content : 2.1 centrifugal pumps 08 marks 2.2 reciprocating ... ch.2.pdf · heads, losses and...

CH.2 Hydraulic Devices / 16 MARKS HAP/17522/AE5G

R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur.

Content : ___________________________________________________________ 2.1 Centrifugal Pumps 08 Marks Types, Construction and working of centrifugal pump. Types of casing. Need of priming. Heads, Losses and Efficiencies of Centrifugal Pump. (No Analytical Treatment.) Net positive suction head. Fault findings and remedies. Pump selection.

2.2 Reciprocating Pumps 08 Marks Construction and Working of single and Double Acting Reciprocating pump. Positive and Negative slip. Air vessels - their function and Advantage. Power and Efficiencies of Reciprocation Pump. (No Analytical Treatment.) Reasons of cavitations and separation. Comparison between Reciprocating and Centrifugal Pump.

Introduction :

Centrifugal Pump : It is a non positive displacement pump. It is a machine or a

device which is used to convert mechanical energy into pressure energy by means of

centrifugal force which are acting on the fluid is called as centrifugal pump.

Types of Centrifugal Pump :

i) Single stage

ii) Two stage

iii) Multi stage

Multi stage

a) to produce a high head (series)

b) To discharge a large quantity of liquid (parallel)



Single stage Centrifugal Pump :

Construction : ( Fig. refer any one)

It consists of casing, impeller, suction pipe, delivery pipe, delivery valve and

electric motor.

Casing : It is an air tight passage made by cast iron used for protection of inner

parts like shaft, Impeller ,bearing etc.

Impeller : It is a rotating part ,heart of centrifugal pump. It consist of backward

curved vanes. It is mounted on one end of shaft while at another side prime mover

connected.

Suction pipe with foot valve and strainer :It is a pipe whose one end is

connected to inlet of pump and other end dip into water sump. A foot valve is a

N.R.V. or one way valve is fitted at lower end of pipe and strainer is used to

separate foreign particles in water.

Delivery pipe :It is a pipe whose one end is connected to outlet of pump and other

end is deliver water at required height.

Priming cup : It is provided for priming purpose which is a placed between casing

and delivery valve.

Working : The first step in the operation of a centrifugal pump is priming so that

no air pocket is left. After pump is primed, the electric motor is started to rotate the

impeller. The rotation of impeller forces the water in radially outward direction in

delivery pipe with high velocity. This high velocity water gets converted into high



pressure when it passes through spiral casing. At the eye of the impeller due to

centrifugal action partial vacuum is created. This causes liquid from the sump to

rush through suction pipe to the eye as sump is at atmospheric pressure. This high

pressure of liquid leaving the impeller is utilized in lifting the liquid to the required

height through the delivery pipe.

Two stage Centrifugal pump :

If the centrifugal pump consists of more than one ie. two impellers then the

pump is called as two stage centrifugal pump. The impellers may be mounted on the

same or different shaft.

Multi stage Centrifugal Pump:-

If the centrifugal pump consists of more than two impellers then the pump is

called as multistage centrifugal pump. The impellers may be mounted on the same

or different shaft.



There are two important functions:-

1) To produce a high head.

2) To discharge large quantity of liquid.

Two stage Centrifugal Pump for high heads :

For developing high head numbers of impellers are mounted in series on the

same shaft which is shown in fig.

1) The water from the suction pipe enters the first impellers at inlet & it

discharged at outlet with increased pressure.

2) Then water with increased pressure from outlet of first impeller is taken by

inlet if second impeller with the help of connecting pipe.

3) At the outlet of second impeller pressure of water will be more than pressure

of water at outlet of first impeller.

4) Thus of more impeller are mounted in the same shaft, pressure at outlet will

be increased.

Let, n = number of impellers are mounted in same shaft.

Hm = head developed by each impeller.

Then total head developed = n x Hm



Discharge passing through each impeller is same.

Two stage Centrifugal Pump for high discharge :

For obtaining high discharge pump should be connected in parallel shown in

figure.

Here each pump lifts the water from same sump & discharge the water to common

pipe. Each of pumps is working against same head.

Let,

n = number of identical pump arranged in parallel.

Q = discharge from one pump.

Total discharge = n x Q

Applications of Centrifugal pump:

1. In domestic purpose for pumping water.

2. For pumping stringy solids and debris-laden liquids.

3. For pumping oil and other viscous liquids.

4. For pumping sewage and waste water.

5. Used for high volume water pumping at low to medium heads.

6. Boiler water feed pumps.

7. Used in handling sugarcane juice in sugar factories

8. Used in milk processing plants.

9. Submerged centrifugal pumps are used to handle acids in chemical plants.



Types of casing :

1) Volute Casing / Chamber :-

It is a spiral type casing in which area between impellor & casing goes on

increasing. The water leaving the impeller flows over circumference of impeller. The

velocity of water decreases with increasing the area of flow when water reaches

delivery pipe its velocity reduced & pressure increases.

A main disadvantage of this casing is there is non useful flow of water in

certain region so it reduces efficiency.

2) Vortex Casing / Chamber :-

This is the modification of volute casing. In this circular chamber is provided in

between impeller and spiral casing. Water is flowing from impeller through

circular chamber into casing. Here circular chamber reduces non useful flow of

water so increase the efficiency.



3) Casing with guide blades :-

To increase the efficiency of the pump this type of casing is used. Here

impeller is surrounded by guide blades mounted on ring called as diffuser ring.



Due to guide vanes area between impeller and casing decreases which helps

in increasing pressure of water. In this casing non useful flow of water totally

reduces.

Priming :

Priming is process of removing the air from centrifugal pump.

Necessity: It is necessary due to the fact that the pressure generated in a centrifugal pump

impeller is directly proportional to the density of the liquid that is in contact with it.

Hence if an impeller is made to rotate in presence of air, only negligible pressure

would be produced with the result that no liquid will be lifted up by the pump. Hence

it is essential to properly prime a centrifugal pump before it can be started.

Priming of Centrifugal pump done by following method : It is the operation in which the suction pipe, casing of the pump and the portion of delivery pipe up to delivery valve is completely field with the liquid which is to be raised by pump. This operation is carried out only once before starting the pump thus air within these parts is removed. Types of Heads :

1) Suction head (hs) :-

It is the vertical distance between centre line of pump & water surface in tank

(or) sump.

2) Delivery head (hd) :-

It is the vertical distance between centre line of pump & water surface in tank

to which water is delivered.

3) Static head (Hs) :-

It is the sum of suction head (hs) & delivery head (hd).

Hs = hs + hd

4) Manometric head :-

It is the head against which centrifugal pump has to work.

Mathematically expressed as;



Hm = hs + hd + hfs + hfd +

Where

hs = suction head. hfs = head loss due to friction in suction.

hd = delivery head. hfd = head loss due to friction in suction.

Efficiencies of centrifugal pump :-

We know in centrifugal pump power is transmitted from shaft of the electric

motor to the shaft of the pump & then to the impeller from impeller it is given to the

water.

Thus power is decreasing from shaft of pump to impeller and then to water.

Following are efficiencies of centrifugal pump :

1) Manometric efficiency (ῃman) :-

It is the ratio of manometric head to the head imparted by the impeller to water

is known as known as manometric efficiency.

ῃman =

2) Mechanical efficiency (ῃm) :-

It is the ratio of power available at impeller to the power at shaft of pump.

ῃm =

3) Overall efficiency (ῃ0) :-

It is the ratio of power output of pump to power input of pump.

ῃ0 =

Also, ῃ0 = ῃmano x ῃmech.



4) Hydraulic efficiency of a centrifugal pump:

It is the ratio of gross work done to the energy supplied to the impeller.

Gross work done Hydraulic efficiency = _____________________ Energy supplied to the impeller

Net Positive Suction Head (NPSH) :

It is defined as difference between suction pressure and vapour pressure measured

at pump suction nozzle when pump is running.

N.P.S.H. = hs+v2/2g - hv

Where, hs = Suction head , v2/2g = Velocity head , hv = Vapour pressure head

Use of N.P.S.H. :

1) To push suction valve from its seat.

2) To overcome frictional losses.

3) To accelerate head within liquid end.

4) To increase the efficiency of pump.

A ) Pump fails to start pumping

Reason Remedy

1 Pump may not be properly primed Re prime the pump

2 Total head against which the pump is working

may be more than the designed head.

Reduce the head or change the

pump

3 Impeller, strainer or suction line may be clogged.

Clean the pump parts.

4 Suction lift may be excessive. Check the vacuum gauge fitted on the suction side.

Reduce the suction lift.

5 Speed may be low. Check the speed with a tachometer and compare it with the design

speed.

Increase the speed.

6 The impeller might be rotating in the wrong direction. Check the direction of the impeller

with that marked on the casing.

Change the direction of

rotation.



B) Pump is not working at the required capacity.

Reason Remedy

1 There may be leakage of air into the pump through the suction line or the stuffing box.

Plug the leakage.

2 There may be excessive wear and tear. Some

of the parts may be damaged. Replace the damaged parts.

C) Pump stop working.

Reason Remedy

1 Air in suction line. This may be due to leakage or improper priming . Sometimes, air enters the

suction pipe from the inlet.

Remove the air by priming and plug the air entry.

2 Suction lift is high. Reduce the suction lift.

D) Pump has very low efficiency.

Reason Remedy

1 Speed may be high. Reduce the speed.

2 Head may be low and discharge may be more. Reduce the discharge or

change the pump

3 Pump may be operating in the wrong direction. Correct the direction of the

impeller.

4 The impeller may be touching the casing, staffing box may not be working properly, shaft may not be properly aligned or there may be

excessive wear.

Repair the affected parts.

(OR)



Possible causes if centrifugal Pump fails to start pumping:

1.Pump may not be properly primed: - due to improper priming delivery of liquid is not obtained as air takes the place of liquid. This causes difference in density of liquid to be pumped and density of air. The remedy is to fill the suction valve, suction pipe, impeller and delivery pipe up to delivery valve with liquid to be pumped.



Total head against which the pump is working may be more than the designed

head:- if working head is more than designed head pump fails to deliver the fluid. In

this case either

1. reduces the head or change pump with pump having higher total head.

2. Impeller, strainer or suction line may be clogged:- it may be due to debris or leaves in tank from where liquid is pumped. Because of this liquid is unable to

suck and hence delivery is not possible. Remedy is to clean the pump parts.

3. Suction lift may be excessive. Check the vacuum gauge fitted on the suction side. If suction height is more than it is not possible to lift the liquid form the underground tank this causes fluid may not be pumped. Remedy is to Reduce the suction lift

4. Speed may be low. Check the speed with a tachometer and compare it with the design speed. If the speed of pump is low impeller will rotate with low speed this causes lower centrifugal force to be developed and hence pump is unable to delivery to fluid. Remedy is Increase the speed.

5. The impeller might be rotating in the wrong direction. Check the direction of the impeller with that marked on the casing. Change the direction of rotation

by changing electric connections.

Criteria (or) Factors for selection of pump :-

1) Pressure :-

We know pump is device which is helpful for pressurize hydraulic oil in hydraulic

system so according to need of pressure we can select pumps as below;

TYPE OF PUMP PRESSURE RANGE (IN BAR)

External gear pump 300

Internal gear pump 350

Fixed vane pump 175

Variable vane pump 125

Screw pump 175

Axial piston type 700 (more than 500 bar )

1) Speed of pump :-

If speed of pump is increase beyond its rayed speed then problems like

cavitations is takes place this problem is directly affect on efficiency & working of

hydraulic system.



2) Flow of pressurized oil :-

Volume of oil at output is measured in litter per minute have flow of oil deciding

speed of actuators.

3) Efficiency of pump :-

Generally efficiency of pump should be better there are following efficiencies.

i) Volumetric efficiency = =

ii) Mechanical efficiency = =

iii) Overall efficiency = It is multiplication of volumetric and mechanical

efficiency.

4) Oil compatibility :-

The meaning of compatibility is nothing but acceptance. Here each pump is

compatible to specific hydraulic oil. If we use different oil then pump will not give

a good performance.

5) Cost of pump :-

There is different variety of pumps available in market according to

application we can choose it by economical aspect cost of pump & its spare

should be less.

Reciprocating pump : It is a positive displacement piston type pump, in which mechanical energy converts into pressure energy by means of reciprocating motion of piston. Single acting reciprocating pump : For getting suction and delivery stroke ie. output by means of single side of piston then it is called as Single acting reciprocating pump.



Fig. Reciprocating pump Construction:

Figure shows a single acting reciprocating pump, which consist of a piston which moves forwards and backwards in a close fitting cylinder. The movement of the piston is obtained by connecting the piston rod to crank by means of connecting rod. The crank is rotated by means of an electric motor. Suction and delivery pipe with suction valve and delivery valve are connected to the cylinder. The suction and delivery valves are one way valves or non return valves, which allow the water flow in one direction only. Suction valve allows water from suction pipe to the cylinder which delivery valve allows water from cylinder to delivery pipe only. Working:

When crank starts rotating from IDC in a clockwise direction (ie.0o to 180o), the piston moves to and fro in the cylinder. When crank is at IDC , the piston is at the extreme left position direction the piston is moving towards right in the cylinder. The movement of the piston towards right creates a partial vacuum in the cylinder. But on the surface of the liquid in the sump atmosphere pressure is acting, which is more than the pressure inside the cylinder. Thus the liquid is forced in the suction pipe from the sump. This liquid opens the suction valve and enters the cylinder.



When crank is rotating from ODC (ie.180o to 360o) in a clockwise direction the piston from its extreme right position starts moving towards left in the cylinder. The movement of piston towards left increases the pressure of the liquid inside the cylinder more than atmosphere pressure. Hence suction valve closes and delivery valve opens. The liquid is forced into the delivery pipe and is raised to required height. Double acting reciprocating Pump :

It consists of two suction pipes with suction valve and two delivery pipe with delivery

valve. Here liquid is in contact with both sides of the piston. In double acting pump

suction and delivery strokes occurs simultaneously. When the crank rotates in

clockwise direction, piston moves from inner dead center to right causing vacuum to

develop on the left side of the piston and the liquid is sucked through suction valve

s1. At the same time the liquid on right side of the piston is compressed to high

pressure causing delivery valve d2 to open and liquid flows through delivery pipe 2.

This process continues till the crank reaches to ODC. As the crank rotates to move

the piston from ODC to IDC, the liquid is sucked from suction valve s2 and delivered

through delivery valve d1. When the graph of discharge Vs crank angle is plotted, it

is observed that during both suction and delivery stroke, discharge is obtained.

During complete one revolution of crank from 0o to 360o in clockwise direction we

get two suction and two delivery strokes.

Slip in reciprocating Pump: Slip of pump means difference between the theoretical discharge and actual discharge of the pump. (i.e. Positive Slip = Qth - Qact.)



Negative slip in reciprocating Pump:

If actual discharge is more than the theoretical discharge, in which case Cd will be

more than one and the slip of pump will be negative. In that case slip of the reciprocating pump is known as negative slip.

Negative slip occurs when delivery pipe is short, suction pipe is too long and pump is running at high speed. This is so because for such pumps the inertia pressure in the suction pipe will be large in comparison to the pressure on the outside of the delivery valve, which may cause delivery valve to open before the suction stroke is completed. Some liquid is thus pushed directly into the delivery pipe even before the delivery stroke is commenced, which results in making the actual discharge more than the theoretical discharge.

Purpose of Air vessel used in the pump: An air vessel is fitted to the suction pipe and to the delivery pipe at a point close to the cylinder of reciprocating pump for the purpose of : 1. To obtain a continuous supply of liquid at uniform rate. 2. To save a considerable amount of work in overcoming the frictional resistance in the suction and delivery pipes. 3. To run the pump at high speed without separation and cavitation. 4. Large amount of power is saved due to low acceleration head.

Function of air vessel: 1. To get more uniform discharge and continuous supply through delivery pipe of reciprocating pump. 2. To save a considerable amount of work in overcoming the frictional resistance in suction and delivery pipe.

Advantages of air vessel in reciprocating pump: 1. By using air vessel we can avoid the fluctuation by maintaining constant velocity and discharge. 2.The velocity being constant, there is no acceleration of water and hence no accelerating head is present. This can permit us to install the pump at a higher level or to run the pump at higher speed without separation occurring inside the cylinder.



Cavitation and Separation in reciprocating pump :

Cavitation: It is nothing but vaporizing liquid when pressure of liquid falls below

vapour pressure. If the speed of pump is high then during suction stroke, there is

possibility of falling the pressure of liquid below vapour pressure then vapour

bubbles are formed in liquid and carried along flow. They explode when they

touches in side surface of cylinder or piston surface during this explosion very high

pressure created and which results in pitting of surface this phenomenon is called as

cavitation. Due to cavitation speed of reciprocating pump is low.

Effects of cavitation :

1.Pitting action on metal surface.

2.Cavities are formed on metallic surface.

3.Noise and vibrations are produced.

4.Efficiency of pump decreases.



Precaution / How we can reduce or minimize the effect of Cavitation :

1.The pressure of flowing liquid in any part of hydraulic system should not allow to

fall below its vapour pressure.

2.Special material or coatings such as aluminium bronze, stainless steel which are

cavitation resistance material should be used.

3.The liquid temperature should be as low as possible to keep vapour pressure down

to obtain an increased NPSH.

4.The velocity of liquid in suction pipe should be as low as practicable.

5.The sharp bends in the suction pipe should be avoided to reduce loss of head.

Separation : Separation means separating piston and water by a gap. If pump runs

at high speed there is possibility of separation during start of suction stroke and end

of delivery stroke there are limitation on speed of pump.

Difference between centrifugal pump and reciprocating pump :

Sr. No.

Centrifugal Pump Reciprocating Pump

1 The discharge is continuous and smooth.

The discharge is fluctuating and pulsating.

2 It can handle large quantity of liquid.

It can handle small quantity of liquid only.

3 It can be used for lifting highly viscous liquids.

It can be used for lifting pure water or less viscous liquids.

4 It is used for large discharge through smaller head

It is used for small discharge and high heads.

5 Cost is less. Cost is more.

6 Runs at high speed Runs at Low speed.

7 Operation is smooth and without much noise.

Operation is complicated and with much noise.

8 Efficiency is high Efficiency is low.



QUESTION BANK

1.Construction and Working of Centrifugal pump with sketch. 6/8m

2. What is priming? Why it is necessary? How it can be done?

3.Define : a) Mechanical efficiency. b) Hydraulic efficiency of a centrifugal pump.

4.What are the different faults that may occur in centrifugal pump and state their

remedies. 6m

5.State and explain possible causes if centrifugal pump fail to start pumping.

6.Draw neat sketch of arrangement of reciprocating pump. Label all the parts.

7.Explain construction and working of reciprocating pump.

8.Explain the construction and working of double acting reciprocating pump.

9.What is negative slip in reciprocating pump and why air vessel is used in the pump?

10.State the function and advantages of air vessel in reciprocating pump.

11.Differentiate between centrifugal pump and reciprocating pump.

content : 2.1 centrifugal pumps 08 marks 2.2 reciprocating ... ch.2.pdf · heads, losses and...

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