) Closed coupled pumps c) Long coupled pumps a) Monoblock pump is simpler and easy to handle. The motor & pump are directly coupled together and therefore base plate and coupling alignment is not required. These pumps are more compact compared to closed coupled and long coupled pumps. These pumps are available for flows up to 200 m3/hr. Long coupled pumps 2) Axially split case pumps: An axial split casing pump is a pump whose casing is split axially, or whose principal joint is parallel to the shaft centerline. The casing is split into the upper half and lower half. An axially split casing is always a horizontal pump. Pump internals can be rectified by simply removing the pump top case - there is no need to remove the motor. MONOBLOCK pumps b) Closed coupled pump comes with a spacer or rigid coupling in between impeller and the pump shaft. These pumps are generally used for flow up to 500 m3/hr. Closed coupled pumps Axially split casing pumps Vertical inline pumps Indian Plumbing Today / February 2017 / 19 3) Vertical inline pumps: This pump is a vertical pump with suction and discharge connections in line with each other. These are generally used for high pressure applications. 4) Submersible pumps: These pumps (pump and motor) are fully submerged in the liquid to be pumped. There are two basic types of submersible pumps based on the application -whether they are used for clear water or waste water. Clear water submersible pumps come with closed impellers. Submersible pumps for drainage, waste water, effluent & sewage application comes with semi open or open impellers based on particle size to be handled. Selection of Centrifugal pumps A pump is selected based on the differential head through which it has to operate and the flow required at the end of the terminal. The differential head would mean the vertical height from the water level in the storage sump to the top most floor to which the water is pumped. Other factors such as the liquid to be transported, the temperature, viscosity, particle size in the liquid will further determine the pump size. Pump characteristic curves The flow of centrifugal pump increases with reduction in head. Head is always mentioned in metre in the pump curve. The pump selection should be such that it is operated at highest efficiency for the required flow rate / head to save energy. Drainage pumps for particle size max 12 mm Mr. CHANDRASHEKHAR SONAWANE is Area Manager with India Liaison Office, Ebara Pumps Europe SpA. He has 15 years experience in Sales & Services of centrifugal pumps. Submersible pumps for waste water, sewage A pump is selected based on the differential head through which it has to operate and the flow required at the end of the terminal. 20 / February 2017 / Indian Plumbing Today