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MINIMUMMINIMUM STARTING SPEEDSTARTING SPEED

When the pump is started, there will no flow of wateruntil the pressure difference in the impeller is largeenough to overcome the gross or manometeric head.Therefore a centrifugal head or pressure head causedby the centrifugal force on the rotating water will be(u2

2-u12)/2g.

mHg

uu

2

21

22

mHNDND

2

1

2

2

6060

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PRIMINGPRIMINGWhen a centrifugal pump is not running for sometime,the water present in the pump casing and suction pipeflows back to the sump and these spaces get filled withair. Now when the pump motor is switched on andpump starts running, the head developed equals H=(Vu2.u2)/g of air.

Since ρair<<<ρwater the head generated cannot producespontaneously the vacuum required to start thepumping action then the water cannot be sucked inalong the suction pipe to reach the impeller , for makingthe pump deliver water, there is need to make the pumpsection free from air and fill these space with water .

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The entire operation of completely filling thesuction pipe, casing and delivery pipe with theliquid to be pumped is called priming. Afterproper priming and keeping the delivery valveclosed, the pump is started, the shaft off head(head generated when no flow of liquid) buildsup and delivery valve is generally opened.

PRIMING

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PRIMING DEVICESPRIMING DEVICESa) Pouring water: Water is poured in the pump through

priming funnel. Air vent is opened to provide exit to theair. It is closed after the priming is over.

b) Connection with main waterline: - The pump maybe connected with the city water main which can beopened to fill the impeller and suction pipe in order toprime the pump.

c) Priming chamber:-in small pumps a priming chambermay be used on the delivery side of the impeller. Whenthe pump is stopped, some water is store in the tank andthis can be used to impeller and the suction line beforerestarting.

d) Vacuum producing devices: - An injector using highpressure water, stream or compressed air is employed tocreate vacuum at the top of the casing. So that water issucked into the suction pipe and the impeller.

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BLADE SHAPEBLADE SHAPE

The shape of impeller blades is changed depending on the blade angle β2 which has a significant influence on the conversion of energy. The different blade and velocity vectors are represented in fig

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Direction of outlet relative velocity is fixed byβ2 andvelocity vector depending upon blade configuration.For backward curved blade the tangential componentVu2 is reduced for a given impeller speed, the impellerwill have a low energy transfer is maximum forforward curved blade. However, the absolute velocityV2 at impeller outlet is also increased. a high value ofV2 is not desirable as its conversion into staticpressure cannot be very efficiently carried out in thecasing section.So losses are maximum for forward curved blade.

Normally backward blade with (22˚-32˚) is employedexcept in the case where the high head is the majorconsideration.

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SLIP FACTORSLIP FACTORThe ideal head developed considering infinitenumber of blades in the impeller through which thewater has to pass.In practice the impeller must haveonly a finite number of blades, due to inertia effects,the liquid which is trapped between the impellerblades is reluctant to move round with the impeller.This results in a difference of pressure force acrossthe blade, there being a high pressure (+) on theleading face and a low pressure (-) on the trailingface. This difference of pressure, called vane loading,increase with an increase in the number of blades.

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Due to deviation in the flow path, the tangentialcomponent gets reduced from Vu2 to Vu2`. The difference(Vu2-Vu2`) is called slip of the impeller. the slip factor isdefined the ratio of whirl component with fluid deviationto the whirl component without fluid deviation further dueto real fluid effects( friction and separation on the wall ofdisc , shrouds, and blades) the radial velocity may notuniform around the periphery of the impeller.

The net effect of slip and non-uniform velocity is to reducethe Euler head.

Slip factor (σ) = By stodola relation

(z is the number of blades)

Slip factors increases with the increase in the number of blades and becomes unity when there are infinite no. of blades.

2

2

u

u

V

V

z

1

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AXIAL AXIAL THRUST IN CETRIFUGAL PUMPSTHRUST IN CETRIFUGAL PUMPSAxial thrust is a force casting parallel to the axis of the pump

shaft, caused due to the following reason:-(a). The water while passing through the impeller is rotating with a

forced vortex , but that the outside the surrounding othercomponent are in the rest condition , this cause a differentialstatic thrust acting parallel to the axis of pump shaft andtowards the impeller inlet.

(b). Liquid enters the pump axially and is then deflected from itsoriginal path to a radial direction. The dynamic action of liquidcauses a force to act on the pump in the direction of flow ofinlet. To enable the pump to withstand the thrust, the followingmethods may be employed.

1.For small pumps(1) Providing a thrust ball bearing in the direction of axial thrust. (2) Inserting a cast iron ring in the casing which should fit in with a similar ring cast integral with impeller.

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2.For large pumpsWhere the axial thrust is heavy.(1) Use of double suction impeller Suction on two sides

of the impeller neutralizes the thrust. But this method can be only for single stage pump.

(2) Provision of reliving holes are provided in the impeller to allow suction pressure to act on both sides. (3) A balance plate fitted at the end of the pump shaft.

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3. For Multi Stage Pump

The number of impeller are made generally even, this will facilitates to arrange the inlets of the half of the impeller in the opposite direction. These will balance the axial thrust produced by the other half numbers of the impellers.

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PERFORMANCE OF PUMPSPERFORMANCE OF PUMPS--CHARACTERISTIC CURVECHARACTERISTIC CURVE

A pump is usually designed for a particular speed,flow rate and head, but in actual practice theoperation may be at some other condition of head,and for the changed condition the behavior of thepump is less efficient than the quantity the value ofvelocity of flow of liquid through impeller will bechanged.

Characteristic curve are usually prepared for thecentrifugal pumps are main and operatingcharacteristic curve, constant efficiency curves,constant head & constant- discharge curve

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Main characteristic curves of centrifugal pump

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Operating characteristics: During operation the pump must run at a constant speed. Normally, this is the designed speed. The particular set of main characteristics which correspond to the designed speed is mostly used in operation and is therefore known as operating characteristics

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MuschalMuschal Curve or Constant Curve or Constant Efficiency CurveEfficiency Curve

With the help of data obtained from the above curve, a series of constant. Efficiency curve can be obtained. They facilitate the job of the salesman and enable the prospective customer to see directly the range of operation with a particular efficiency,

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Constant Head and Constant Constant Head and Constant Discharge CurveDischarge Curve

It is quite possible that a pump may be required to deliver water at a certain height, in which case it is fixed. If for some reason the speed varies, discharge will also be affected.