-
Mr M J Rajale
-
Power Unit
-
Introduction to Pumps:
Primary flow control device in any circuit is the
pump.
The function of a pump is to convert mechanical
energy into Hydraulic energy.
It is the heart of any hydraulic system because it
generates the force necessary to move the load.
Mechanical energy is delivered to the pump using a
prime mover such as an electric motor
-
Working-
Partial vacuum is created at the inlet due to the
mechanical rotation of pump shaft.
Vacuum permits atmospheric pressure to force
the fluid through the inlet line and into the pump.
The pump then pushes the fluid mechanically into
the fluid power actuated devices such as a motor or
a cylinder
-
Classification of Pumps
-
Non-positive displacement pumps
(Hydrodynamic pumps)-
Primarily velocity-type units that have a great deal of
clearance between rotating and stationary parts.
Characterized by a high slip that increases as the back
pressure increases, so that the outlet may be completely
closed without damage to the pump or system.
Do not develop a high pressure but move a large volume of
fluid at low pressures.
Because of large clearance space, these pumps are not self-
priming.
-
Non-positive displacement pumps(cont)
Displacement between the inlet and the outlet
is not positive.
Therefore, volume of fluid delivered by a pump
depends on the speed at which the pump is
operated and the resistance at the discharge
side.
Eg: centrifugal pump
-
Advantages :
1. Fewer moving parts.
2. Initial and maintenance cost is low.
3. Give smooth continuous flow.
4. Suitable for handling almost all types of fluids including
slurries and sledges.
5.Operation is simple and reliable.
Disadvantages
1. Not self-priming and hence they must be positioned below
the fluid level.
2. Discharge is a function of output resistance.
3. Low volumetric efficiency.
-
.
Construction of Centrifugal Pumps
-
How C.P. Works?
-
1. Centrifugal pumps are used to transport fluids by the
conversion of rotational kinetic energy to the
hydrodynamic energy of the fluid flow
2. The rotational energy typically comes from an engine or
electric motor.
3. The fluid enters the pump impeller along or near to the
rotating axis and is accelerated by the impeller, flowing
radially outward into a diffuser or volute chamber (casing),
from where it exits.
Applications:
Common uses include water, sewage, petroleum and
petrochemical pumping.
https://en.wikipedia.org/wiki/Volute_(pump)
-
Positive displacement pumps (Hydrostatic pumps).
Have very little slips, are self-priming and pump against very
high pressures, but their volumetric capacity is low.
Have a very close clearance between rotating and
stationary parts and hence are self-priming.
Eject a fixed amount of fluid into the hydraulic system per
revolution of the pump shaft.
Equipment must always be protected by relief valves to
prevent damage to the pump or system.
-
Advantages of positive displacement pumps
over non-positive displacement pumps are as
follows:
1. They can operate at very high pressures of up to 800 bar
(used for lifting oils from very deep oil wells).
2. They can achieve a high volumetric efficiency of up to 98%.
3. They are highly efficient and almost constant throughout
the designed pressure range.
4. They are a compact unit, having a high power-to-weight
ratio.
5. They can obtain a smooth and precisely controlled motion.
6. They have a great flexibility of performance. They can be
made to operate over a wide range of pressures and speeds.
-
Positive Displacement Pumps Non-positive Displacement
Pumps
The flow rate does not change with
head
The flow rate decreases with head
The flow rate is not much affected by
the viscosity of fluid
The flow rate decreases with the
viscosity
Efficiency is almost constant with
head
Efficiency increases with head at first
and then decreases
High maintenance low
High and low pressure areas
completely separated
Not separated, (connected
hydraulically)
Clean fluid Not necessary
High cost Low cost
Self priming Not Self priming
Volumetric capacity less Volumetric capacity high
-
External Gear pumps-
-
External Gear Pumps
-
Construction-Uses two external Spur gears made from Alloy steel .
Casing & shaft made from -C.I.
Assembly made with great Precision & very fine radial
& Axial clearances.
Gear pumps are fixed displacement type, which means
that the amount of fluid displaced for each revolution of
the drive shaft is theoretically constant.
The swept volume or displacement of gear pumps for
hydraulics will be between about 1 and 200 millilitres.
For lubrication, pump uses a small amount of oil from
the pressurized side of the gears, bleeds this through the
(typically) hydrodynamic bearings
-
Theoretical discharge is,
When the system pressure increases, imbalance occurs.
This imbalance increases mechanical friction and the bearing
load of the two gears
When the outlet flow is resisted, pressure in the pump
outlet chamber builds up rapidly and forces the gear diagonally
outward against the pump inlet.
Volume displacement is,
-
Advantages :
1.They are self-priming.
2.They give constant delivery for a given speed.
3.They are compact and light in weight.
4. Volumetric efficiency is high.
Disadvantages:
1. The liquid to be pumped must be clean, otherwise it will
damage pump.
2. Variable speed drives are required to change the delivery.
3. If they run dry, parts can be damaged because the fluid to be
pumped is used as lubricant.
4.Noisy in operation than either vane or piston pumps.
-
Application of External Gear Pumps :
Gear pumps are also widely used in chemical installations to
pump high viscosity fluids.
Petrochemicals: diesel oil, crude oil etc.
Chemicals: Sodium silicate, acids, mixed chemicals
Paint and ink.
Resins and adhesives.
Pulp and paper: acid, soap, lime, sludge etc.
Food: Chocolate, cacao butter, sugar, vegetable fats and oils,
animal food etc.
-
Internal Gear Pumps
-
The crescent placed in between these acts as a
seal between the suction and discharge
Applications-•All varieties of fuel oil and lube oil
•Resins and Polymers
•Alcohols and solvents
•Polyurethane foam
•Food products such as corn syrup, chocolate, and peanut
butter
•Paint, inks, and pigments
•Soaps and surfactants
•Glycol
-
Gerotor Pumps-The name gerotor is derived from "Generated Rotor"
-
Gerotor Pumps
The gerotor has one tooth less than the outer idler gear
The pockets of increasing size are suction pockets and
those of
decreasing size are discharge pockets
b is the tooth height,
Z is the number of rotor teeth,
Amax is the maximum area between male and
female gears (unmeshed – occurs at I/L) and
Amin is the minimum area between male and female gears
(meshed – occurs at O/L).
-
The close tolerance between the gears acts as a seal between
the suction and discharge ports.
-
Working-
1. During the assembly's rotation cycle, each of
these volumes changes continuously, so any
given volume first increases, and then
decreases.
2. An increase creates a vacuum. This vacuum
creates suction, and hence, this part of the
cycle is where the intake is located.
3. As a volume decreases compression occurs.
During this compression period, fluids can
be pumped, or compressed (if they are
gaseous fluids).
-
Advantages-
High Speed
Only two moving parts
Constant and even discharge regardless of pressure conditions
Operates well in either direction
Quiet operation
Disadvantages-
Medium pressure limitations
Fixed clearances
No solids allowed
One bearing runs in the product pumped
Overhung load on shaft bearing
-
Materials of Construction / Configuration Options
Externals (head, casing) - Cast iron
Internals (rotor, idler) - Steel
Bushing - Carbon graphite, bronze, and other materials as
needed
Applications-
Common gerotor pump applications include, but are not limited to:
Light fuel oils
Lube oil
Cooking oils
Hydraulic fluid
-
Lobe Pumps
-
Similar to that of external gear pump, but they generally have
a higher volumetric capacity per revolution
Solid particle size can be much larger in lobe pumps than in
other positive displacement types. Because lobes do not make
contact, and clearances are not as close as in other positive
displacement pumps.
-
Working-1. As the lobes come out of mesh, they create expanding
volume on the inlet side of the pump. Liquid flows into
the cavity and is trapped by the lobes as they rotate.
2. Liquid travels around the interior of the casing in the
pockets between the lobes and the casing , it does not
pass between the lobes.
3. Finally, the meshing of the lobes forces liquid through
the outlet port under pressure.
4. Because lobes do not make contact, and clearances are
not as close as in other positive displacement pumps,
this design handles low-viscosity liquids with
diminished performance
5. High-viscosity liquids require reduced speeds to
achieve satisfactory performance.
-
The advantages of lobe pumps are as follows: 1. Lobe pumps can handle solids, slurries, pastes and many
liquid.
2. No metal-to-metal contact.
3. Long-term dry run (with lubrication to seals).
4. Non-pulsating discharge at high speed
Disadvantages1. Require timing gears.
2. Reduced lift with thin liquids.
Rotary lobe pump applications:
Lobe pumps are frequently used in food applications because
they handle solids without damaging the product.
1. Paper coatings. 2. Soaps and surfactants.
3. Paints and dyes. 4. Rubber and adhesives.
5. Pharmaceuticals.
-
Screw Pumps-
-
Constuction-
Pumps have two or more gear-driven helical meshing screws
in a close fitting
The driving screw and driven screw are connected by means of
timing gears.
Working-
When the screws turn, the space between the threads is
divided into compartments.
As the screws rotate, the inlet side of the pump is flooded with
hydraulic fluid because of partial vacuum.
When the screws turn in normal rotation, the fluid contained
in these compartments is pushed uniformly along the axis
toward the center of the pump, where the compartments
discharge the fluid.
Fluid does not rotate but moves linearly along threads ,
Thus, there are no pulsations at a higher speed; it is a very quiet
operating
-
Advantages
1. Self-priming and more reliable.
2. Handle liquids containing gases and vapor.
3. Have long service life.
4. Wide range of flows and pressures
5. Wide range of liquids and viscosities
6. Built-in variable capacity
7. High speed capability allowing freedom of driver selection
8. Low mechanical vibration, pulsation-free flow, and quiet
operation
9. Compact design — easy to install and maintain
Disadvantages
1. Relatively high cost because of close tolerances and running
clearances
2. Performance characteristics sensitive to viscosity change
3. High pressure capability requires long pumping elements
4. Bulky and heavy.
5. Low volumetric and mechanical efficiencies.
-
Vane pump:
1.Unbalanced vane pump:
I. Unbalanced vane pump with fixed delivery.
II.Unbalanced vane pump with pressure-compensated
variable delivery.
2. Balanced vane pump.
-
I) Unbalanced vane pump with fixed delivery-
-
Parts of vane pump
-
Construction-
The main components of the pump are the cam surface
and the rotor.
The rotor contains radial slots splined to drive shaft. The
rotor rotates inside the cam ring.
Each radial slot contains a vane, which is free to slide in
or out of the slots due to centrifugal force.
The vane is designed to mate with surface of the cam
ring as the rotor turns.
The cam ring axis is offset to the drive shaft axis.
-
Working-
When the rotor rotates, the centrifugal force pushes the vanes
out against the surface of the cam ring.
The vanes divide the space between the rotor and the cam ring
into a series of small chambers.
During the first half of the rotor rotation, the volume of
these chambers increases, thereby causing a reduction of
pressure. This is the suction process, which causes the fluid to
flow through the inlet port.
During the second half of rotor rotation, the cam ring pushes
the vanes back into the slots and the trapped volume is
reduced. This positively ejects the trapped fluid through the
outlet port.
-
Working (Cont…….)
In this pump, all pump action takes place in the
chambers located on one side of the rotor and shaft,
and so the pump is of an unbalanced design.
The delivery rate of the pump depends on the
eccentricity of the rotor with respect to the cam ring.
-
II) Pressure-Compensated Variable Displacement Vane Pump (an Unbalanced Vane Pump with Pressure-Compensated Variable Delivery)-
-
Construction-Variable displacement feature can be brought into vane pumps
by varying eccentricity between the rotor and the cam ring.
Here in this pump, the stator ring is held against a spring
load.
Working-The system pressure acts directly through a hydraulic
piston on the right side. This forces the cam ring against a
spring-loaded on the left side.
If the discharge pressure is large enough, it overcomes the
compensated spring force and shifts the cam ring to the left.
This reduces the eccentricity and decreases the flow.
If the pressure continues to increase, there is no
eccentricity and pump flow becomes zero.
-
L -The width of rotor in m
Eccentricity is + ve = Q is max
Eccentricity is –ve = direction of flow get reverse
Eccentricity is zero = Q is zero
The maximum value of eccentricity produces the maximum
volumetric displacement ,
-
Advantages of vane pumps are as follows:1. Self-priming, robust and supply constant delivery at a given
speed.
2. Their vanes are self-compensating for wear and vanes can be
replaced easily.
4. They are light in weight and compact.
5. They can handle liquids containing vapours and gases.
6. Volumetric and overall efficiencies are high.
7.Discharge is less sensitive to changes in viscosity and
pressure variations.
Disadvantages of vane pumps are as follows:1.Relief valves are required to protect the pump in case of sudden
closure of delivery.
2.They are not suitable for abrasive liquids.
3.They require good seals.
4.They require good filtration systems and foreign particle can
severely damage pump.
-
2.Balanced Vane Pump with Fixed Delivery-
-
Construction-The rotor and vanes are contained within a double eccentric cam
ring and there are two inlet segments and two outlet segments
during each revolution.
Working-Double pumping action not only gives a compact design, but
also leads to another important advantage: although pressure
forces acting on the rotor in the outlet area are high, the forces
at the two outlet areas are equal and opposite, completely
canceling each other.
As a result, there are no net loads on shaft bearings.
Consequently, the life of this type of pump in many
applications has been exceptionally good.
Operating times of 24000 h or more in industrial applications
are widespread.
In more severe conditions encountered in mobile vehicles, 5000–
10000 h of trouble-free operation is frequently achieved.
-
Advantages of balanced vane pumps are as follows:
1. Eliminates the bearing side loads and therefore high
operating pressure can be used.
2.The service life is high compared to unbalanced type
due to less wear and tear.
Disadvantages of balanced vane pumps are as
follows:
1.They are fixed displacement pumps.
2.Design is more complicated.
3.Manufacturing cost is high compared to unbalanced
type.
-
Piston Pumps:Piston pumps are of the following two types:
1. Axial piston pump: These pumps are of two designs:
a) Bent-axis-type piston pump.
b) Swash-plate-type piston pump.
2. Radial piston pump.
-
1. Axial inline piston pump.
-
Components of piston pump
-
2. Bent axis piston pump.
-
Construction-
It contains a cylinder block rotating with a drive shaft.
However, the centerline of the cylinder block is set at an offset
angle relative to the centerline of the drive shaft.
The cylinder block contains a number of pistons arranged along
a circle. The piston rods are connected to the drive shaft flange by
a ball and socket joints.
Working-
The pistons are forced in and out of their bores as the distance
between the drive shaft flange and cylinder block changes.
A universal link connects the cylinder block to the drive shaft to
provide alignment and positive drive.
The volumetric displacement of the pump depends on the
offset angle θ.
No flow is produced when the cylinder block is centerline
θ can vary from 0 to a maximum of about 30 degree
-
Swash-Plate-Type Piston Pump
This type of pump can also be designed to have a variable
displacement capability.
The maximum swash plate angle is limited to 17.5° by
construction.
-
Construction-
The cylinder block and drive shaft are located on the
same centerline.
The pistons are connected to a shoe plate that bears
against an angled swash plate.
Working-
As the cylinder rotates , the pistons reciprocate because
the piston shoes follow the angled surface of the swash
plate.
The outlet and inlet ports are located in the valve plate
so that the pistons pass the inlet as they are being pulled
out and pass the outlet as they are being forced back in.
This type of pump can also be designed to have a
variable displacement capability.
The maximum swash plate angle is limited to 17.5°
by construction.
-
Radial piston pump
The stroke of each piston is caused by an eccentric
drive shaft or an external eccentric tappet (e.g., stroke
ring).
-
When filling the workspace of the pumping pistons from
"inside" (e.g., over a hollow shaft) it is called an inside
impinged (but outside braced) radial piston pump
-
Working-The general mode of operation will be explained at the
movement of one pumping piston:
The outer ring for bracing of the pumping pistons is in
eccentric position to the hollow shaft in the center. This
eccentricity determines the stroke of the pumping
piston.
The piston starts in the inner dead center (IDC) with
suction process. After a rotation angle of 180° it is finished
and the workspace of the piston is filled with the moved
medium.
The piston is now in the outer dead center (ODC). From
this point on the piston displaces the previously sucked
medium in the pressure channel of the pump.
-
If the workspace is filled from "outside" it's called
an outside impinged radial piston pump (but inside
braced)
-
Advantages:high efficiency
high pressure (up to 1,000 bar)
low noise level
very high load at lowest speed due to the
hydrostatically balanced parts possible
no axial internal forces at the drive shaft bearing
high reliability
Disadvantage:Bigger radial dimensions in comparison to
the axial piston pump.
-
Applications-Due to the hydrostatically balanced parts it is possible
to use the pump with various hydraulic fluids like
mineral oil, biodegradable oil, oil in water, water-glycol)
synthetic ester or cutting emulsion.
That implies the following main applications for a
radial piston pump:
machine tools (e.g., displace of cutting emulsion,
supply for hydraulic equipment like cylinders)
high pressure units (HPU) (e.g., for overload protection
of presses)
automotive sector (e.g., automatic transmission,
hydraulic suspension control in upper-class cars)
-
Comparison of Hydraulic Pumps
-
Pump Performance-
Volumetric efficiency :
Ratio of Actual flow rate of the pump to the Theoretical flow
rate of the pump.
For gear pumps = 80%–90%.
For vane pumps = 92%.
For piston pumps = 90%–98%.
-
Mechanical efficiency:
Ratio of the pump output power assuming no leakage to
Actual power delivered to the pump
o/p Power from pump = Pressure * flow rate
Actual power delivered to pump =actual torque* pump speed
-
Overall efficiency :
Ratio of actual power delivered by the pump to actual power
delivered to the pump.
Overall effi = mech effi * vol effi
-
Pump Performance Curve
-
Discharge pressure,
-
Pump Selection:
The main parameters affecting the selection of a particular type of
pump are as follows:
1. Maximum operating pressure- max P increase cost of
component
2. Maximum delivery- compensate leakage losses choose pump
having capacity 10% higher than required
3. Type of control- manual servo control, pressure compensated
control, constant power control and constant flow control
4. Pump drive speed-fluid delivery rate is proportional to the speed
of rotation. Faster the pump runs, the shorter its life.
5. Type of fluid- Pumps are designed to operate within a particular
range of fluid viscosity ,
-hydraulic fluid to lubricate the bearings and moving parts
-
6. Fluid Contamination-Whichever type is used, a
strainer must be fitted in the suction line.
7. Pump noise- sound generated increases with speed
and pressure
8. Size and weight of a pump- power to weight ratio high
9. Pump efficiency- pump consider best when have higher
overall efficiency
10. Cost- lower maintenance cost and total cost low
12. Maintenance and spares.