LECTURE ¹11
HYDRAULIC SYSTEMS. INTRODUCTION
11.1. INTRODUCTION
The controlled movement of
parts or a controlled application of force is a common requirement in the
industries. These operations are performed mainly by using electrical machines
or diesel, petrol and steam engines as a prime mover. These prime movers canprovide
various movements to the objects by using some mechanical attachments like
screw jack, lever, rack and pinions etc. However, these are not the only prime
movers. The enclosed fluids (liquids and gases) can also be used as prime
movers to provide controlled motion and force to the objects or substances. The
specially designed enclosed fluid systems can provide both linear as well as
rotary motion. The high magnitude controlled force can also be applied by using
these systems. This kind of enclosed fluid based systems using pressurized
incompressible liquids as transmission media are called as hydraulic systems.
The hydraulic system works on the principle of Pascal’s law which says that the
pressure in an enclosed fluid is uniform in all the directions. The Pascal’s law
is illustrated in figure 11.1.1. The force
given by fluid is given by the multiplication of pressure and area of cross
section. As the pressure is same in all the direction, the smallerpiston feels
a smaller force and a large piston feels a large force. Therefore, a large
force can be generated with smaller force input by using hydraulic systems.
Fig. 11.1.1. Principle of
hydraulic system
It consists of:
·
a movable piston
connected to the output shaft in an enclosed cylinder;
·
storage tank;
·
filter;
·
electric pump;
·
pressure
regulator;
·
control valve;
·
leak proof
closed loop piping.
Fig. 11.1.2. Schematic of
hydraulic system
The hydraulic systems
consists a number of parts for its proper
functioning. These include storage tank, filter, hydraulic pump,
pressure regulator, control valve, hydraulic cylinder, piston and leak proof
fluid flow pipelines. The schematic of a simple hydraulic system is shown in
figure 11.1.2.
The output shaft transfers
the motion or force however all other parts help to control the system. The
storage/fluid tank is a reservoir for the liquid used as a transmission media.
The liquid used is generally high density incompressible oil. It is filtered to
remove dust or any other unwanted particles and then pumped by the hydraulic
pump. The capacity of pump depends on the hydraulic system design. These pumps
generally deliver constant volume in each revolution of the pump shaft.
Therefore, the fluid pressure can increase indefinitely at the dead end of the
piston until the system fails. The pressure regulator is used to avoid such
circumstances which redirect the excess fluid back to the storage tank. The movement
of piston is controlled by changing liquid flow from port A and port B. The
cylinder movement is controlled by using control valve which directs the fluid
flow. The fluid pressure line is connected to the port B to raise the piston
and it is connected to port A to lower down the piston. The valve can also stop
the fluid flow in any of the port. The leak proof piping is also important due
to safety, environmental hazards and economical aspects. Some accessories such
as flow control system, travel limit control, electric motor starter and
overload protection may also be used in the hydraulic systems which are not
shown in figure 11.1.2.
Applications of
hydraulic systems
The hydraulic
systems are mainly used for precise control of larger forces. The main
applications of hydraulic system can be classified in five categories:
Industrial: Plastic processing machineries, steel making and primary metal
extraction applications, automated production lines, machine tool industries,
paper industries, loaders, crushes, textile machineries, R & D equipment
and robotic systems etc.
Mobile hydraulics: Tractors, irrigation system, earthmoving equipment, material handling
equipment, commercial vehicles, tunnel boring equipment, rail equipment,
building and construction machineries and drilling rigs etc.
Automobiles: It is used in the systems like breaks, shock absorbers, steering
system, wind shield, lift and cleaning etc.
Marine applications: It mostly covers ocean going vessels, fishing boats and navel
equipment.
Aerospace equipment: There are equipment and systems used for rudder control, landing gear,
breaks, flight control and transmission etc. which are used in airplanes,
rockets and spaceships.
11. 2. HYDRAULIC
PUMP
The combined
pumping and driving motor unit is known as hydraulic pump. The hydraulic pump
takes hydraulic fluid (mostly some oil) from the storage tank and delivers it
to the rest of the hydraulic circuit. In general, the speed of pump is constant
and the pump delivers an equal volume of oil in each revolution. The amount and
direction of fluid flow is controlled by some external mechanisms. In some
cases, the hydraulic pump itself is operated by a servo controlled motor but it
makes the system complex. The hydraulic pumps are characterized by its flow
rate capacity, power consumption, drive speed, pressure delivered at the outlet
and efficiency of the pump. The pumps are not 100% efficient. The efficiency of
a pump can be specified by two ways. One is the volumetric efficiency which is
the ratio of actual volume of fluid delivered to the maximum theoretical volume
possible. Second is power efficiency which is the ratio of output hydraulic
power to the input mechanical/electrical power. The typical efficiency of pumps
varies from 90-98%.
The hydraulic
pumps can be of two types:
·
centrifugal
pump;
·
reciprocating
pump.
Centrifugal pump
uses rotational kinetic energy to deliver the fluid. The rotational energy
typically comes from an engine or electric motor. The fluid enters the pump
impeller along or near to the rotating axis, accelerates in the propeller and flung out to the
periphery by centrifugal force as shown in figure 11.2.1. In centrifugal pump
the delivery is not constant and varies according to the outlet pressure. These
pumps are not suitable for high pressure applications and are generally used
for low-pressure and high-volume flow applications. The maximum pressure
capacity is limited to 20-30 bars and the specific speed ranges from 500 to
10000. Most of the centrifugal pumps are not self-priming and the pump casing
needs to be filled with liquid before the pump is started.
Fig. 11.2.1. Centrifugal pump
The reciprocating pump is a
positive plunger pump. It is also known as positive displacement pump or piston
pump. It is often used where relatively small quantity is to be handled and the
delivery pressure is quite large. The construction of these pumps is similar to
the four stroke engine as shown in figure 11.2.2. The crank is driven by some
external rotating motor. The piston of pump reciprocates due to crank rotation.
The piston moves down in one half of crank rotation, the inlet valve opens and luid
enters into the cylinder. In second half crank rotation the piston moves up,
the outlet valve opens and the fluid moves out from the outlet. At a time, only
one valve is opened and another is closed so there is no fluid leakage.
Depending on the area of cylinder the pump delivers constant volume of fluid in
each cycle independent to the pressure at the output port.
Fig. 11.2.2. Reciprocating or
positive displacement pump
Pump Lift
In general, the pump is
placed over the fluid storage tank as shown in figure 11.2.3. The pump creates
a negative pressure at the inlet which causes fluid to be pushed up in the
inlet pipe by atmospheric pressure. It results in the fluid lift in the pump
suction. The maximum pump lift can be determined by atmospheric pressure and is
given by pressure head as given below:
(11.2.1)
Theoretically, a pump lift
of 8 m is possible but it is always lesser due to undesirable effects such as
cavitation. The cavitation is the formation of vapor cavities in a liquid. The
cavities can be small liquid-free zones («bubbles» or «voids») formed due to
partial vaporization of fluid (liquid). These are usually generated when a
liquid is subjected to rapid changes of pressure and the pressure is relatively
low. At higher pressure, the voids implode and can generate an intense
shockwave. Therefore, the cavitation should always be avoided. The cavitation
can be reduced by maintaining lower flow velocity at the inlet and therefore
the inlet pipes have larger diameter than the outlet pipes in a pump. The pump
lift should be as small as possible to decrease the cavitation and to increase
the efficiency of the pump.
Fig. 11.2.3. Pump lift
11.3. PRESSURE
REGULATION
The pressure regulation is
the process of reduction of high source pressure to a lower working pressure
suitable for the application. It is an attempt to maintain the outlet pressure
within acceptable limits. The pressure regulation is performed by using
pressure regulator. The primary function of a pressure regulator is to match
the fluid flow with demand. At the same time, the regulator must maintain the
outlet pressure within certain acceptable limits.
The schematic of pressure
regulator and various valves placement is shown in figure 11.3.1. When the
valve V1 is closed and V2 is opened then the load moves
down and fluid returns to the tank but the pump is dead ended and it leads to a
continuous increase in pressure at pump delivery. Finally, it may lead to
permanent failure of the pump. Therefore some method is needed to keep the
delivery pressure P1 within the safe level. It can be achieved by
placing pressure regulating valve V3 as shown in figure 11.3.1.This
valve is closed in normal conditions and when the pressure exceeds a certain
limit, it opens and fluid from pump outlet returns to the tank via pressure
regulating valve V3. As the pressure falls in a limiting range, the
valve V3 closes again.
When valve V1 is
closed, the whole fluid is dumped back to the tank through the pressure
regulating valve. This leads to the substantial loss of power because the fluid
is circulating from tank to pump and then pump to tank without performing any
useful work. This may lead to increase in fluid temperature because the energy
input into fluid leads to the increase in fluid temperature. This may need to
the installation of heat exchanger in to the storage tank to extract the excess
heat. Interestingly, the motor power consumption is more in such condition
because the outlet pressure is higher than the working pressure.
Fig. 11.3.1. Schematic of
pressure regulation
Advantages and
Disadvantages of Hydraulic system
Advantages:
·
the hydraulic
system uses incompressible fluid which results in higher efficiency;
·
it delivers
consistent power output which is difficult in pneumatic or mechanical drive
systems;
·
hydraulic
systems employ high density incompressible fluid. Possibility of leakage is
less in hydraulic system as compared to that in pneumatic system. The
maintenance cost is less;
·
these systems
perform well in hot environment conditions.
Disadvantages:
·
the material of
storage tank, piping, cylinder and piston can be corroded with the hydraulic
fluid. Therefore one must be careful while selecting materials and hydraulic
fluid;
·
the structural
weight and size of the system is more which makes it unsuitable for the smaller
instruments;
·
the small
impurities in the hydraulic fluid can permanently damage the complete system,
therefore one should be careful and suitable filter must be installed;
·
the leakage of
hydraulic fluid is also a critical issue and suitable prevention method and
seals must be adopted;
·
the hydraulic
fluids, if not disposed properly, can be harmful to the environment.
