Technology
Leaving Certificate
PNEUMATICS
© t4 Galway Education Centre
Contents
Introduction
4
Pneumatic systems
4
Compressor types
5
Positive displacement
5
Single piston
6
Single acting double stage
7
Rotary compressors
7
Rotary screw compressors
8
Dynamic compressors
8
Sliding vane compressors
9
Axial compressors
10
Storage of compressed air
10
Additional system components
11
Separators
11
Dryers
12
Filters
12
Flow controllers
12
Lubricators
12
Coolers
13
Compressed air uses
13
Actuators
15
Single acting
15
Double acting
16
Actuator mountings
17
Energy stored in compressed air
18
Worked examples
19
Pneumatic valves
22
Pneumatic symbols
22
Spool and Poppet valves
27
3/2 directional control valves
29
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5/2 directional control valve
30
Actuator piston speed control
31
Flow regulators
31
Bidirectional flow control
31
Quick exhaust valve
32
Speed control using a 5/2 valve
33
Throttling
34
Shuttle valve
34
Two pressure valve
35
Logic circuits
35
Logic circuit diagram (OR)
36
Logic circuit diagram (AND)
37
Electric control
38
Reed switch
39
Simple solenoid controlled circuit
40
Solenoid delay circuit
41
Circuit stages
42
Pneumatic control using PIC Logicator
44
Simple programme for Solenoid control
45
Programmable logic controllers
46
Ladder diagrams
48
Digital logic functions
50
Principles in selecting control strategies
51
Safety requirements
53
Sample questions
57
Suggested project applications
60
Glossary of compressed air Technology
62
Notepad
66
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Pneumatics
Introduction
Pneumatics is a subsection of an area known as fluid power. It uses Air which is a
colourless, odourless and tasteless gas consisting of approximately 78% Nitrogen and
20% Oxygen. The remaining 2% consists of about 1% Argon and a mixture of other trace
elements such as helium, hydrogen and neon. Pneumatic power is widely used in Industry
where it uses pressurised air, more commonly called compressed air to do work and effect
mechanical motion, which may be linear or rotary. It is used worldwide in the
construction and mining industries, transport systems, diving and dentistry to name but a
few. It is often the preferred system of use because of its availability and safety attributes.
Although compressed air may be used directly from a pump some sort of storage system
is preferable.
Pneumatic Systems
Components:
Pneumatic systems are made up of a number of different components; the main
ones are shown below. All systems have a pump or compressor driven by an electric
motor or for site work, petrol or diesel engine and a reservoir or tank to store the
compressed air. Additional components used are regulators, filters, lubricators, pressure
gauges, control switches and valves. In portable or fairly small systems most of the
components listed above are to be found mounted directly to the air reservoir as shown
below. In large Industrial systems there are a number of additional components which we
will look at later.
4
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Electric Motor
Compressor
Reservoir
Air Intake Filter
Pressure Control Switch
Reservoir Pressure Gauge
Pressure Release Valve
Safety Guard
Belt Drive System
Air Takeoff Point
1
8
9
10
5
6
7
3
4
2
Fig 1.
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Compressor types
From the diagram above we see that there are two basic types of compressor, positive
displacement and dynamic. The positive displacement type the air is trapped in a
compression chamber and the volume occupied by the trapped air is mechanically reduced
which give a rise in pressure before being discharged at the outlet.
At constant speed the airflow remains constant with slight variations in the output
pressure.
Dynamic compressors operate by giving an extra energy to continuously flowing air by
means of impellers rotating at very high speeds. The velocity of the flowing air is
converted into pressure energy by both the impellers and the volutes or diffusers. In the
Centrifugal type of dynamic compressor it is the shape of the impeller blades that give the
relationship between the air flow and the generated pressure.
Positive displacement Compressors
There are two types: Reciprocating and rotary. Reciprocating compressors work on the
same principle as you would find in a bicycle or foot pump. The commonest small scale
compressor is the single acting single stage type and the single acting double stage. These
compressors only compress in one direction only as shown in Fig 2a. and Fig 2b. Double
acting compressors have a compression stroke in both directions of piston travel. Most
industrial compressors are double acting and two stages or multi stage. Two stage means
that the outlet pressure from the first cylinder is then the inlet stage for the next cylinder
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theoretically doubling the usable outlet pressure. Fig 4. Multistage compressors increase the
outlet pressure even further. They are available in sizes from about 1hp to approximately
600 hp.
Single Piston Pump/Compressor
Compression
Discharge
Compression
Suction
Expansion
Piston and Cylinder
Head Clearance
Suction Stroke
Piston at Top dead Centre
Piston at Bottom Dead Centre
Fig 2b.
In the above figure one full compression cycle is shown. Piston A is at Top Dead Centre,
the clearance between the Cylinder Head and Piston is shown. As the Piston begins the
downward movement the Inlet Valve begins to open and Air at Atmospheric pressure
starts to enter the Cylinder. This continues until the Piston reaches Bottom Dead Centre.
The Cylinder is now full of air, the Piston
begins to move upwards in the cylinder, the
Cylinder Head
Air Inlet
inlet valve closes and the air is now being
Compressed
Air Outlet
compressed into the available space at the top
Cooling Fins
of the cylinder. Just as the Piston reaches the
top of its stroke the outlet valve opens
Crankcase
allowing the now pressurised air out to the
storage reservoir or tank. The typical
Crankshaft Drive
Pulley
appearance of a single acting single cylinder
Oil Filler
compressor is shown in Fig 3.
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Single acting double stage
Intercooler
Compressed air Outlet
Air Inlet
1st Stage
2nd Stage
Crankshaft
Fig 2b.
Double acting double stage
Inlet valve
Intercooler
Compressed Air
Outlet
Inlet valve
Crankshaft
Fig 4.
Rotary compressors
Smaller in size and range from 3 hp to 600 hp. The commonest type of Rotary compressor
is the Twin helical screw type also known as a Rotary screw operates by trapping air
between the revolving screw rotors thus reducing the volume of air along the screw and
increasing pressure at the outlet. Other less common types of Rotary Screw compressors
are mentioned on the compressor chart.
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Rotary Screw Compressor
Outlet port
The air is compressed as it is
moved along the rotor helix
Compressed air entering the
Outlet Port on the body
Air shown in blue entering the
cutaway Inlet port and rotor
A cutaway section of a Rotary Screw type compressor is shown in the above diagram. It
consists of a body with Inlet and Outlet ports and two meshing helical screw rotors. The
helical rotors turn in opposite directions Air enters the Inlet port and is trapped between
the two rotors moving it along the helix reducing the volume and increasing its pressure
until it reaches the outlet port at the other end of the compressor body.
Dynamic Compressors
Dynamic compressors are continuous rotary machines that accelerate the air as it passes
through the rotating components thus converting the air velocity into pressure. They are
divided into two types Centrifugal and Axial.
The Centrifugal type is the commonest in industry, where impellers at very high speeds
impart velocity energy to the air before being passed through diffusers to convert it to
pressure energy.
Axial compressors are made with multiple rows of blades with matching rows of
stationary vanes. The rotating blades impart the initial velocity while the stationary vanes
then act as diffusers to give the final output pressure. Normally used with a very high flow
air source.
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Sliding vane Rotary compressor
Air
outlet
Compression
Sealing
Outer casing
Bending
stresses
Compressed Air Out
Maximum Volume
at this point
Pressure drop Air intake
Rotating vane
Stationary vane
Direction
of rotation
Air Intake
Axial Compressor
Sliding vane
These pumps have a number of vanes that are free to slide into or out of slots in the pump
rotor. When the pump driver turns the rotor, centrifugal force, causes the vanes to move
outward in their slots and bear against the inner bore of the pump casing forming
compression chambers. As the rotor revolves, Air flows into the area between the vanes
when they pass the Intake or suction port. The Air is transported around the pump casing
by the sliding vanes, decreasing the available space thus compressing the Air until it
reaches the discharge outlet port. From this point the Air is transported through the piping
to the reservoir / tank ready to do useful work.
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Axial Compressors
Axial compressors consist of rotating and stationary vanes. A shaft drives a central drum,
retained by bearings, which has a number of annular vanes rows attached. These rotate
between similar numbers of stationary vane rows attached to a stationary tubular casing.
The rows alternate between the rotating vanes (rotors) and stationary vanes (stators),
shown in red in the diagram, with the rotors imparting energy into the air, and the stators
converting the increased rotational energy into static pressure through diffusion. A pair of
rotating and stationary vanes is called a stage. The cross-sectional area between rotor
drum and casing is reduced in the direction of the Air flow to maintain axial velocity as it
is compressed.
Storage of Compressed Air
After compression the air is normally stored in some sort of Tank or Reservoir. Reservoirs
are cylindrical in shape with semi-circular or domed ends to withstand the pressures
involved. They may be placed horizontally under the compressor as shown in the previous
diagram, or have a vertical configuration, or placed somewhere else some distance from
the compressor. Standard industrial tanks are available from 300 litres to 20,000 litre
capacities. The pressure at which the air is stored in the tank is dependent on the
compression ratio of the actual compressor. The compression ratio is the maximum
pressure the compressor can deliver related to atmospheric pressure and it is measured in
bar. (1 bar = 14.5 psi, 0.1N/mm2) In some parts of the world underground caverns and
abandoned mines are being used by power companies as storage facilities for compressed
air. This stored Potential energy is then mixed with a small amount of gas to fuel turbines
and produce electricity.
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Additional system components
A number of additional pieces of equipment are required to treat the compressed air to
maintain pressure and quality before it is put to use. These include Filters, coolers,
Separators, Dryers, Flow controllers, Lubricators and Traps and drains. The diagram
below shows a typical industrial Pneumatic arrangement.
Pipe distribution
Filters
Reservoir
Water drain points
Pipe curved upwards to
prevent water entering the
air supply
Dryer
Slope 1.5%
Compressor
Industrial Pneumatic components and Pipe
distribution layout.
Separators Air in
Separators
They remove contamination from the Air (dirt, water, oils, etc.) before it enters the
Compressor. They may be installed after every intercooler to remove condensed moisture.
Lubricant injected rotary compressors have a separator immediately after the compressor
to remove the injected oil before it is cooled and re-circulated for a second compression
stage.
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Dryers
There are three common types of Dryer.
Refrigerant type
This type cools the air to 35 / 40°F to remove the condensed moisture before the air is
reheated and discharged.
Deliquescent type
These use a desiccant material to absorb water vapour and then dissolve it in the liquid
formed.
Regenerative type
These are normally of a twin tower configuration. The water vapour in the air stream
collects in the thousands of small holes in the desiccant. The desiccant itself is not
changed and the moisture is removed in a regenerative process by applying hot dry air.
One tower dries the air from the compressor while the desiccant from the other is being
regenerated.
Filters
These include particle filters to remove solids, coalescing filters to remove lubricant and
moisture and absorbent filters to remove very fine particles
Flow controllers
They regulate the pressure and deliver varying volumes of air in response to the changing
demands on the system.
Traps and Drains
Mechanical and Electrical traps are used to allow for the removal of the contaminants but
not the compressed air. Mechanical types use a float type device and the electrical traps
use a timed solenoid type or a liquid level sensing device to do the same job.
Lubricators
Compressor lubricants are designed to cool, seal and lubricate moving parts. Lubricators
may also be installed on air lines close to the point of use for pneumatic tool such as
drills, grinders, Chisels, etc.
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Coolers
There are two types Intercoolers and Aftercoolers.
Intercoolers
Nearly all multistage compressors use intercoolers which are heat exchangers that remove
the heat generated by compression in the initial stage before going on to the next
compression stage. They have an affect on the overall compression efficiency.
Aftercoolers
They are installed at the final stage of compression to reduce the air temperature.
Compressed Air use
Most industrial facilities use compressed air to do a multitude of operations. These
include: Packing equipment, the movement of goods from one conveyer to another in a
factory, Pneumatic tools, refrigeration and aeration. Some examples are given in the table
below.
Compressed air applications in Industry
Industry
Compressed Air Uses
Food
Dehydration, bottling, controls and actuators, conveying, spraying coatings, cleaning, vacuum
packing
Automotive
Tool powering, stamping, control and actuators, forming, conveying
Furniture
Air piston powering, tool powering, clamping, spraying, controls and actuators
General
Manufacturing
Clamping, stamping, tool powering and cleaning, control and actuators
Textiles
Mixing liquids, clamping, conveying, automated equipment, controls and actuators, loom jet
weaving, spinning.
Lumber and Wood
Sawing, hoisting, clamping, pressure treatment, controls and actuators
Metals Fabrication
Assembly station powering, tool powering, controls and actuators, injection moulding, spraying
Chemicals
Conveying, controls and actuators
Petroleum
Process gas compressing, controls and actuators
Primary Metals
Forming, controls and actuators, hoisting
Rubber and Plastics
Tool powering, clamping, controls and actuators, forming, mould press powering, injection
moulding
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Non Industrial applications of compressed air
Non-Industrial
Compressed Uses
Mining
Pneumatic tools, hoists, pumps, controls and actuators
Agriculture
Farm equipment, materials handling, spraying of crops, dairy machines
Power Generation
Starting gas turbines, automatic control, emissions controls
Transportation
Pneumatic tools, hoists, air brake systems
Service Industries
Pneumatic tools, hoists, air brake systems, garment pressing machines, hospital
respiration systems, climate control
Wastewater Treatment
Vacuum filters, conveying
Recreation
Amusement parks - air brakes, air mechanisms
Underwater exploration - air tanks
Cinemas - projector cleaning
Ski resorts - snow making
Hotels - elevators, sewage disposal
It is now time to look at pneumatic Valves and Actuators. The physical components that
perform the various tasks in an industrial situation.
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Actuators
Single Acting actuator
Single acting actuators/cylinders use compressed air to provide a power stroke in one
direction only. The return stroke is provided by means of a spring. The spring is normally
fitted to the return or instroke of the piston rod but it may also be fitted on the outstroke
side. In the spring type the rod is forced out to perform some task and all the spring has to
do is return the piston. The end of the piston rod is not physically attached to any other
component. For example the end of the rod may move a package on to a conveyer belt or
attach a label to a box. Single acting actuators are low cost, simple to assemble devices
that provide linear movement over a wide range of applications. The bore diameter
determines the maximum force that the actuator can exert and the stroke the maximum
linear travel. Most actuators are fairly tolerant of adverse working conditions such as high
humidity, dirty or dusty environments where it is normal to use high pressure hoses to
clean down equipment. Shown below is a cut away diagram of a single acting spring
return actuator with the typical physical appearance of what you would expect to
encounter in a real life situation. The pneumatic symbol is also shown; we will look at the
pneumatic symbols in more detail later.
Seal
Return Spring
Compressed air Inlet
Symbol
Piston Rod
Outlet opens to the
Atmosphere
Single acting actuator typical
physical appearance
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Double acting Actuator
The physical appearance is very similar to the single acting actuator. These actuator use
compressed air on both the outstroke and the instroke. Therefore they are useful for
pushing and pulling operations. Speed control may be achieved by fitting flow control
valves to the actuator. They are available in cushioned and non cushioned types. The non
cushioned type is only used in applications where a slow speed is required as the end of
the piston would make metal to metal contact within the cylinder.
Symbol
Seals
Piston rod
Compressed air may be
applied to both ports
Flow control
valves
Mounting bracket
Additional components may be added to the actuators to provide sensing and feedback
information to control the operation of the pneumatic circuit. The piston has a band of
magnetic material around its circumference; the cylinder is made from a non magnetic
material. Magnetically operated reed switches are placed at either or both ends of the
cylinder and the switches are operated once with each stroke of the piston.
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In this way much of the operation may be partially or fully automated. The reed switches
may directly control the pneumatic circuit or they may operate Solenoid switch to the
same job. One such device is the magnetic reed switch shown below.
Symbol
Reed switches
Magnetic
material
Piston
The actuators are rigidly mounted or allowed to swivel as part of a larger assembly. The
mounting points are the actuator body and the end of the piston rod. There are a variety of
devices available for this purpose, a number of which are shown below:
Actuator mountings
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Swivel mountings for larger actuators.
The Energy stored in compressed air.
Energy = Pressure x Volume
Work = Change in Pressure x Volume
Power = Change in Pressure x Volume x Time
Ignoring all losses, calculate the Potential energy of compressed air at
25°C for different Pressures:
Volume: 1.0 cubic meter at 10 bar, and at 20 Bar.
Atmospheric pressure (P0 = 1) and air acting as an ideal gas (PV = RT), in kJ/kg.
Available Energy,
(A.E.) = RT [(P0/P1)-1+ln (P1/P0)]
For air R = 0.287 kJ/kg, P0 = 1 ata, P1 = 10 ata, 20 ata.
A.E. = (0.287) (273+25) [(1/10)-1+ln (10/1)] = 119.96 kJ/kg.
A.E. = (0.287) (273+25) [(1/20)-1+ln (20/1)] = 174.96 kJ/kg.
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As mentioned earlier the bore diameter determines the maximum force that the actuator
can exert in both the outstroke and instroke. Taking both sides of the piston, the piston
face outstroke (D) and piston rod instroke side (d) the forces acting on both may be easily
calculated as seen below.
D
d
Outstroke side
Instroke side
The theoretical thrust of the cylinder is calculated by multiplying the effective area of the
piston by the working pressure. On the outstroke side, this is the full cylinder bore but on
the instroke it is reduced by the cross-sectional area of the piston rod. The bore is in mm
and the pressure (P) is in bar. To get your answers in Newton per square mm (P) is
divided by 10. (1 bar = 0.1 N/mm2).
The theoretical thrust/Force F is given as:
F = pD2P
4
Example 1:
F = p(D2 – d2 )P
4
Thrust
(Outstroke)
Pull
(Instroke)
Find the theoretical thrust and pull on a 20mm diameter piston with a 6mm rod
supplied with a pressure of 5 bar.
Applying the above formula:
Thrust F = 3.1412 x 20 x 20 x 5
4 x 10
F = 6282.4 / 40
Thrust F = 157.06 Newton’s
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Pull
F = 3.1412 x (202 - 62) x 5
4 x 10
Pull
F = 3.1412 x ( 364 ) x 5
40
Pull
F = 142.92 Newton’s
Example 2:
Find the theoretical thrust and pull on a 50mm diameter piston with a 12mm rod
supplied with a pressure of 8 bar.
Applying the previous formula:
Thrust F = 3.1412 x 50 x 50 x 8
4 x 10
Thrust F = 62824
40
Thrust F = 1570.6 Newton’s
Pull
F = 3.1412 x (502 - 122) x 8
4 x 10
Pull
F = 3.1412 x (2356) x 8
4 x 10
Pull
F = 3.1412 x (2356) x 8
4 x 10
Pull
F = 59205.3376
40
Pull
20
F = 1480.13 Newton’s
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Sometimes you may be asked to calculate the bore size of the actuator required to
perform a certain task.
A certain application requires 112 kg force to perform a required operation. Compressed
air is supplied at a pressure of 6 bar. We will use an actuator with a piston stroke of
100mm. What size cylinder bore size should be used for this application?
Formulae: F = P x A
F = force required
P = pressure available
A = area of the piston/bore
Force = pD2P or
pr2P
4
p = 3.1416
r = radius of the piston/bore
D = diameter of the piston/bore
P = pressure available
The first operation is to get the pressure in bar to kg/force per mm2.
(1 bar = 0.01 kg force/mm2 ). Using the above formula:
112 kg = 6 x 0.01 x A
112 = 0.06A
112
0.06
1866.67
=
=
A
A
Now that we have the area of the piston/bore in mm2 we can workout the diameter of the
bore using either of the formulae for area given above.
A = pD2
4
1866.67
= 3.1416 x D2
4
1866.67 x 4 = 3.1416 x D2
1866.67 x 4
= D2
3.1416
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2376.75
2376.75
=
=
D2
D
48.75 mm
= D
(Bore diameter of 48.75 mm required, 50mm the closest actuator size available.)
Pneumatic Valves.
The pneumatic valve is one of the most important components in the circuit or system.
They are grouped according to their function, signal type and construction. Valves are
sub-divided into the following:
•
Directional control valves
•
Flow control valves
•
Non-return valves
•
Pressure control valves
•
Combinational valves
•
Solenoid valves
As valve types are too many and varied we will be focusing on the 3/2 Valve and the 5/2
directional control valves. So that we might understand how valves are specified and
described we have firstly to look at their symbolic representation.
Reading Pneumatic symbols
Squares represent the valve
switching positions.
The number of squares
represents the number of
switching positions.
Lines drawn at right angles
in boxes indicate shut off
positions.
Inlet and outlet ports are shown
by lines on the outside of the box
and in their initial positions.
Lines indicate flow path and
arrows the direction of flow.
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Exhaust port
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Air pressure supply
from the compressor
Number of Ports.
Number of Ports.
Number of Positions
Number of Positions
2/2 Way directional
control valve
4/2 Way directional
control valve
3/2 Way directional control
valve normally closed
5/2 Way directional
control valve
3/2 Way directional control
valve normally open
5/3 Way directional
control valve centre off.
The designation of the ports is important when interpreting the circuit symbols and valves
as fitted to a physical system. To make sure that the correct lines, connections and valves
are physically in place, there has to be a relationship between the circuit and the
components in use. Therefore all of the components used are labelled with the correct
designated symbol. A numbering system is now used to designate directional control
valves in accordance with ISO standards. Before this a lettering system was in use. Both
are shown below.
The numbers and lettering are shown on the valves in the previous symbols chart. In
addition to valve symbols we also have energy transmission symbols, control symbols and
other devices.
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Flow and pressure control symbols.
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Control symbols continued
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Logic and other symbols
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Spool and Poppet valves
Spool Valve.
2
2 Initial state
Activated state
To actuator
From actuator
1
3
Spool 1
Seals
3
2
Return spring
Open to the
atmosphere
Spool
Exhaust
Poppet Valve.
Initial state
Air inlet
Activated state
To actuator
2
2
From actuator
Seals
1
1
3
3
Return spring
Seals
Air inlet
Exhaust
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Cutaway section of a Spool valve
Cutaway section of a Poppet valve
There are two main types of valve. The spool valve which seals on the outside diameter of
the valve. Poppet valves are usually smaller valves because they cannot be balanced.
Nearly all the directional control valves are of the spool type. Spool valves are balanced
as both ends of the spool are vented to the atmosphere and the pressure acting on the
spool is equal all around its diameter. As the spool is balanced the only force required to
operate the spool is the spring force. Therefore the operating force is low.
This is not the case for the poppet valve as you have to overcome the spring force and the
supplied air pressure keeping the seal against the internal face of the valve. Therefore
operating force is much higher. That is why poppet valves are restricted to low air flows.
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3 Port 2 Way directional valve normally closed,
(Pushbutton spring return)
Physical appearance
Pneumatic symbol
Initial state
Activated state
Single acting actuator
Piston rod
extended under air
Piston rod retracted
under spring pressure
2
1
2
3
3/2 valve in
initial position
Pushbutton
pressed
1
3
Exhaust valve
open
Air in
3/2 valve pushbutton
pressed
Exhaust valve closed
Air in
We will now start to build some simple pneumatic circuits using the 3/2 valve, the 5/2
valve and later a combination of both to enable more complicated and powerful
operations.
In the above diagram we have a 3 port 2 way valve (3 ports and 2 possible positions) in
its initial position. Compressed air is connected to port1, port2 is connected to the inlet
port of the actuator and port3 is the exhaust or outlet. In its initial state port1 is closed, the
piston is retracted under the pressure of the spring and any air in the actuator is forced out
through to port2 which in turn is connected to the exhaust port3 and out to the
atmosphere. When the pushbutton is pressed and the valve is activated, port1 is now
connected to port2 and to the inlet port on the actuator forcing the piston/rod out under the
air pressure from the compressor. At this stage the exhaust port3 is closed. It will remain
in this state until the pushbutton is released when it returns to its initial position. Both
states are shown above.
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5 Port 2 Way directional valve
(Pushbutton spring return)
Physical appearance
Pneumatic symbol
Double acting actuator
Piston retracted under
air pressure
4
2
Piston extended
under air pressure
4
2
5/2 valve in initial
position
5
Exhaust
3
1
Air inlet
Pushbutton
pressed
5/2 valve in
activate position
5
3
1
Exhaust
Air inlet
The above diagram shows a 5/2 pneumatic valve (5 ports, 2 possible positions) firstly in
its initial position and then activated. Compressed air entering port 1 exits at port 2
retracting the piston rod, air exhausts through port 5 as shown in the diagram initial
position. On pushing the button the valve changes over, compressed air still enters port 1
but now it exits at port 3 and the piston rod is extended ass shown in the activated
position. On releasing the button it returns to its initial position.
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Piston speed control
Cylinders are allowed to operate at their maximum speed for many applications. This can
result in high noise and premature wear and tear on the piston and other components.
Therefore some sort of speed control is required. Piston speed may be increased or more
commonly decreased; this decrease is achieved by fitting flow control valves. The most
common types of valve are shown below.
Bidirectional Flow control
One of the commonest metered bidirectional flow control valves is the needle valve.
This valve uses a fine tapered needle to provide precise metered air flow in both
directions.
The control knob has micrometer precision graduations around the barrel so that very
accurate settings of the tapered needle in the body orifice are possible. The required
settings are maintained by a set screw in the side of the control knob.
Graduated control Knob
O-Ring seal
Locking grub screw
Tapered
needle
Simplified diagram
Airflow possible in both
directions
Cross-section of a tapered
bidirectional needle valve
Conventional flow control/ regulator
This is a unidirectional line mounted flow control regulator that may be mounted
anywhere in the line between the valve and the actuator ports. It consists of a body, a
tapered screw and a flexible disc valve shown in red.
As the screw is turned the tapered point moves further into the orifice restricting the air
flow. If the flow is in the direction of arrow 1 to arrow 2 as shown, then the air flows past
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Conventional flow
control / regulator
Adjustable screw
Lock
Rubber valve seals
Mounted directly
to the Actuator
Physical appearance
Cross-section of an actuator
flow control valve
the tapered screw and out the other side of the valve. The flow of air in this direction also
maintains a pressure on the red disc valve keeping it shut. Reverse the direction of flow;
the air now flows from arrow 2 to arrow 1, lifting the red disc valve thus allowing
unrestricted flow in the opposite direction.
Quick exhaust valve
In some applications piston speed may be increased by using a quick exhaust valve. When
the compressed air supply is flowing from port 1 to port 2 the poppet valve is kept closed
under the supplied pressure. When the control valve is operated to reverse the piston
direction the lower pressure on the side 2 of the valve allows the poppet to open rapidly
allowing the air to flow quickly through the large exhaust orifice and silencer.
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This bypasses the normal exhaust route through the main control valve and tubing and
resulting in a faster than normal piston movement. A speed improvement of 50% is
normal, but to achieve this quick exhaust valve must be fitted directly to the actuator port.
Physical appearance
Cutaway section showing poppet
valve positions
Speed control using 5/2 valve.
The speed of the actuator piston may be controlled using a 5/2 valve and a flow control
valve on each of the actuator ports between the valve and the actuator. This valve is
shown in the diagram on page 28. As shown in the diagram air is controlled entering the
actuator on the power stroke passing the tapered point from port 1 to port 2, this is known
as throttling the flexible disc shown in the valve diagram is kept closed under the applied
pressure., For small single acting actuators the supply air is throttled and exhaust
throttling is used for double acting actuators. On the return stroke the flexible valve lifts
allowing unrestricted flow in the opposite direction. If the control flow is set to low the
operation of the piston in the actuator becomes very jerky, this is known as the stick slip
effect. In many applications it is common to speed up the retraction of the piston, this is
achieved by the fitting of a quick exhaust valve as described above. The following circuit
diagrams show examples of where how these flow or throttle valves are used.
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Supply and exhaust air throttling.
The circuit diagram below shows the positioning of the flow control valves in the circuit
with different orientations and individual valve flow restrictions.
Restricted exhaust flow, 20%
from the actuator
Unrestricted flow, 100% into
the actuator
Unrestricted
flow 100%
4
2
4
5
2
Restricted flow, 20% into
actuator,
Unrestricted flow, 100% on
exhaust from the actuator.
3
5
1
3
1
Air in
Shuttle valve
Before progressing to logic functions there are two other important valves to consider.
They are the shuttle valve (shown below) and the two pressure valve.
The shuttle valve switches based on the pressures entering either of the inputs (port 1) and
exiting at port 2. If both input ports 1 start to receive compressed air, the connection with
the higher pressure takes precedence and it is output to 2, (OR function).
Air output 2
Symbol
High pressure
Air input 1
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Air input 1
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Two pressure valve.
The two pressure valve is based on compressed air entering both input ports (ports1) and
exiting via port 2. This time if both ports receive compressed air then the connection with
the lower pressure takes precedent an is output. (AND function). The physical appearance
of shuttle valve and the two pressure valve are identical, but are identified by the symbol
markings on the outer body.
1
Compressed air
inlet ports 1.
Output port 2
2
1
2
Output port 2.
Physical appearance
1
1
High pressure in
Compressed air
inlet ports 1
Logic circuits
All logic functions may be represented using truth tables. The truth table for the logic OR
function is shown below. For a logic OR function at least one input device has to be
activated in order to achieve an output. The components needed are: an actuator, two 3/2
control valves, a 5/2 control valve, a shuttle valve and a compressed air supply. The
component positioning and circuit diagram are shown below.
Truth Table
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Logic circuit diagram (OR function)
Double acting actuator
Double acting actuator
4
2
4
5/2 Control valve 5
2
3
5/2 Control valve 5
1
3
1
Shuttle valve
Shuttle valve
2
1
2
1
1
3/2 Control valve
Pushbutton 1
3/2 Control valve
2
1
3
Pushbutton 2
2
1
3/2 Control valve
2
3
Pushbutton 1
1
1
3/2 Control valve
2
Pushbutton 2
1
3
3
Pushbutton 2 pressed,
actuator rod extending
Pushbutton 1 pressed,
actuator rod extending
Double acting actuator
The two
3/2 control valves are
connected to the inlet ports 1 of
4
2
the shuttle valve. This in turn is
5/2 Control valve 5
connected to the 5/2 control
valve pilot port and then to the
Shuttle valve
2
1
actuator ports. On pressing
either of the pushbuttons 1 or 2
3
1
3/2 Control valve
2
Pushbutton 1
1
the compressed air is delivered
1
3/2 Control valve
2
Pushbutton 2
1
3
3
by the shuttle valve to the pilot
Pushbuttons released,
actuator exhausting
port of the 5/2 valve. This in
turn activates the valve
delivering air to the actuator port and extending the actuator piston and rod. Both
pushbutton circuits are shown above. The red lines are the pressurised lines. When the 3/2
valves return under spring pressure, they remove the pilot pressure from the 5/2 valve, it
also retracts under spring pressure allowing the valve to return to its initial position and
exhaust the actuator as shown in the third circuit diagram. A 4/2 control valve could be
used in place of the 5/2 valve if necessary.
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Logic circuit diagram (AND function)
Truth Table
Double acting actuator
4
Double acting actuator
2
Pilot port
4
2
5
3
Pilot port
5/2 control valve
5
1
Two pressure valve
Pushbutton 1 1
1
2
1
3/2 control valve 2
3/2 Control valve 2
3
1
Two pressure valve
2
1
3/2 control valve 2
5/2 control valve
3
Pushbutton 2 1
Pushbutton 1 1
3
Compressed air
1
3/2 Control valve 2
3
Pushbutton 2 1
3
Compressed air
Circuit in its initial state,
neither pushbutton activated
Pushbutton 1 activated,
no change in actuator
Double acting actuator
4
Double acting actuator
2
Pilot port
4
2
5
3
Pilot port
5/2 control valve
5
Two pressure valve
1
3/2 control valve 2
Pushbutton 1 1
1
2
Two pressure valve
1
1
3/2 control valve 2
3/2 Control valve 2
3
5/2 control valve
3
Pushbutton 2 1
Compressed air
Pushbutton 2 activated,
no change in actuator
3
Pushbutton 1 1
1
2
1
3/2 Control valve 2
3
Pushbutton 2 1
3
Compressed air
Both pushbuttons activated, compressed air
delivered to the actuator under the action of
the two pressure valve. Piston rod begins to
extend.
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Logic circuit diagram (AND function).
The circuit connection is similar to the OR function configuration, except that the shuttle
valve is replaced by a two pressure valve. If pushbutton 1 is pressed the signal is blocked
by the valve giving precedent to the lower pressure side. As this side of the two pressure
valve is exhausted through the 3/2 valve, nothing happens (no signal pressure to operate
the pilot port of the 5/2 valve). On pressing pushbutton 2 a similar condition exists, again
nothing happens (see diagram). Now if both pushbutton 1 and pushbutton 2 are pressed
the side of the valve getting the signal first is blocked, but this time there is signal
pressure on the other side of the valve. This signal pressure enters the pilot port operating
the 5/2 control valve and activating the actuator. The actuator piston rod now begins to
extend. When either of the 3/2 control valves are released the two pressure valve looses its
pilot pressure signal and the actuator will exhaust through the two pressure valve and the
3/2 control valve returning the circuit to its initial position.
Electrical control
Electro-pneumatics
In recent years the totally pneumatic control systems have been replaced by electrical
/electronic control systems and the sequencing of applications. Totally pneumatic systems
are still used in the more hazardous situations and where external conditions may interfere
with the proper operation of electrical / electronic circuitry. Totally pneumatic
applications are normally used in less complex systems. Electronic systems are now used
in the management and control of all aspects of industrial installations, compressor
control, pressure and flow control, and the use of reed switches and solenoids to control
valve and circuit operation. In this section the emphasis is on the use of reed switches and
the Solenoid valve (Diagrams below). In simple applications reed switches are attached to
the outside of the actuators to provide feedback, and tell the controller the position of the
piston in the actuator of when it reaches a certain position. This feedback information is
then used to possibly bring in another actuator or perform some other operation.
The reed switch is made up of two contacts mounted on pieces of spring steel within a
sealed enclosure. When the magnetic material around the piston comes within the range of
the reed switch it is operated closing the contacts under the influence of the magnet.
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Magnetic field lines
Reed switch contacts closed
Reed switch contacts open
N
S
Magnetic material
around the piston
Piston
Actuator outer casing
Section of actuator showing magnetic material and reed switch.
As the piston moves away again from the reed switch it looses its magnetic influence and
the contacts spring apart again. In hazardous environments you have the reed switch
directly operating and switching air flow from one port to the other. In normal situations
the outputs go to a controller and are connected to solenoid directional control valves.
Electrical suppression is required with solenoid coils as the collapsing magnetic field tries
to keep current flowing in the coil producing a back emf causing arcing across the reed
switch contacts. This causes interference, inaccurate operation and early damage to the
reed switch. The simplest method of preventing this back emf is to attach a diode across
the coil terminals. This does not effect the normal operation of the coil but it effectively
connects the ends of the coil together allowing current to flow around the coil at a very
low voltage until the solenoid valve has closed.
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Symbol for a Solenoid
valve with a spring return
Solenoid coil
Two physically different
Solenoid valves
Solenoid plunger
Poppet valve
Internal pilot
Cutaway of a 5/2 solenoid spool valve
Simple solenoid controlled circuit
The diagram shows the electrical
control side and the operational
Switch,
mechanical or reed.
+24V
side of the circuit firstly in its
initial state.
2
This is the simplest solenoid
1S1
1
arrangement possible as all that is
involved is a single acting actuator
a 3/2 solenoid directional valve and
1S1
3
Solenoid
0V
Operational side
Electrical control side
an air supply. If the switch is
Initial state
activated then it brings in the
solenoid coil and operates the
+24V
valve. The compressed air now
flows in port 1 through the valve
2
and out port 2 operating the
1S1
1S1
1
actuator. When the switch is
released the circuit to the solenoid
is broken and the 3/2 valve returns
0V
Solenoid switch operated
to its initial position under spring
pressure.
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3
It is important to remember that for the circuit to function properly the solenoid coil
numbering on the electrical side (1S1) in this example, must be the same as the
operational side valve numbering, again (1S1) as shown in the diagram. Sometimes it is
necessary to operate a number of solenoids together from a proximity switch. A relay is
needed to do this as the reed proximity switch cannot handle the currents required. Using
a relay the proximity switch can be used to operate a number of independent contacts on
the relay to switch much larger currents. Much more complex circuits and systems are
possible with a combination of switches and valves. The operating circuit diagram below
shows the various stages in a relay controlled 5 second delay solenoid operated 4/2
directional valve. The separate stages (1 – 4) are explained on the next page. (Switch
symbols on page 22).
Solenoid delay
circuit.
+24V
K1
SW1
K2
1Y2
K1
K2
1Y1
5
4
2
1
3
1Y1
1Y2
0V
Stage 1
+24V
K1
SW1
K2
1Y2
K1
K2
1Y1
5
4
2
1
3
1Y1
0V
1Y2
Stage
+24V
K1
SW1
K2
1Y2
K1
K2
2.4
1Y1
4
2
1
3
1Y1
0V
1Y2
Stage 3
+24V
K1
SW1
K2
1Y2
K1
K2
5
1Y1
4
2
1
3
1Y1
0V
1Y2
Stage 4
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Stage 1
This is the circuit layout in its initial position. On the electrical side you have a
pushbutton switch, a relay K1, a proximity switch SW1, a second relay K2 with a 5
second delay set and two solenoids 1Y1 and 1Y2. In this case the supply voltage is 24
volts DC. The operational side has a double acting actuator, two flow controls and a 4/2
directional control valve with two solenoids. In its initial state the electrical circuit is
inactive and there is a supply of compressed air flowing from port 1 to port 2 keeping the
actuator piston retracted.
Stage 2
In this state the pushbutton has been pressed, there is now current flow through relay K1
which in turn brings in contacts K1 and the solenoid 1Y1, the red part of the circuit in the
diagram. When 1Y1 is energised it activates the left side of the operational circuit
connecting the air supply now from port 1 to port 4 and forcing the actuator piston rod out
of the actuator. Both circuits are shown in the diagram.
Stage 3
In this part of the circuit the piston rod has now reached the end of its travel and operates
the proximity switch SW1. Current now flows through Switch SW1 and the relay K2. As
there is a 5 second delay set on this relay nothing happens until the 5 seconds have
elapsed. The operational side of the circuit has not changed either.
Stage 4
In this diagram, after the 5 second delay, the current flows through the relay K2 and this
brings in contacts K2 and operates the solenoid 1Y2. On the operational side of the circuit
1Y2 has now operated and switched over the 4/2 valve to its initial position as shown in
the diagram for stage 1. As you now know the flow control valves in the circuit are to
control the operational speed of the piston in the actuator.
This circuit used a 24 volt supply, but 6 volts or 9 volts could also be used in the control
system with the appropriate relays and solenoid coils and a 5/2 directional control valve
could be used in place of the 4/2 valve in the circuit above.
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In the above circuit there is an electrical time delay of 5 seconds used before the solenoid
valve was operated. I would also like to mention at this stage that a mechanical time delay
is also frequently used in pneumatic circuits. (Sample circuit below)
Mechanical delay
Roller activated
Double acting
actuator
Small air
reservoir
Pilot operated 5/2
control valve
3/2 valve, pushbutton
Spring return
3/2 valve roller,
spring return
Most pilot operated valves need about 2.5 bar to operate the pilot piston. The flow of air
to the pilot may be reduced by the included flow control valve giving a small delay before
the valve is activated retracting the piston. To increase this time delay a small reservoir is
fitted between the flow control and the pilot valve. As the reservoir takes time to fill up
with air to the required pressure before the pilot valve will operate, this introduces a
further delay into the circuit. This is also referred to as a dwell control. In practice an
actuator casing or a piece of coiled pressure pipe is often used as a reservoir to give the
required delay in the circuit.
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Pneumatic control using PIC Logicator software and Interface board.
Power supply cable
Output from PIC control
Programming cable from
board to Solenoid
Input to PIC control
board from Reed
Single acting
Actuator
Reed
switch
Double
acting
actuator
PIC Control
T Piece
Electronic
Compressed Air supply
Manifold
5/2 Solenoid Control valve
3/2 Roller lever
Pneumatic control
In the above Image we have the PIC control board and the PIC Logicator Software
controlling a solenoid 5/2 control valve. This arrangement is using a combination of
Pneumatic and Electronic control. In the arrangement shown we have a Laptop running
the Logicator control Software connected to the control board with a programming cable
and a power supply cable as shown. There is also a connection from the output side of the
control board to the Solenoid switch on the 5/2 control valve. The Input side of the control
board has a feedback connection from the Reed sensor switch located on the double acting
actuator as shown. In the arrangement shown there is a supply of compressed air through
the solenoid valve to the right side of the actuator forcing the piston out of the actuator.
The simple control programme then activates the solenoid from the feedback information
receive from the reed sensor switch connecting the compressed air to the left side of the
actuator forcing the piston and rod back into the actuator again. When the piston is in the
retracted state it now presses on the roller lever valve connecting the compressed air to the
single acting actuator forcing the piston out of the actuator.
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The control programme waits one second and then removes the signal from the solenoid
and the piston is again forced out of the double acting actuator, removing the air from the
single acting actuator bringing the system to its initial starting position ready to start over
again. Any number of different control arrangements are possible using a combination of
Electronic and pneumatic control.
Simple programme for Solenoid control
Logicator for PIC micro’s
The programme starts and goes
to the decision box, checks to
see if Input 7 is High. If it is low it
goes around the right hand loop
and checks again until it gets a
High. When Input 7 is High, it
makes Output 1 High. It then
waits 1 second and makes
Output 1 low. It follows to loop
around to the top of the decision
box and checks again for the
next cycle. This simple
programme switches on and off
the Solenoid controlling the flow
of compressed air to the
Actuator.
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PLC programming
Programmer connected to a Laptop,
controlled using a programming
Language known as Ladder Logic.
Standalone PLC programmer
Software circuit layout to
control a Double acting actuator
with a Solenoid valve using a
PLC programmer.
Reed
sensor
PLC
Solenoid valve
Solenoid valve
Programmable logic controllers
A PLC has many "input" terminals, through which it interprets "high" and "low" logical
states from sensors and switches. It also has many output terminals, through which it
outputs "high" and "low" signals to power lights, solenoids, contactors, small motors, and
other devices lending themselves to on/off control. In an effort to make PLCs easy to
program, their programming language was designed to resemble ladder logic diagrams.
Thus, an industrial electrician or electrical engineer accustomed to reading ladder logic
schematics would feel comfortable programming a PLC to perform the same control
functions.
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What makes a PLC special? PLC's are used to automate machinery in assembly lines.
We use the computer link feature that allows a PLC to take commands and communicate
with a computer. If something goes wrong with ladder logic, every 'rung' of the code is
multithreaded. Normally in a programming language things happen in order. The
command or line of code on top is executed before the command on the bottom until you
hit the end of a loop. This is not so in ladder logic. Everything happens at the same time.
So what is ladder logic programming really like? Ladder logic programming looks, like a
ladder. It's more like a flow chart than a program. There are two vertical lines coming
down the programming environment, one on the left and one on the right. Then, you have
rungs of conditionals on the left that lead to outputs on the right.
For example:
x0001 x0002
Y0001
|---| |-----|/|---------( )-----|
|
|
|
|
| x0001
Y002 |
|---| |--[01000 TON T012]--( )--|
|
|
|
|
|
R001 |
Sample Ladder Logic programme
|--[D0140 = 0001]--------( )--|
|
| R001
|
Y004 |
|--| |---------------------( )--|
|
|
|-{END}-------------------------|
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Ladder diagrams
Ladder diagrams are specialized schematics commonly used to document industrial
control logic systems. They are called "ladder" diagrams because they resemble a ladder,
with two vertical rails (supply power) and as many "rungs" (horizontal lines) as there are
control circuits to represent. If we wanted to draw a simple ladder diagram showing a
lamp that is controlled by a hand switch, it would look like this:
The "L1" and "L2" designations refer to the two poles of a 24V DC supply, unless
otherwise noted. L1 is the positive conductor, and L2 is the neutral conductor.
Typically in industrial relay logic circuits, but not always, the operating voltage for the
switch contacts and relay coils will be 120 volts AC. Lower voltage AC and even DC
systems are sometimes built and documented according to "ladder" diagrams:
So long as the switch contacts and relay coils are all adequately rated, it really doesn't
matter what level of voltage is chosen for the system to operate with.
Note the number "1" on the wire between the switch and the lamp. In the real world, that
wire would be labelled with that number, using heat shrink or adhesive tags, wherever it
was convenient to identify. Wires leading to the switch would be labelled "L1" and "1,"
respectively. Wires leading to the lamp would be labelled "1" and "L2," respectively.
These wire numbers make assembly and maintenance very easy. Each conductor has its
own unique wire number for the control system that it's used in.
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Wire numbers do not change at any junction or node, even if wire size, colour, or length
changes going into or out of a connection point. Of course, it is preferable to maintain
consistent wire colours, but this is not always practical. What matters is that any one,
electrically continuous point in a control circuit possesses the same wire number. Take
this circuit section, for example, with wire No.25 as a single, electrically continuous point
threading too many different devices:
In ladder diagrams, the load device (lamp, relay coil, solenoid coil, etc.) is almost always
drawn at the right-hand side of the rung. While it doesn't matter electrically where the
relay coil is located within the rung, it does matter which end of the ladder's power supply
is grounded, for reliable operation.
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Digital logic functions
We can construct simply logic functions for our hypothetical lamp circuit, using multiple
contacts, and document these circuits quite easily and understandably with additional
rungs to our original "ladder." If we use standard binary notation for the status of the
switches and lamp (0 for not actuated or de-energized; 1 for actuated or energized), a truth
table can be made to show how the logic works:
Now, the lamp will come on if either contact A or contact B is actuated, because all it
takes for the lamp to be energized is to have at least one path for current from wire L1 to
wire 1. What we have is a simple OR logic function, implemented with nothing more than
contacts and a lamp.
We can mimic the AND logic function by wiring the two contacts in series instead of
parallel:
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Now, the lamp energizes only if contact A and contact B are simultaneously actuated. A
path exists for current from wire L1 to the lamp (wire 2) if and only if both switch contacts
are closed.
Principles in selecting control strategies.
Pneumatic systems are very popular in a wide range of work applications. Many of the
existing manufacturing companies already have a combination of different systems in
their factories. All of the control system, totally electronic, electro pneumatic and totally
pneumatic have their place in modern industry; therefore why select one system in
preference to another. In many instances the working environment will dictate which
system is preferable. Some of the most important points in deciding on a control strategy
are listed below:
Decision points:
•
Technical capabilities: are they capable of accomplishing the required task.
•
Initial system cost and simplicity: which is cheapest and easiest to implement?
•
Ease of use: is the system easy to understand, time and cost required for technical
training and maintenance.
•
Production times: how fast does the system operate, how will it affect output.
•
Size/Space restrictions: what are the physical sizes of machinery and
components?
•
Availability of equipment: will the components, spare parts, be available for
many years into the future.
•
Energy consumption: which system is most cost effective?
•
Accuracy/Reliability: how precisely can products or item be placed and how
many time before accuracy is compromised.
•
Cleanliness: what environment will the equipment operate in (Clean room?)
•
Safety: which system is safest to use and therefore safest for employees.
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Pneumatic control
Reasonably cheap to implement, readily available power supply, no need for electrical
power or cabling on the factory floor. Air is a clean medium; if a leak develops it will not
do any damage. With the use of air motors there is no risk of fire as they do not heat up if
overloaded. Safer for employees as they may be used in hazardous conditions where there
may be a risk of explosions or in very wet conditions without fear of electrocution.
Electronic control systems
Electronic control systems are more flexible and more precise and faster control is
possible. Electronic systems use solid state components in electronic circuits to create
control signals in response to returned sensor feedback information at various stages in
the system. Centralized control makes it easier to monitor and operate more complex
operations. Electronic systems also offer high reliability, very compact size, and reliable
speeds across a wide range, easy to control and interface and coordinate with other
machinery in the production line.
Electro pneumatic systems
Electrical control systems use electricity as the power source for the control device. This
type of control uses relays, solenoids and motors and normally has a two position action,
for example in the control of solenoid switches, (On or OFF, activated or at rest).The
control is simple and reliable and can use low voltages.
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Compressed Air Safety requirements.
General workshop requirements
•
All pipes, hoses, and fittings must have a rating of the maximum pressure of
the compressor. Compressed air pipelines should be identified (psi) as to
maximum working pressure.
•
Air supply shutoff valves should be located (as near as possible) at the pointof-operation.
•
Air hoses should be kept free of grease and oil to reduce the possibility of
deterioration.
•
Hoses should not be left lying on the floors where they are likely to cause
personnel to trip and fall. When possible, air supply hoses should be suspended
overhead, or otherwise located to afford efficient access and protection against
damage.
•
Hose ends must be secured to prevent whipping if an accidental cut or break
occurs.
•
Pneumatic impact tools, such as riveting guns, should never be pointed at a
person.
•
Before a pneumatic tool is disconnected (unless it has quick disconnect plugs),
the air supply must be turned off at the control valve and the air in the tool
exhausted.
•
Compressed air must not be used under any circumstances to clean dirt and
dust from clothing or off a person’s skin. Workshop air used for cleaning
should be regulated to 15 psi unless equipped with diffuser nozzles to provide
lower pressure.
•
Goggles or face shields or other eye protection must be worn by personnel
using compressed air for cleaning equipment.
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•
Static electricity can be generated through the use of pneumatic tools. This
type of equipment must be grounded if it is used where fuel, flammable
vapours or explosive atmospheres are present.
Safety Requirements for Operating & Maintaining Compressed Air Machinery:
All components of compressed air systems should be inspected regularly by qualified and
trained personnel. Operators need to be aware of the following:
Air receivers/reservoirs:
•
The maximum allowable working pressures of air receivers should never be
exceeded except when being tested.
•
Air tanks and receivers should be equipped with inspection openings.
•
The intake and exhaust pipes of small tanks, similar to those used in workshops
and garages, should be made removable for interior inspections.
•
No tank or reservoir should be altered or modified by unauthorised persons.
•
Air reservoirs should be fitted with a drain cock that is located at the bottom of the
reservoir.
•
Reservoir should be drained frequently to prevent accumulation of liquid inside
the unit.
•
Air tanks should be located so that the entire outside surfaces can be easily
inspected. Air tanks should not be buried or placed where they cannot be seen
for frequent inspection.
•
Each air reservoir should be equipped with at least one pressure gauge.
•
A safety (spring loaded) release valve should be installed to prevent the
reservoir from exceeding the maximum allowable working pressure.
•
Only qualified personnel should be permitted to repair air tanks, and all work
must be done according to established safety standards.
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Air Distribution Lines:
•
Air lines should be made of high quality materials, fitted with secure
connections.
•
Only standard fittings should be used on air lines.
•
Compressed air lines should be identified as to maximum working pressures.
•
Operators should avoid bending or kinking air hoses.
•
Hoses should be checked to make sure they are properly connected to pipe
outlets before use.
•
Air hoses should not be placed where they will create tripping hazards.
•
Air lines should be inspected frequently for defects, and any defective
equipment repaired or replaced immediately.
Pressure regulation Devices:
•
Only qualified personnel should be allowed to repair or adjust pressure
regulating equipment.
•
Valves, gauges and other regulating devices should be installed on compressor
equipment in such a way that cannot be made inoperative.
•
Air tank safety valves should be set no less than 15 psi or 10 percent
(whichever is greater) above the operating pressure of the compressor but
never higher than the maximum allowable working pressure of the air
reservoir.
•
Air lines between the compressor and receiver should not usually be equipped
with stop valves.
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Compressor Operation:
•
The air intake should be from a clean, outside, fresh air source. Screens or
filters should be used to clean the air.
•
Air compressors should never be operated at speeds faster than the
manufacturer’s recommendation.
•
Equipment should not become overheated.
•
Moving parts, such as compressor flywheels, pulleys, and belts that could be
hazardous should be effectively guarded.
Compressed Air Equipment Maintenance:
•
Only authorised and trained personnel should service and maintain air
compressor equipment.
•
Exposed, non current-carrying, metal parts of compressor should be effectively
earthed.
•
High flash point lubricants should not be used on compressors because of its
high operating temperatures that could cause a fire or explosion.
•
Equipment should not be over lubricated.
•
Petrol or diesel fuel powered compressors shall not be used indoors.
•
Equipment placed outside but near buildings should have the exhausts directed
away from doors, windows and fresh air intakes.
•
Soapy water solutions should be used to clean compressor parts of carbon
deposits, but kerosene or other flammable substances should not be used.
•
The air systems should be completely purged after each cleaning.
•
During maintenance work, the switches of electrically operated compressors
should be locked open and tagged to prevent accidental operation.
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Sample questions.
Q.1
.
Describe compressed air installations.
(a) Draw a typical compressed air installation system block diagram showing the
relative position of the following components:
• compressors
• coolers
• air receiver/reservoirs
• relief valves
• dryers
• filters
• water traps
(b)
(i) State the function of the components listed in (a) above
(ii) List air compressor types in common use and select and describe any one type.
Q.2
.
Describe the application of the fundamental principles relating to:
(a) Control of Flow
• directional
• flow control, bi-directional
• flow control with by-pass
• non-return
(b) Control of movement
• speed
• stopping or preventing movement
• changing direction
Q.3.
Identify the main features and state typical applications of the
following types of cylinder:
• single-acting
• double-acting
(a) State the main reasons for the following special features in cylinders
• cushioning
• magnetic piston
(b) Explain with the aid of a simple sketch the main features and operation of a spool and
poppet valve.
(c) Identify the different methods of valve actuation
(d) State the function of a reservoir in a pneumatic circuit
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Q.4.
Electro-Pneumatic Components
State the function of the listed components:
(i) Solenoids
(ii) solenoid-pilot operated
(iii) Reed switches
(iv) Proximity sensors
(v) Relays
Q.5
.
1) What is A?
2) What is B?
A
3) What is C?
4) What will happen when the valve is
in this position (normal position)?
(i) Piston rod expands
(ii) Piston rod contracts
(iii) Nothing happens
B
5) What will happen when the valve is
in this position (switched position)?
(i) Piston rod expands
(ii) Piston rod contracts
(iii) Nothing happens
C
Q.6.
The schematic diagram in fig 1 shows:
•
•
•
•
a time-delay valve
a pressure-sequencing valve
a duel-pressure valve
a shuttle valve
Fig 1.
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Q.7.
Study the circuit diagram below and answer the questions below:
Which airlines (A, B C D E F) are power sources? __________________________
Which airlines (A, B C D E F) are signals? _________________________________
What is G? __________________________________________________________
What is H? _________________________________________________________
H
A
B
G
C
F
E
D
Q.8
.
Find the theoretical thrust and pull on a 40mm diameter piston with a 10mm
rod supplied with a pressure of 6 bar.
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Suggested project applications
Project application:
A stacking device is required to supplies blanks to a machine for stamping.
•
The piston should advance a blank when a
pushbutton is pressed.
•
On releasing the pushbutton the piston should
retract ready to advance the next blank.
Develop a suitable pneumatic control circuit for the above operation.
Single acting actuator
2
1
3
Pushbutton, 3/2 valve
Compressed air
(Suggested application solution using a 3/2 valve and a single acting actuator).
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Project application:
Opening and Closing a door of a warehouse has to be controlled using a pneumatic
cylinder under the following conditions:
•
It should be possible to either open or close the door using first push button
located out side the ware house
•
It should be possible to either open or close the door using a second push button
located inside the ware house
Develop a suitable pneumatic control circuit using pilot operated controls.
(Suggested application solution using 5/2 valves and a double acting actuator.)
Double acting actuator
Pilot 5/2 valve
with spring return
Shuttle valve
Two pressure valve
5/2 Pushbutton,
outside the warehouse
5/2 Pushbutton,
inside the warehouse.
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Glossary of Compressed Air Technology
Absolute Pressure - Total pressure measured from zero.
Absorption - The chemical process by which a hygroscopic desiccant, having a high
affinity with water, melts and becomes a liquid by absorbing the condensed moisture.
Actual Capacity - Quantity of gas actually compressed and delivered to the discharge
system at rated speed and under rated conditions. Also called Free Air Delivered (FAD).
Adsorption - The process by which a desiccant with a highly porous surface attracts and
removes the moisture from compressed air. The desiccant is capable of being regenerated.
Aftercooler - A heat exchanger used for cooling air discharged from a compressor.
Resulting condensate may be removed by a moisture separator following the aftercooler.
Atmospheric Pressure - The measured ambient pressure for a specific location and
altitude.
Capacity - The amount of air flow delivered under specific conditions, usually expressed
in cubic feet per minute (cfm).
Capacity, Actual - The actual volume flow rate of air or gas compressed and delivered
from a compressor running at its rated operating conditions of speed, pressures, and
temperatures. Actual capacity is generally expressed in actual cubic feet per minute
(acfm) at conditions prevailing at the compressor inlet.
Capacity Gauge - A gauge that measures air flow as a percentage of capacity, used in
rotary screw compressors
Compression, Adiabatic - Compression in which no heat is transferred to or from the gas
during the compression process.
Compression, Isothermal - Compression is which the temperature of the gas remains
constant.
Compression, Polytrophic – Compression in which the relationship between the pressure
and the volume is expressed by the equation PVn is a constant.
Compression Ratio - The ratio of the absolute discharge pressure to the absolute inlet
pressure.
Constant Speed Control - A system in which the compressor is run continuously and
matches air supply to air demand by varying compressor load.
Critical Temperature – The highest temperature at which well-defined liquid and vapour
states exist. Sometimes it is defined as the highest temperature at which it is possible to
liquefy a gas by pressure alone.
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Cubic Feet per Minute (cfm) - Volumetric air flow rate.
Standard cfm - Flow of free air measured and converted to a standard set of reference
conditions (14.5 psia, 68oF, and 0% relative humidity).
Cut-In/Cut-Out Pressure - Respectively, the minimum and maximum discharge
pressures at which the compressor will switch from unload to load operation (cut in) or
from load to unload (cut out).
Cycle Time - Amount of time for a compressor to complete one cycle.
Deliquescent - Melting and becoming a liquid by absorbing moisture.
Desiccant - A material having a large proportion of surface pores, capable of attracting
and removing water vapour from the air.
Dew Point - The temperature at which moisture in the air will begin to condense if the air
is cooled at constant pressure. At this point the relative humidity is 100%.
Demand - Flow of air at specific conditions required at a point or by the overall facility.
Diffuser – A stationary passage surrounding an impeller, in which velocity pressure
imparted to the flowing medium by the impeller, is converted into static pressure.
Displacement – The volume swept out by the piston or rotor(s) per unit of time, normally
expressed in cubic feet per minute.
Dynamic Type Compressors – Compressors in which air or gas is compressed by the
mechanical action of rotating impellers imparting velocity and pressure to a continuously
flowing medium. (Can be centrifugal or axial design)
Exhauster – A term sometimes applied to a compressor in which the inlet pressure is less
than atmospheric pressure.
Filters – Devices for separating and removing particulate matter, moisture or entrained
lubricant from air.
Free Air - Air at atmospheric conditions at any specified location, unaffected by the
compressor.
Full-Load - Air compressor operation at full speed with a fully open inlet and discharge
delivering maximum air flow.
Gas – One of the three basic phases of matter. While air is a gas, in pneumatics the term
gas normally is applied to gases other than air.
Gauge Pressure - The pressure determined by most instruments and gauges, usually
expressed in psig.
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Horsepower, Brake - Horsepower delivered to the output shaft of a motor or engine, or
the horsepower required at the compressor shaft to perform work.
Horsepower, Indicated – The horsepower calculated from compressor indicator
diagrams. The term applies only to displacement type compressors.
Horsepower, Theoretical or Ideal. - The horsepower required to isothermally compress
the air or gas delivered by the compressor at specified conditions.
Humidity, Specific - The weight of water vapour in an air vapour mixture per pound of
dry air.
Hysteresis – The time lag in responding to a demand for air from a pressure regulator.
Impeller – The part of the rotating element of a dynamic compressor which imparts
energy to the flowing medium by means of centrifugal force. It consists of a number of
blades which rotate with the shaft.
Inlet Pressure - The actual pressure at the inlet flange of the compressor.
Intercooling - The removal of heat from air or gas between compressor stages.
Multi-stage compressors – Compressors having two or more stages operating in series.
Piston Displacement - The volume swept by the piston; for multistage compressors, the
piston displacement of the first stage is the overall piston displacement of the entire unit.
Pneumatic Tools - Tools that operate by air pressure.
Positive displacement compressors – Compressors in which successive volumes of air
or gas are confined within a closed space and the space mechanically reduced, resulting in
compression. These may be reciprocating or rotating.
Pressure- Force per unit area, measured in pounds per square inch (psi).
Pressure, Absolute – The total pressure measured from absolute zero (i.e. from an
absolute vacuum).
Pressure Dew Point - For a given pressure, the temperature at which water will begin to
condense out of air.
Pressure, Discharge – The pressure at the discharge connection of a compressor. (In the
case of compressor packages, this should be at the discharge connection of the package)
Pressure Drop - Loss of pressure in a compressed air system or component due to
friction or restriction.
Pressure, Intake – The absolute total pressure at the inlet connection of a compressor.
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Pressure Range - Difference between minimum and maximum pressures for an air
compressor. Also called cut in-cut out or load-no load pressure range.
Required Capacity - Cubic feet per minute (cfm) of air required at the inlet to the
distribution system.
Receiver - A vessel or tank used for storage of gas under pressure. In a large compressed
air system there may be primary and secondary receivers.
Reciprocating compressor – Compressor in which the compressing element is a piston
having a reciprocating motion in a cylinder.
Rotor – The rotating element of a compressor. In a dynamic compressor, it is composed
of the impeller(s) and shaft, and may include shaft sleeves and a thrust balancing device.
Seals – Devices used to separate and minimize leakage between areas of unequal
pressure.
Sequence - The order in which compressors are brought online.
Specific Humidity - The weight of water vapour in an air-vapour mixture per pound of
dry air.
Speed – The speed of a compressor refers to the number of revolutions per minute (rpm)
of the compressor drive shaft or rotor shaft.
Stages – A series of steps in the compression of air or a gas.
Start/Stop Control - A system in which air supply is matched to demand by the starting
and stopping of the unit.
Surge - A phenomenon in centrifugal compressors where a reduced flow rate results in a
flow reversal and unstable operation.
Temperature, Absolute - The temperature of air or gas measured from absolute zero. It
is the Fahrenheit temperature plus 459.6 and is known as the Rankine temperature. In the
metric system, the absolute temperature is the Centigrade temperature plus 273 and is
known as the Kelvin temperature.
Theoretical Power - The power required to compress a gas isothermally through a
specified range of pressures.
Vacuum pumps – Compressors which operate with an intake pressure below atmospheric
pressure and which discharge to atmospheric pressure or slightly higher.
Valves – Devices with passages for directing flow into alternate paths or to prevent flow.
Volute – A stationary, spiral shaped passage which converts velocity head to pressure in a
flowing stream of air or gas.
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Notepad:
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