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INTRODUCTION 1.1 PRINCIPLE AND CLASSIFICATION OF PUMPS 1.1.1 Principle Newton’s First law states that “Energy can neither be created nor be destroyed, but can be transformed from one form of energy to another form.” Different forms of energy exists namely, electrical, mechanical, fluid, hydraulic and pneumatic, pressure, potential, dynamic, wave, wind, geothermal, solar, chemical, etc. A machine is a contrivance, that converts one form of energy to another form. An electric motor converts electrical energy to mechanical energy. An internal combustion engine converts chemical energy to mechanical energy, etc. A pump is a machine which converts mechanical energy to fluid energy, the fluid being incompressible. This action is opposite to that in hydraulic turbines. Most predominant part of fluid energy in fluid machines are pressure, potential and kinetic energy. In order to do work, the pressure energy and potential energy must be converted to kinetic energy. In steam and gas turbines, the pressure energy of steam or gas is converted to kinetic energy in nozzle. In hydraulic turbine, the potential energy is converted to kinetic energy in nozzle. High velocity stream of fluid from turbine nozzle strikes a set of blades and makes the blades to move, thereby fluid energy is converted into mechanical energy. In pumps, however, this process is reversed, the movement of blade system moves the fluid, which is always in contact with blade thereby converting mechanical energy of blade system to kinetic energy. For perfect conversion, the moving blade should be in contact with the fluid at all places. In other words, the moving blade system should be completely immersed in fluid. 1.1.2 Classification of Pumps 1.1.2.1 Classification According to Operating Principle Pumps are classified in different ways. One classification is according to the type as positive displacement pumps and rotodynamic pumps. This classification is illustrated in Fig. 1.1. In positive placement pumps, fluid is pushed whenever pump runs. The fluid movement cannot be stopped, otherwise the unit will burst due to instantaneous pressure rise theoretically to infinity, practically exceeding the ultimate strength of the material of the pump, subsequently breaking the material. The motion may be rotary or reciprocating or combination of both. 2 ROTODYNAMIC PUMPS (CENTRIFUGAL AND AXIAL) P U MP S P os itive Dis placement P umps Reciprocating Type Piston plunger Rotary Type Other P umps R otodynamic P umps Jet Pump Hydraulic Ram Centrifugal, Mixed and Axial Flow Regenerative Vane, Lobe Screw, Gear Perialistic, Metering, Diaphram, Radial piston, Axial piston F ig. 1.1. P ump clas s ification The principle of action, in all positive displacement pumps, is purely static. These pumps are also called as ‘static pumps’. The pumps, operated under this principle, are reciprocating, screw, ram, plunger, gear, lobe, perialistic, diaphram, radial piston, axial piston etc. In rotodynamic pumps, however, the energy is transferred by rotary motion and by dynamic action. The rotating blade system imparts a force on the fluid, which is in contact with the blade system at all points, thereby making the fluid to move i.e., transferring mechanical energy of the blade system to kinetic energy of the fluid. Unlike turbine, where pure pressure or potential energy is converted to kinetic energy, in pumps, the kinetic energy of the fluid is converted into either, pressure energy or potential energy or kinetic energy or the combination of any two or all the three forms depending upon the end use in spiral or volute casing, which follows the impeller. In domestic, circulating and in agricultural pumps, the end use is in the form of potential energy i.e., lifting water from low level to high level. In process pumps, used for chemical industries, the fluid is pumped from one chamber under pressure to another chamber under pressure. These chambers may be at the same level (only pressure energy conversion) or may be at different levels (pressure and potential energy conversion). Pumps used for fire fighting, for spraying pesticides, must deliver the liquid at very high velocity i.e., at very high kinetic energy. These pumps convert all available energy at the outlet of the impeller into very high kinetic energy. In turbines, the fluid is water or steam or chemical gas-air mixture at constant pressure and temperature, whereas, pumps deal with fluid at different temperatures and viscosities such as water, acids, alkaline, milk, distilled water, and also cryogenic fluids, like liquid hydrogen, liquid oxygen, liquid nitrogen, liquid ammonia, which are in gaseous form under normal temperatures. Pumps are also used to pump solid-liquid, liquid-gas or solid-liquid-gas mixtures, with different percentage of concentration called ‘consistency’. Hence pumps are applied in diversified field, the pumping fluid possessing different property, namely, viscosity, density, temperature, consistency, etc. 3 INTRODUCTION A third category of pump, called jet pump, wherein, the fluid energy input i.e., high head low discharge of fluid is converted into another form of fluid energy i.e., low head and high discharge. These pumps are used either independently or along with centrifugal pumps. The reverse of Jet pump is ‘Hydraulic Ram’ wherein low head and high discharge of water is converted into high head and low discharge. Hydraulic Rams are installed at hills near a stream or river. The natural hill slope is the low head input energy. Large quantity of water at low head is taken from the river. A portion of water is pumped at high pressure and is supplied to a nearby village as drinking water. Remaining water is sent back to the river. This system does not need any prime mover like diesel or petrol engine or electric motor. Repair and maintenance is easy, in hydraulic ram since moving part is only the ram. 1.1.2.2 Classification According to Head and Discharge Another classification of pump is according to the head and discharge or quantity of flow to be pumped. Any customer, who is in need of a pump specifies only these two parameters. A quick selection of the pump is made referring standard charts for selecting the pump. Fig.1.2 gives the selection of pump according to head and discharge. 10000 H.m PISTON 1000 CENTRIFUGAL 100 10 AXIAL 1 100 10 1000 10000 100000 3 Q.m /hr Fig. 1.2. Pump selection as per head and discharge 1.1.2.3 Classification According to Specific Speed Most accurate method of pump selection is based on the non-dimensional parameter called ‘specific speed’ which takes into account speed of the pump along with head and discharge. Specific speed, ns = 3.65 n H Q 3/ 4 ...(1.1) where ns–specific speed, n–speed in rpm, Q–discharge in m3/sec, H–head in m. If pressure rise is known instead of total head then p = γH, where p–pressure rise of pumping fluid in N/m2 and γ–specific weight of the fluid at the given temperature in N/m3. It is essential that all parameters must be 4 ROTODYNAMIC PUMPS (CENTRIFUGAL AND AXIAL) converted to equivalent water parameters before substituting them in equation 1.1. Fig.1.3, illustrates the pump selection according to the specific speed of the pump. Centrifugal (radial flow) Low Medium High Diagonal and mixed flow ns = 50 ÷ 80 n s = 80 ÷ 150 ns = 150 ÷ 300 ns = 300 ÷ 500 b2 b2 b2 Propeller and axial flow n s = 500 ÷ 1000 b D2 = 2 to 1,8 D0 D2 = 1,8 to 1,4 D0 D2 D0 D2 D0 D0 D0 D0 D2 = 2,5 to 1,8 D0 D2 D2 D2 2 D2 = 1,4 to 1,2 D0 D2 = 0,8 D0 H–Q H–Q N– Q Q D– Q N– D– Q H–Q N– H–Q Q Q D– D N– –Q Q H–Q N–Q Q D– Fig. 1.3. Classification according to specifc speed From Fig.1.3, it is evident that, at low specific speeds, centrifugal pumps; at medium specific speeds, mixed flow pumps and at high specific speeds, axial flow pumps are used. All of them are classified as rotodynamic pumps. At very low specific speeds, however, positive displacement pumps are used. Referring to the equation (1.1), it is seen that positive displacement pumps are used for very high head-very low discharge conditions. Ship propellers and aircraft propellers are of very high specific speed units beyond 1200 i.e., used for very low head-very high discharge conditions. 1.1.2.4 Classification According to Direction of Flow in Impeller Another classification of pumps is according to the direction of flow of fluid in impeller of the pump such as radial or centrifugal flow, mixed or diagonal flow and axial flow. Fig.1.4, illustrates the position of blade system in the impeller passage of a pump. Considering the flow of fluid in impeller, (Fig.1.4) if the flow direction is radial (2-1) and (3-1) i.e., perpendicular to the axis of rotation, the pump is called radial flow centrifugal pump. If the flow is axial (6-5) i.e., parallel to the axis of rotation, the pump is called axial flow pump. If the flow is partly axial and partly radial (4-2) and (4-3) i.e., diagonal, it is called mixed flow pump or diagonal flow pump. It is evident, from the Fig.1.4, that all these pumps are rotodynamic pumps i.e., rotary blade passage and dynamic action of blade system in the fluid passage. 5 INTRODUCTION b 2′′ b2 Outlet, Delivery of water a2 a2 III D 2′ IV 6 (b) Mixed 1 2 3 Ds a1 D1 (a) Radial Inlet, entry of water Ds II D3 D ′3′ a1 D2 I (c) Axial 5 4 Shaft 90° axis (d) Relative location Fig. 1.4. Position of blade system in different types of impellers 2–1 Centrifugal — Radial flow — very high head and very low flow. 3–1 Centrifugal — Radial flow — high head and low flow. 4–2 Mixed flow — Medium head and medium flow — low range. 4–3 Diagonal flow — Medium head and medium flow — higher range. 6–5 Axial flow, propeller — low head and high flow. Radial type centrifugal pumps have higher impeller diameter ratio (outlet to inlet diameter) and the blade is longer. Mixed flow pumps have medium diameter ratio and axial flow pumps have equal inlet and outlet diameters. This indicates that radial flow pumps work mostly by centrifugal force and partly by dynamic force, whereas, in axial flow pumps, the pressure rise is purely by hydrodynamic action. In mixed and diagonal flow pumps, however, the pressure rise is partly by centrifugal force and partly by hydrodynamic force.