Types of Pumps

Pumps are machines that move fluids by transferring mechanical energy to a fluid.The term pump is used for machines that transport incompressible fluids. Compressor is the term used for machines used for handling compressible fluids or gases such as air, oxygen, etc. In a pumping operation, the pressure of the fluid will increase and cause it to flow downstream in a direction of low pressure. In order to transport the fluid, it is necessary that enough energy be added to overcome frictional losses in the piping system. The selection of pump is dependent on following factors:

1) the delivery pressure required

2) volume flow rate

3) properties of the fluid handled such as density and viscosity

4) temperature of fluid

The pumps used for moving water can be broadly classified as positive displacement pumps and kinetic pumps. Kinetic pumps can be further subdivided into centrifugal pumps (the vast majority of kinetic pumps) and special effects pumps. Jet pumps,gas lift and electromagnetic pumps are examples of special effect pumps, which are not widely used. The operating principle of centrifugal pumps and positive displacement pumps are quite different. Positive displacement pumps discharge constant volume of water without depending on delivery pressure. These generally produce low flow rates against high pressures. In case of centrifugal pumps, the discharge flow rate will be high for low pressure and the flow rate decreases as discharge pressure increases. Before attempting to select a pump for a particular application, it is important to understand the types of pumps available and how they operate.

i. Centrifugal Pumps

Centrifugal pumps are the most common type of pumps found. Centrifugal pumps enjoy widespread application partly due to their ability to operate over a wide range of flow rates and pump heads. Developing centrifugal force to move up liquid is basis of operation of these pumps. When circular motion is induced in water, it has a spiral flow towards the axis of rotation.In a centrifugal pump, the water is admitted to the centre of rotation by the suction pipes, and is then rotated with the help of spiral vanes. This results in increased pressure head due to centrifugal force and the water can be pushed ahead in delivery pipe.

For these pumps, the discharge flow rate, discharge pressure, power required for pumping and efficiency of pump are interrelated. These pumps are popular and have lot of applications where flow rate is high and delivery head is low. Ease of operation and maintenance are advantages of these pumps.

The volute type centrifugal pump consists of two basic parts; the impeller on a rotating shaft and a stationary pump casing.The pump casing provides a pressure boundary for the pump and contains channels to properly direct the suction and discharge flow. The pump casing has suction and discharge penetrations for the main flow path of the pump and normally has small drain and vent fittings to remove gases trapped in the pump casing. From the suction inlet of casing, the fluid moves to the centre of impeller, from where it is moved by the revolving blades or vanes of the impeller. Because the impeller blades are curved, the fluid is pushed in a tangential and radial direction by the centrifugal force. This force acting inside the pump is the same one that keeps water inside a bucket that is rotating at the end of a string. Due to this centrifugal force, the liquid moves to the outer periphery of the pump casing where it is collected in the outer part of the pump casing called the volute. The volute is a region that expands in cross- sectional area as it wraps around the pump casing. The purpose of the volute is to collect the liquid discharged from the periphery of the impeller at high velocity and gradually cause a reduction in fluid velocity by increasing the flow area. This converts the velocity head to static pressure. The fluid is then discharged from the pump through the discharge connection. By changing the form of the vanes, different characteristics are obtained.

Some centrifugal pumps contain diffusers. A diffuser is a set of stationary vanes that surround the impeller.The purpose of the diffuser is to increase the efficiency of the centrifugal pump by allowing a more gradual expansion and less turbulent area for the liquid to reduce in velocity. The diffuser vanes are designed in a manner that the liquid exiting the impeller will encounter an ever increasing flow area as it passes through the diffuser. This increase in flow area causes a reduction in flow velocity, converting kinetic energy into flow pressure.

ii. Positive Displacement Pump
A Positive displacement pump has an expanding cavity on the suction side of the pump and a decreasing cavity on the discharge side. Liquid is allowed to flow into the pump as the cavity on the suction side expands and the liquid is forced out of the discharge as the cavity collapses. This principle applies to all types of Positive Displacement Pumps whether the pump is a rotary lobe, gear within a gear, piston,diaphragm, screw, progressing cavity, etc.A positive displacement pump, unlike a centrifugal pump, will produce the same flow at a given RPM no matter what the discharge pressure is. A positive displacement pump cannot be operated against a closed valve on the discharge side of the pump, i.e. it does not have a shut-off head like a centrifugal pump does. If a positive displacement pump is allowed to operate against a closed discharge valve it will continue to produce flow which will increase the pressure in the discharge line until either the line bursts or the pump is severely damaged or both. These pumps find wide applications in process industries where accurate flow rate is needed or when the pump acts against high discharge pressure. These pumps are generally not used for pumping water in water supply systems.

iii. Vertical Turbine Pump

A vertical turbine pump is vertical axis centrifugal or mixed flow pump. The pump is located below the ground water level in the well. It is driven by a long shaft,which connects the pump to motor drive on ground surface. Relatively small diameter of pump makes it more suitable for tube wells. If the tube wells are deep, the length suction pipe of a centrifugal pump becomes very long and the centrifugal pumps cannot pump water from deep tube wells. A more effective solution in such a case is to move the pump down into the well so, instead of lifting the water, it’s pushing water up. Vertical turbine pumps are constructed to operate in this way. These pumps have three major parts: the pump element, the discharge column and head assembly as shown in figure 17.4. The pump element consists of one or more stages or bowls. Each bowl consists of an impeller and a diffuser. A screen is  provided at the bottom of pump for entry of sand free water. The discharge column connects the pump element and pump head assembly and convey water. It has a discharge pipe and a line shaft of pump with couplings and bearings. The shaft is located at the center of the discharge pipe. The lubrication of pump is done either by water or oil. For oil lubricated pumps, the drive shaft is covered by another pipe which transports the oil to parts which need lubrication. The pump head assembly is a base from which the discharge column pump assembly, shaft assembly are suspended. Motor or gear drive is generally mounted on it. The performance curves are similar to those of volute type of centrifugal pumps. The limitations of this pump are that it needs proper alignment.

The correct alignment of pump with both well casing and motor or drive is the most important requirement for trouble free operation of vertical turbine pump.Otherwise, parts of the pump may touch well casing and get damaged. To achieve this proper alignment, a strong foundation is used for mounting the motor or drive and for suspending the discharge pipe and pump with shaft. The pump bowl assembly is placed well below the maximum draw down level so that it is always under water.

iv. Submersible Pump

With ever falling ground water levels, the tube wells are drilled deeper and deeper to fetch water. Running a vertical turbine pump with long drive shaft in such very deep wells is a tough engineering challenge. A more effective solution in such a case is to closely couple the motor and pump and move them together down into the well. In this way, the requirement of long shaft is avoided. A vertical turbine pump close coupled to a small diameter submersible electric motor is called submersible pump. Submersible pumps are designed to be fully immersed within a well or tank. As shown in figure 17.5, the submersible pump consists of a pump and motor assembly, a delivery pipe and other accessories such as water proof cable. These pumps are single stage or multi stage centrifugal pumps with impellers of mixed or radial flow. The motors of are normally sealed in oil-filled cavities that are protected from contact with the liquid.

Submersible pumps can be applied to many distinct applications, from pumping water from bore wells, to transferring wastewater at large flow rates or high pressures, to simply pumping up water off the floor or the bottom of a tank; depending upon submersible pump design.Some of the more common submersible pumps designed for specific applications include water submersible pumps, sewage submersible pumps, sand submersible pumps, irrigation submersible pumps, and solar submersible pumps.There are four main specifications to consider when selecting between the available submersible pump types (beyond specific pump type or application).
These submersible pump specifications include :

1. Maximum discharge flow, which is the maximum flow the pump is designed to generate. This value is dependent on the system or pressure head the pump must enter.

2. Maximum discharge pressure, which is the maximum pressure the pump is designed to generate.3. Horsepower (hp), which is used to express the rate at which mechanical energy is expended. Horsepower is the work done at the rate of 745.7 watts

4. Discharge size, which is the size of the submersible pumps’ discharge or outlet connection.

In these pumps both the pump assembly and motor are of same size which easily fit in to a tube well. Also these pumps can tolerate deviations in vertical alignment of tube well casing. Since there is no working part above the ground, these pumps are suitable for flood prone areas and public grounds where an above ground pump house would be inconvenient. Although the pumps run for years without trouble in case of proper installation, withdrawal and overhauling of after every two or three years is recommended.

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