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.