Centrifugal pumps

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CENTRIFUGAL PUMPS 1. INTRODUCTION Centrifugal pumps According to Reti, the first machine that could be characterized as a centrifugal pump was a mud lifting machine which appeared as early as 1475 in a treatise by the Italian Renaissance engineer Francesco di Giorgio Martini.[2] True centrifugal pumps were not developed until the late 17th century, when Denis Papin built one using straight vanes. The curved vane was introduced by British inventor John Appold in 1851. The hydraulic machines which convert the mechanical energy into hydraulic energy are called pumps,the hydraulic energy is in the form of pressure energy. If the mechaniçal energy is çonverted, into pressure energy by means of centrifugal force acting on the fluid, the hydraulic machine is called centrifugal pump. The centrifugal pump acts aš a reversed of an inward radial flow reaction turbine.This means that the flow in centrifugal pumps is in the radial outward directions. The centrifugal pump works on the principle of forced vortex flow which means that when a certain mass of liquid is rotated by an external torque, the rise in pressure head of the rotating liquid takes place. The rise in pressure head at any point of the rotating liquId is Department of Mechanical Engineering , S.I.E.T. Dhenkanal. Page 1

Transcript of Centrifugal pumps

Page 1: Centrifugal pumps

CENTRIFUGAL PUMPS

1.INTRODUCTIONCentrifugal pumpsAccording to Reti, the first machine that could be characterized as a centrifugal pump was a mud lifting machine which appeared as early as 1475 in a treatise by the Italian Renaissance engineer Francesco di Giorgio Martini.[2] True centrifugal pumps were not developed until the late 17th century, when Denis Papin built one using straight vanes. The curved vane was introduced by British inventor John Appold in 1851.

The hydraulic machines which convert the mechanical energy into hydraulic energy are called pumps,the hydraulic energy is in the form of pressure energy. If the mechaniçal energy is çonverted, into pressure energy by means of centrifugal force acting on the fluid, the hydraulic machine is called centrifugal pump.The centrifugal pump acts aš a reversed of an inward radial flow reaction turbine.This means that the flow in centrifugal pumps is in the radial outward directions. The centrifugal pump works on the principle of forced vortex flow which means that when a certain mass of liquid is rotated by an external torque, the rise in pressure head of the rotating liquid takes place. The rise in pressure head at any point of the rotating liquId is proportional to the square of tangential velocity of the liquid at that point (rise in pressure head= V 2

2g or ω

2 r2

2 g). Thus at the outlet of the impeller,

where radius is more, the rise in pressure head will be more and the liquid will be discharged at the outlet with a high pressure head. Due to this high pressure head, the liquid can be lifted to a high level.

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2 . MAIN PARTS OF CENTRIFUGAL PUMPS

The followings are the main parts of a centrifugal pump :

1. Impeller2. Casing 3. Suction pipe with a foot valve and a strainer. 4. Delivery pipe

1. Impeller:- The rotating part of a centrifugal pump is called ‘impeller’ .It consists of a series of backward curved vanes. The impeller is mounted on a shaft which is connected to the shaft of an electric motor.

2. Casing:-The casing of a centrifugal pump is similar to the casing of a reaction

turbine. It is an airtight passage surrounding the impeller and is designed in such a way that the kinetic energy of the water discharged at the outlet of the impeller is converted into pressure energy before the water leaves the casing and enters the delivery pipe. The following three types ofthe casings are commonly adopted:

a) Volute casingb) Vortex casing c) Casing with guide blade

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3. Suction pipe with a foot valve and a strainer:- A pipe whose one end is connected to the inlet of the pumps and oher end dips into water in a sump is know as suction pipe.A foot valve which is a non-return valve or one-way type of valve is fitted at the lower end of the suction pipe.The foot valve opens only in the upward direction.A strainer is also fitted at the lower end of the suction pipe.

4. Delivery pipe A pipe whose one end is connected to the outlet of the pump and other of

the pump and other end delivers the water at a required height is known as delivery pipe.

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3 . WORK DONE BY THE CENTRIFUGAL PUMPS

In case of the centrifugal pump, work is done by the impeller on the water. The expression for the work done by the impeller on the water is obtained by drawing velocity triangles at inlet and outlet of the impeller in the same way as for a turbine. The water enter the impeller radially at inlet for best efficiency of the pump which means the absolute velocity of water at inlet makes an angle of 90° with, the direçtlon of motion of the impeller at inlet. Hence angle α = 90° and Vw1 = 0. For drawing the velocity triangles, the same notations are used as that for turbines.The fig shown that the velocity triangles at the inlet and outlet tips of the vanes fixed to an impeller.

Let N = Speed of the impeller in r.p.m.

D1 = Diameter of impeller at inlet

u1 = Tangential velocity of impeller at inlet =π D1N60

D2 = Diameter of impeller at outlet

u2 = Tangential velocity of impeller at inlet =π D2N60

V1 = Absolute velocity of water at inlet

Vr1 = Relative velocity of water at inlet

α = Angle made by absolute velocity at inlet with the direction of motion of vane

θ = Angle made by relative velocity at inlet with the direction of motion of vane

Vr2 ,β and Φ are the corresponding values at outlet

The work done by the water on the runner per second per unit weight of the water of

the striking per second is = 1g [Vw1 u1 - Vw2 u2]

The work done by the impeller on the water per second per unit weight of water

striking per second = 1g [ Vw2 u2]

Work done by impeller on the water per second = Wg ×Vw2 u2

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Q = Volume of water = π D2 B2 Vf2

4. DEFINITIONS OF HEADS AND EFFICIENCIES

1.Suction Head(hs) :- It is the vertical height of the centre line of the centrifugal pumps above the water surface in the tank or pumps from which water is to be lifted .this height is called suction lift and is denoted by ‘hs’.

2.Delivery Head(hd) :- The vertical distance between the centre line of the pump and the water surface in the tank to which water is delivered is known as delivery head. This is denoted by ‘hd’.

3. Static Head (HS) :- The sum of suction head and delivery head is known as static head. This is represented by ‘HS’ and written as

HS = hs + hd

4. Manometric Head(Hm) :- The manometric head i is defined as the head against which a centrifugal pumps has to work.It is denoted by ‘Hm’.

a) Hm=Head imparted by the impeller to the water – Loss of head in the pumps

= V w 2u2

g – Loss of head in impeller and casing

= V w 2u2

g ….if loss of pump is zero

b) Hm = total head at outlet of the pump –total head at the inlet of the pump

=¿ + V o2

2g + Zo) – ¿ + V i2

2g + Zi)

Po

ρg = Pressure head at outlet of the pump

V o2

2g = Velocity head at outlet of the pump

Zo = Vertical height of the outlet of the pump from datum line Zi = datum head at the inlet of the pump

c) Hm = hs + hd + hfs + hfd + V o2

2g hfs = Frictional head loss in suction pipe, hfd = frictional head loss in delivery pipe

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Efficiencies of a centrifugal pump :- In case of a centrifugal pump, the power is transmitted from the shaft of the electric motor to the shaft of the pump and then to the impeller. From the impeller, the power is given to the water. Thus power is decreasing from the shaft ofthe pump to the impeller and then to the water. The followings are the important efficiencies of a centrifugal pump:

a) Manometric efficiency(ɳman) b) Mechanical efficiency(ηm)

c) Overall efficiency(ηo)

a) Manometric efficiency(ɳman) :-

The ratio of the manometric head to the head imparted by the impellar to the water is known as manometric efficiency.

ɳman=Manometric head

head imparted by impeller ¿water¿

=g Hm

V m2u2

Power at the impeller =Work doneby impeller per second

1000 KW

=Wg ×V w 2u2

1000KW

b) Mechanical efficiency(ηm)

Mechanical efficiency of a centrifugal pump (ηm) is the ratio of theoretical power that must be supplied to operate the pump to the actual power delivered to the pump.

ηm = Power at the impeller

Power at the shaft

=Wg

×V w 2u21000

S . P ,S.P=Shaft power

c) Overall efficiency(ηo)

Overall efficiency of a centrifugal pump (ηo) is the ratio of power output of the pump to the power input to the pump.

The power output of the pump in KW= Weight of water lifted× Hm

1000 = W H m

1000

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ηo = ɳman × ηm5. 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).

6. . PRIMING OF A CENTRIFUGAL PUMPS

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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

7. CHARACTERISTIC CURVES OF CENTRIFUGAL PUMPS

Characteristic curves of centrifugal pumps are defined those curves which are plotted from the results of a number of tests on the centrifugal pump.these curves are necessary to predict the behaviour and performance of the pump when the pump is working under different flow rate,head and speed.the followings are the important characteristic curves for pumps :-

1. Mains characteritics curves 2. Operating characteritics curves3. Constant efficiency or muschel curves

1. Mains characteritics curves

The pump is usually designed to run at the same speed as the driving unit (i.e., prime mover), which is generally an electric motor of the AC induction type. When the electric power is not available, the pump may be driven by a diesel engine, or may be coupled to the tractor engine. In such circumstances, it is necessary to know the performance of a pump at different speeds, which can be best seen from the main characteristic curves of a pump.

In order to obtain the main characteristic curves of a pump, it is operated at different speeds. For each speed, the pump discharge (Q) is varied by means of a delivery valve and for the different values of Q, the corresponding values of manometric head (Hm), shaft power (SP) and overall efficiency (ho) are measured or calculated. Thereafter, Hm vs Q; SP vs Q, and ho vs Q curves for different speeds are plotted as shown in Fig. 28.1, which represent the main characteristics of a pump. Clearly, these curves are useful in indicating the performance of a pump at different speeds.

2. Operating characteritics curves

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During operation of a pump, the pump must run constantly with the speed of the prime mover; this constant speed is usually the design speed. The set of main characteristics curves which corresponds to the design speed is mostly used in pump operation, and hence such curves are known as the operating characteristics curves. A typical set of such characteristics of a centrifugal pump is shown in Fig. 28.2, which consists of four curves at a constant speed viz., head versus discharge (Hm vs Q) curve, efficiency versus discharge (ho vs Q) curve, power versus discharge (BP or SP vs Q) curve, and net positive suction head required versus discharge (NPSHR vs Q) curve. From these characteristic curves, it is possible to determine whether the pump will handle the necessary quantity of liquid against the desired head and what will happen if the head is increased or decreased. In addition, these characteristic curves illustrate what size motor will be required to operate the pump at the required conditions and whether or not the motor will be overloaded under any other operating conditions. A brief description of these curves is provided below.

3. Constant efficiency or muschel curves

The constant efficiency curves or Muschel curves (Fig. 28.6) help determine the range of pump operation for a particular efficiency. As shown in Fig. 28.6, the constant or iso-efficiency curves may be obtained from Hm vs Q and ho vs Q curves of main characteristic curves. In order to plot the iso-efficiency curves, horizontal lines representing constant efficiencies are drawn on the ho vs Q curves. The points at which these lines cut the efficiency curves at various speeds are transferred to the corresponding Hm vs Q curves. The points corresponding to the same efficiency are then joined by smooth curves, which represent the iso-efficiency curves or Muschel curves.

8. CAVITATION

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Cavitation is one of the disadvantages of the centrifugal pump. It can occur in centrifugal pumps and other devices where a sudden pressure reduction occurs. It is most often associated with handling liquids that are close to their boiling point, when the reduction in pressure causes the boiling point of a liquid to be reduced and hence boiling can occur This will result in the production of vapour bubbles. In a centrifugal pump this is most likely to happen at the suction [inlet] of the pump where the pressure is at its lowest value. The vapour bubbles formed pass along the impeller to the discharge side of the pump replacing liqiud and reducing the output of the pump. Once the bubbles reach the discharge the greater pressure there causes the bubbles to collapse and burst. This collapse produces forces so large that small pieces of metel can be physicaly knocked out of the impeller or case leaving small holes [cavities ñ hence the name!]. The bubbles can also cause the impeller to run out of balance creating excessive viabration. Three signs that cavitation is occurring in a pump are a destinctive crackling noise due to the collapse of the bubbles; excessive vibration; reduced output. Cavitation can often be reduced or overcome by partly closing the discharge valve, which increases the internal pressure in the pump and thus helps prevent the formation of vapour bubbles. A more permanent solution is to redesign the system so that the pump will operate under conditions that less favourable to the formation of bubbles. The simplest way of achieving this is to situate the pump at a lower level so that there is a positive pressur e on the suction due to the hieght of liquid in the suction line [pressure in a liquid increases with the depth of liquid]. Effects of cavitation

1) The metallic surfaces are damaged and cavities are formed on the surfaces.2) Due to sudden collapse of vapour bubble,considerable noise and vibrations

are produced3) The efficiency of a turbine decrease due to cavitation

9. NET POSITIVE SUCTION HEAD (NPSH)

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In a pump, cavitation will first occur at the inlet of the impeller. Denoting the inlet by i, the NPSHA at this point is defined:

The Net Positive Suction Head - NPSH - can be defined as

the difference between the Suction Head, and

the Liquids Vapor Head

and can be expressed as

NPSH = hs  - hv                (3)

or, by combining (1) and (2)

NPSH = ps  / γ + vs2  / 2 g - pv  / γ                (3b)

where

NPSH = Net Positive Suction Head (m, in)

This is the standard expression for the Available NPSH at point. Cavitation will occur at the point i when the Available NPSH is less than the NPSH required to prevent cavitation (NPSHR). For simple impeller systems, NPSHR can be derived theoretically,but very often it is determined empirically.Note NPSHA and NPSHR are in absolute units and usually expressed in "m abs" not "psia".

10. ADVANTAGE & DIS ADVANTAG OF CENTRIFUGAL PUMPS

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Centrifugal pump is a kind of machinery used for pumping waterthrough the centrifugal movement

Positive displacement pumps will always be more efficientthan centrifugal pumps

. As a conclusion, we had obtained a performance curve at 3different speed of pump by avariahie characteristic. The efficiencyof pump have related to the losses mean energy during the process.This efficiency will be increase if less loss occur.

REFERENCES

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1. A Textbook of Fluid Mechanics and Hydraulic Machines by Dr.R.K.Bansal

2. https://en.wikipedia.org/wiki/Centrifugal_pump

3. http://nptel.ac.in/courses/112105171/1

4. A Textbook of Fluid Mechanics and Hydraulic Machines by Dr.R.K.rajput

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