CH.2 Hydraulic Devices / 16 MARKS HAP/17522/AE5G R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 1 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)
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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)
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.
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
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.
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.)
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.