pumps Principle of operation of a pump: Classifications of pumps: Hydraulic machines in which energy is transmitted from the working member to the flowing liquid and the energy of the liquid at the outlet of the hydraulic machine is less than the outlet energy are referred to as pumps. Principle of operation of a pump: Pumps operate on the principle whereby a partial vacuum is created at the pump inlet due to the internal operation of a pump. This allows atmospheric pressure to push the fluid out of the oil tank (reservoir) and into the pump intake. The pump then mechanically pushes fluid out of the discharge line. Classifications of pumps: Classification based on displacement Non positive displacement pumps (hydrodynamic pumps) Positive displacement pumps (hydrostatic pumps) 2. Classification based on delivery Constant delivery pumps Variable delivery pumps 3. Classification based on motion Rotary pump Reciprocating pump.

Rotodynamic PUMPS Centrifugal Pumps A rotodynamic pump is a device where mechanical energy is transferred from the rotor to the fluid by the principle of fluid motion through it. The energy of the fluid can be sensed from the pressur and velocity of the fluid at the delivery end of the pump. Therefore, it is essentially a turbine in reverse. Like turbines, pumps are classified according to the main direction of fluid path through them like (i) radial flow or centrifugal, (ii) axial flow and (iii) mixed flow types. Centrifugal Pumps The pumps employing centrifugal effects for increasing fluid pressure have been in use for more than a century.The centrifugal pump, by its principle, is converse of the Francis turbine. The flow is radially outward, and the hence the fluid gains in centrifugal head while flowing through it. Because of certain inherent advantages,such as compactness, smooth and uniform flow, low initial cost and high efficiency even at low heads, centrifugal pumps are used in almost all pumping systems.

Working of Centrifugal Pumps

COMPONENTS OF Centrifugal Pumps

Net Head Developed by a Pump At any point in the system, the elevation or potential head is measured from a fixed reference datum line. The total head at any point comprises pressure head, velocity head and elevation head. For the lower reservoir, the total head at the free surface is  = to the elevation of the free surface above the datum line since the velocity and static pressure at A are zero Similarly the total head at the free surface in the higher reservoir is = to the elevation of the free surface of the reservoir above the reference datum. The liquid enters the intake pipe causing a head loss  for which the total energy line drops to point B corresponding to a location just after the entrance to intake pipe. The total head at B can be written as

The variation of total head as the liquid flows through the system is shown in Fig. As the fluid flows from the intake to the inlet flange of the pump at elevation  The total head drops further to the point C due to pipe friction and other losses equivalent to  The fluid then enters the pump and gains energy imparted by the moving rotor of the pump. This raises the total head of the fluid to a point D at the pump outlet In course of flow from the pump outlet to the upper reservoir, friction and other losses account for a total head loss or  down to a point E . At E an exit loss  occurs when the liquid enters the upper reservoir bringing the total heat at point F  to that at the free surface of the upper reservoir. If the total heads are measured at the inlet and outlet flanges respectively, as done in a standard pump test

The head developed H is termed as manometric head Total inlet head to the pump =  Total outlet head of the pump =  Where , are the velocities in suction and delivery pipes respectively. Therefore, the total head developed by the pump, The head developed H is termed as manometric head 

Velocity triangles for centrifugal pump Impeller Figure shows an impeller of a centrifugal pump with the velocity triangles drawn at inlet and outlet. The blades are curved between the inlet and outlet radius. A particle of fluid moves along the broken curve shown in Figure Work done on the fluid per unit weight =  Now, considering the operation under design conditions, the inlet whirl velocity  and accordingly the inlet angular momentum of the fluid entering the impeller is set to zero Work done on the fluid per unit weight  = 

EFFICIENCIES OF CENTRIFUGAL PUMPS It represents the effectiveness of the pump in increasing the total energy of the fluid from the energy given to it by the impeller. Therefore, we can write The overall efficiency   of a pump is defined as where, Q is the volume flow rate of the fluid through the pump, and P is the shaft power, i.e. the input power to the shaft The energy required at the shaft exceeds  because of friction in the bearings and other mechanical parts Thus a mechanical efficiency is defined as so that,

WORK DONE BY IMPELLER ON WATER R1 = radius of impeller at inlet R2 = radius of impeller at outlet U1 & u2 – tangential blade velocity at inlet and outlet V = absolute velocity Vf = velocity of flow Vr = relative velocity Vw = Velocity of whirl W.D = 1/g [ (Vw2 u2) – (Vw1 u1)] Above equation But , Vw1 = 0 , as entry is radial. Therefore , W.D = 1/g [ (Vw2 u2) ]

Losses in a Centrifugal Pump Mechanical friction power loss due to friction between the fixed and rotating parts in the bearing and stuffing boxes. Disc friction power loss due to friction between the rotating faces of the impeller (or disc) and the liquid. Leakage and recirculation power loss. This is due to loss of liquid from the pump and recirculation of the liquid in the impeller. The pressure difference between impeller tip and eye can cause a recirculation of a small volume of liquid, thus reducing the flow rate at outlet of the impeller as shown in Figure

Multistage centrifugal pumps A centrifugal pump containing two or more impellers is called a multistage centrifugal pump. The impellers may be mounted on the same shaft or on different shafts. For higher pressures at the outlet, impellers can be connected in series. For higher flow output, impellers can be connected parallel. A common application of the multistage centrifugal pump is the boiler feedwater pump. For example, a 350 MW unit would require two feedpumps in parallel. Each feedpump is a multistage centrifugal pump producing 150 l/s at 21 MPa. All energy transferred to the fluid is derived from the mechanical energy driving the impeller. This can be measured at isentropic compression, resulting in a slight temperature increase (in addition to the pressure increase).

Priming Most centrifugal pumps are not self-priming. In other words, the pump casing must be filled with liquid before the pump is started, or the pump will not be able to function. If the pump casing becomes filled with vapors or gases, the pump impeller becomes gas-bound and incapable of pumping. To ensure that a centrifugal pump remains primed and does not become gas-bound, most centrifugal pumps are located below the level of the source from which the pump is to take its suction. The same effect can be gained by supplying liquid to the pump suction under pressure supplied by another pump placed in the suction line.

Performance characteristic curves of centrifugal pumps (a) Main characteristic curves of a centrifugal pumps (b) Operating characteristic curves of a centrifugal pumps

Cavitation Cavitation is the formation, growth and rapid collapse of vapour bubbles in flowing liquids. Bubbles form at low pressures when the absolute pressure drops to the vapour pressure and the liquid spontaneously boils. (Bubbles may also arise from dissolved gases coming out of solution.) When the bubbles are swept into higher-pressure regions they collapse very rapidly, with large radial velocities and enormous short-term pressures. The problem is particularly acute at solid surfaces. Cavitation may cause performance loss, vibration, noise, surface pitting and, occasionally, major structural damage. Besides the inlet to pumps the phenomenon is prevalent in marine current turbines, ship and submarine propellers and on reservoir spillways. The best way of preventing cavitation in a pump is to ensure that the inlet (suction) pressure is not too low. The net positive suction head (NPSH) is the difference between the pressure head at inlet and that corresponding to the vapour pressure.

The net positive suction head must be kept well above zero to allow for further pressure loss in the impeller. The inlet pressure may be determined from Bernoulli: Where, Z inlet – is the maximum permissible suction lift (limited by cavitation) To avoid cavitation one should aim to keep pinlet as large as possible by: keeping z inlet small or, better still, negative (i.e. below the level of water in the sump; keeping V small (large-diameter pipes); keeping hf small (short, large-diameter pipes). The first also assists in pump priming

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