Introduction to Pneumatics

© t4 Galway Education Centre

Technology Leaving Certificate

PNEUMATICS

2 © 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


3

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


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.

1. Electric Motor

2. Compressor

3. Reservoir

4. Air Intake Filter

5. Pressure Control Switch

6. Reservoir Pressure Gauge

7. Pressure Release Valve

8. Safety Guard

9. Belt Drive System

10. Air Takeoff Point

1

2

3

4

5

6

7

8

9

10

10 Fig 1.


5

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


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.

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

inlet valve closes and the air is now being

compressed into the available space at the top

of the cylinder. Just as the Piston reaches the

top of its stroke the outlet valve opens

allowing the now pressurised air out to the

storage reservoir or tank. The typical

appearance of a single acting single cylinder

compressor is shown in Fig 3.

Single Piston Pump/Compressor

Pis

ton

an

d C

ylin

der

Hea

d C

lea

ran

ce

Piston at Top dead Centre

Ex

pan

sio

n

Piston at Bottom Dead Centre C

om

pre

ssio

n

Dis

charg

e

Su

ctio

n

Suction StrokeSuction StrokeSuction StrokeSuction Stroke Compression Compression Compression Compression

Fig 2b.

Cylinder Head

Air Inlet Compressed

Air Outlet

Cooling Fins

Crankshaft Drive

Pulley

Oil Filler

Crankcase


7

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.

Single acting double stage

Crankshaft

Fig 2b.

Intercooler Air Inlet Compressed air Outlet

1st Stage 2

nd Stage

Double acting double stage

Compressed Air

Outlet

Fig 4.

Inlet valve

Inlet valve

Crankshaft

Intercooler


Rotary Screw Compressor

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.

Air shown in blue entering the

cutaway Inlet port and rotor

The air is compressed as it is

moved along the rotor helix

Compressed air entering the

Outlet Port on the body

Outlet port


9

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.

Air Intake

Compressed Air Out

Axial Compressor

Direction

of rotation

Outer casing

Rotating vane

Stationary vane

Compression

Air

outlet

Sealing

Pressure drop Air intake

Maximum Volume

at this point

Bending

stresses

Sliding vane Rotary compressor


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.


11

Pipe distribution

Compressor

Separators

Reservoir

Dryer

Filters

Pipe curved upwards to

prevent water entering the

air supply

Water drain points

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.

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.

Industrial Pneumatic components and Pipe

distribution layout.

Air in

Slope 1.5%


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.


13

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


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

Amusement parks - air brakes, air mechanisms

Underwater exploration - air tanks

Cinemas - projector cleaning

Ski resorts - snow making

Recreation

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.


15

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.

Return Spring

Single acting actuator typical

physical appearance

Piston Rod Compressed air Inlet

Outlet opens to the

Atmosphere

Symbol Seal


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.

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.

Symbol

Compressed air may be

applied to both ports

Seals

Piston rod

Flow control

valves

Mounting bracket


17

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.

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:

Symbol

Piston Magnetic

material

Reed switches

Actuator mountings


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.

Swivel mountings for larger actuators.


19

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.

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:

Thrust F = pD2P Pull F = p(D

2 – d

2 )P

4 4

Example 1: 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

Outstroke side Instroke side

D

d

(Outstroke) (Instroke)


Pull F = 3.1412 x (202 - 6

2) 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 - 12

2) 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 F = 1480.13 Newton’s


21

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 pr

2P p = 3.1416

4 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 = A

0.06

1866.67 = 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


2376.75 = D2

75.2376 = 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.

Squares represent the valve

switching positions.

The number of squares

represents the number of

switching positions.

Lines indicate flow path and

arrows the direction of flow.

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.

Reading Pneumatic symbols

Air pressure supply

from the compressor Exhaust port


23

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.

2/2 Way directional

control valve

3/2 Way directional control

valve normally closed

3/2 Way directional control

valve normally open

4/2 Way directional

control valve

5/2 Way directional

control valve

5/3 Way directional

control valve centre off.

Number of Ports. Number of Positions

Number of Ports. Number of Positions


Flow and pressure control symbols.


25

Control symbols continued


Logic and other symbols


27

Spool and Poppet valves

Poppet Valve.

Spool Valve.

To actuator

Initial state Activated state

Exhaust

Air inlet

To actuator

From actuator

Seals

Seals

Return spring

2

1 31 3

2

Spool

Open to the

atmosphere

Initial state Activated state

Seals

Exhaust

Spool

Air inlet

From actuator

Return spring

2

1 3 1

2

3

2


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.

Cutaway section of a Spool valve

Cutaway section of a Poppet valve


29

3 Port 2 Way directional valve normally closed, (Pushbutton spring return)

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.

Air in

3/2 valve pushbutton

pressed

Piston rod

extended under air

Activated state

Exhaust valve closed

Air in

3/2 valve in

initial position

Piston rod retracted

under spring pressure

Exhaust valve

open

Single acting actuator Initial state

Pushbutton

pressed

Physical appearance Pneumatic symbol

2

1 3

2

1 3


5 Port 2 Way directional valve

(Pushbutton spring return)

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.

4 2

5

1

3

4 2

5

1

3

5/2 valve in initial

position

5/2 valve in

activate position

Air inlet Air inlet

Exhaust Exhaust

Piston retracted under

air pressure

Piston extended

under air pressure

Double acting actuator

Pushbutton

pressed

Pneumatic symbol

Physical appearance


31

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.

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

Tapered

needle

O-Ring seal

Cross-section of a tapered

bidirectional needle valve

Graduated control Knob

Simplified diagram

Airflow possible in both

directions

Locking grub screw


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.

Conventional flow

control / regulator

Physical appearance

Adjustable screw

Mounted directly

to the Actuator

Cross-section of an actuator

flow control valve

Lock

Rubber valve seals


33

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.

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.

Physical appearance Cutaway section showing poppet

valve positions


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.

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).

Unrestricted

flow 100%

4 2

5

1

3

4 2

5

1

3

Restricted exhaust flow, 20%

from the actuator

Unrestricted flow, 100% into

the actuator

Restricted flow, 20% into

actuator,

Unrestricted flow, 100% on

exhaust from the actuator.

Air in

Air input 1 Air input 1

Air output 2

Symbol

High pressure


35

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.

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.

Compressed air

inlet ports 1.

Output port 2.

1

1

2

1 1

2

Output port 2

Compressed air

inlet ports 1

High pressure in

Physical appearance

Truth Table


Logic circuit diagram (OR function)

The two 3/2 control valves are

connected to the inlet ports 1 of

the shuttle valve. This in turn is

connected to the 5/2 control

valve pilot port and then to the

actuator ports. On pressing

either of the pushbuttons 1 or 2

the compressed air is delivered

by the shuttle valve to the pilot

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.

1 1

2

2

1 3

4 2

5

1

3

Pushbutton 1Pushbutton 2

Shuttle valve

5/2 Control valve

3/2 Control valve 3/2 Control valve


2

1 3

1 1

2

2

1 3

4 2

5

1

3


Shuttle valve

5/2 Control valve



2

1 3

Pushbutton 1 pressed,

actuator rod extending

Pushbutton 2 pressed,

actuator rod extending

1 1

2

2

1 3

4 2

5

1

3


Shuttle valve

5/2 Control valve



2

1 3

Pushbuttons released,

actuator exhausting


37

Logic circuit diagram (AND function)

Truth Table

1 1

2

2

1 3

2

1 3

4 2

5

1

3


Pushbutton 1 Pushbutton 2

Two pressure valve

Compressed air

5/2 control valve

3/2 control valve3/2 Control valve

Pilot port

1 1

2

2

1 3

2

1 3

4 2

5

1

3



Two pressure valve

Compressed air

5/2 control valve


Pilot port

1 1

2

2

1 3

2

1 3

4 2

5

1

3



Two pressure valve

Compressed air

5/2 control valve


Pilot port

1 1

2

2

1 3

2

1 3

4 2

5

1

3



Two pressure valve

Compressed air

5/2 control valve


Pilot port

Circuit in its initial state,

neither pushbutton activated

Pushbutton 1 activated,

no change in actuator

Pushbutton 2 activated,

no change in actuator

Both pushbuttons activated, compressed air

delivered to the actuator under the action of

the two pressure valve. Piston rod begins to

extend.


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.


39

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.

N S

Reed switch contacts closed

Magnetic material

around the piston

Magnetic field lines

Piston

Actuator outer casing

Reed switch contacts open


Simple solenoid controlled circuit

The diagram shows the electrical

control side and the operational

side of the circuit firstly in its

initial state.

This is the simplest solenoid

arrangement possible as all that is

involved is a single acting actuator

a 3/2 solenoid directional valve and

an air supply. If the switch is

activated then it brings in the

solenoid coil and operates the

valve. The compressed air now

flows in port 1 through the valve

and out port 2 operating the

actuator. When the switch is

released the circuit to the solenoid

is broken and the 3/2 valve returns

to its initial position under spring

pressure.

2

1 3

1S1

0V

+24V

1S1

2

1 3

1S1

0V

+24V

1S1

Initial state

Solenoid switch operated

Switch,

mechanical or reed.

Solenoid

Electrical control side

Operational side

Two physically different

Solenoid valves

Symbol for a Solenoid

valve with a spring return

Solenoid coil

Solenoid plunger

Poppet valve

Internal pilot Cutaway of a 5/2 solenoid spool valve


41

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).

1Y1

+24V

0V

SW1

K1

K1

4 2

1 3

1Y1 1Y2

1Y2

K2

K2 5

1Y1

+24V

0V

SW1

K1

K1

4 2

1 3

1Y1 1Y2

1Y2

K2

K2 2.4

1Y1

+24V

0V

SW1

K1

K1

4 2

1 3

1Y1 1Y2

1Y2

K2

K2 5

1Y1

+24V

0V

SW1

K1

K1

4 2

1 3

1Y1 1Y2

1Y2

K2

K2 5

Stage 1

Stage 3

Stage

Stage 4

Solenoid delay

circuit.


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.


43

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)

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.

Small air

reservoir

Roller activated

Double acting

actuator

Pilot operated 5/2

control valve

3/2 valve roller,

spring return

3/2 valve, pushbutton

Spring return

Mechanical delay


Pneumatic control using PIC Logicator software and Interface board.

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.

Programming cable from

Power supply cable

Compressed Air supply Manifold 5/2 Solenoid Control valve

Single acting Actuator


3/2 Roller lever Pneumatic control

PIC Control

T Piece

Output from PIC control board to Solenoid

Input to PIC control board from Reed

Electronic

Reed

switch


45

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.


PLC programming

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.

Standalone PLC programmer

Programmer connected to a Laptop, controlled using a programming Language known as Ladder Logic.

Software circuit layout to control a Double acting actuator with a Solenoid valve using a PLC programmer.

PLC

Solenoid valve

Reed sensor

Solenoid valve


47

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 |

|--[D0140 = 0001]--------( )--|

| |

| R001 Y004 |

|--| |---------------------( )--|

| |

|-{END}-------------------------|

Sample Ladder Logic programme


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.


49

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.


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:


51

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.


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.


53

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 point-

of-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.


• 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.


55

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.


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.


57

Sample questions.

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.

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

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

Q.1

.

Q.2

.

Q.3.


Electro-Pneumatic Components State the function of the listed components:

(i) Solenoids

(ii) solenoid-pilot operated

(iii) Reed switches

(iv) Proximity sensors

(v) Relays

1) What is A?

2) What is B?

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

5) What will happen when the valve is

in this position (switched position)?

(i) Piston rod expands

(ii) Piston rod contracts

(iii) Nothing happens

The schematic diagram in fig 1 shows:

• a time-delay valve

• a pressure-sequencing valve

• a duel-pressure valve

• a shuttle valve

Fig 1.

Q.4.

B

C

A

Q.5

.

Q.6.


59

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? _________________________________________________________

Find the theoretical thrust and pull on a 40mm diameter piston with a 10mm

rod supplied with a pressure of 6 bar.

A

C

E

D

B

F

G

H

Q.7.

Q.8

.


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.

2

1 3

(Suggested application solution using a 3/2 valve and a single acting actuator).

Pushbutton, 3/2 valve

Single acting actuator

Compressed air


61

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.)


5/2 Pushbutton,

outside the warehouse

5/2 Pushbutton,

inside the warehouse.

Two pressure valve

Pilot 5/2 valve

with spring return

Shuttle valve


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.


63

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.


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.


65

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.


Notepad:

Documents

Introduction to Pneumatics