Pneumatic systems

Pneumatic systems require a supply of clean compressed air to motivate cylinders, tools, valve gear, instruments, delicate air controls and other equipment. Most factory and plant installations operate between 5.5 and 7 bar.

A typical compressor installation is shown in Fig. 27.23.

Compressors are sized according to the amount of free air delivered. Air flow is measured in cubic decimetres per second (dm3/s) at standard atmospheric conditions of 1013 mbar and 20°C as specified in ISO 554. The compressed air is stored in an air receiver and, for a system operating at pressures in the region of 7 bar gauge, the size of the receiver in litres should be approximately equal to 30 times the rated free air delivery of the compressor in dm3/s. Thus, a compressor rated at 50 dm3/s free air delivery requires a receiver of approximately 1500 litres capacity.

Compressed air in any normal supply mains contains contaminants which need to be either completely or partially removed depending on the ultimate use of the compressed air. Naturally, the cleaner the air has to be the greater the expense. The contaminants are:

(a) water in liquid and vapour form;

(b) oil which can exist in three forms, oil/water emulsions, minute droplets suspended in the air as an aerosol and oil vapours;

(c) atmospheric dirt particles and solid particles formed by the heat of compression.

Having considered the types of contaminant present in an air system, one can decide upon the degree of cleanliness needed for any particular process and the means required to obtain this. These include After-coolers, Receivers, Air line filters, Air dryers, Coalescing filters, Vapour adsorbers and Ultra high efficiency dirt filters. Each application must be considered on its merits. Figure 27.24 shows a typical air line installation for a factory. Further cooling may occur in the distribution mains themselves; these should be installed with a pitch in the direction of the air flow, so that both gravity and air flow will carry water to drain legs at appropriate intervals. These legs should be fitted with automatic drain valves. Note that take-off points are connected to the top of the distribution mains to prevent water entering the take-off lines.

The quality of air required for plant use will now dictate which accessories are to be fitted at each take off point. These range from a selection of filters and pressure regulators; and if lubrication is required in air actuated components, then lubricant can be metered and atomized in the air line in the form of a fine fog to coat all operating parts with a thin protective film. The equipment is lubricated automatically through its operating cycle.

Regular maintenance will ensure trouble-free production facilities.

Industrial processes include: air agitation, air bearings, air conveying of foodstuffs and powders, air

Safety valve

Intercooler

Stop valve

Pressure gauge

Air receiver

Safety valve

Stop valve

Pressure gauge

Air receiver

Pneumatic Symbol For Air Receivers

Moisture separator

Relief valve

Moisture separator

Intercooler

Relief valve

Pressure gauges

-v Air intake filter

-v Air intake filter

Automatic drain valves

Two stage double acting air compresser

Bs2917 Pneumatic Circuit Diagram

Fig. 27.24

1. Air intake filter

17. Primary filter*

2. Ar compressor

18. Precision

3. Air compressor

regulator*

water to air

19. 'Ultraire' filter

heat exchanger

A.

Pitch with flow

4. Air receiver

B.

To machine

5. Safety valve

shop

6. Isolating valve

C.

Wide pattern

7. Main line air

return bends

filter

D.

To gauging

8. Automatic air

equipment

receiver drain

E.

Dry air to

9. Drip-leg drain

process control

10. Dryer

F.

Olympian

11. 'Puraire' after-

'plug-in'

filter

Vitalizer unit

12. Filter-regulator

G.

Olympian

13. Automatic

'plug-in' lubro-

drain filter

control unit

14. Lubricator

H.

To future

15. 'Ultraire' filter*

extensions

16. Precision

controller*

*Precision air sets

Fig. 27.24

motors i.e. rotary, reciprocating and linear air cylinders, blow guns, cleaning and cooling nozzles, breathing masks and protective clothing, fluidics, food and drink processing, general machinery, instrumentation, pneumatic circuits and valves, and spray guns.

Figure 27.25. Circuit symbols can be drawn, stored in the computer database, and used repeatedly on other diagrams. Neat layout work manually often involves changes and a lot of repositioning. This is time consuming, but the computer handles alterations with speed and accuracy.

Laptop Ports Symbols
Fig. 27.25 Circuit symbols can drawn, stored in computer database, and used repeatedly on other diagrams. Neat layout work manually often involves changes and a lot of repositioning. This is time consuming, but the computer handles alterations with speed and accuracy

Pneumatic circuit design

The first requirement in circuit design is a thorough understanding of the symbols to be used. The most important and frequently used symbols are the five port and three port valves. They are also the most frequently misunderstood symbols, and therefore we start by showing the build-up of a typical five port valve. In Fig. 27.26 a double-acting cylinder is shown connected to a five port valve. The square envelope represents the valve body and there are three ports on the bottom edge and two on the top edge. A compressed air supply is connected to the centre port 1. Air exhausts to atmosphere at ports 3 and 5. Air outlets to power the cylinder at ports 2 and 4. The lines within the envelope show the passages within the valve for the current valve state. The air supply 1 is connected to outlet 4 and outlet 2 is connected to exhaust 3. Exhaust 5 is sealed. This means that the cylinder has air pressure

Piston rod in the 'minus' position

Piston rod in the 'minus' position

Hydraulic Engineering Symbols

Table 27.2 Selected symbols for fluid power systems (from BS 2917)

NOTE 1. The symbols for hydraulic and pneumatic equipment and accessories are functional and consist of one or more basic symbols and in general of one or more functional symbols. The symbols are neither to scale nor in general orientated in any particular direction. NOTE 2. In circuit diagrams, hydraulic and pneumatic units are normally shown in the unoperated position

NOTE 3. The symbols show connections, flow paths and the functions of the components, but do not include constructional details. The physical location of control elements on actual components is not illustrated.

Description

Symbol

Description

Symbol

Description

Symbol

General symbols

Basic symbols

Restriction: affected by viscosity unaffected by viscosity

Functional symbols hydraulic flow pneumatic flow or exhaust to atmosphere

Energy conversion

Pumps and compressors

Fixed capacity hydraulic pump:

with one direction of flow with two directions of flow

Motors

Fixed capacity hydraulic motor:

with one direction of flow

Oscillating motor: hydraulic

Cylinders

Air-oil actuator (transforms pneumatic pressure into a substantially equal hydraulic pressure or vice versa)

Directional control valves v a

Flow paths: one flow path two closed ports two flow paths two flow paths and one closed port two flow paths with cross connection one flow path in a by-pass position, two closed ports m

Directional control valve 2/2: with manual control controlled by pressure against a return spring

Single acting:

returned by an unspecified force

Double acting: with single piston rod

Cylinder with cushion: Single fixed

Detailed

Simplified

Direction control valve 5/2:

controlled by pressure in both directions

NOTE. In the above designations the first figure indicates the number of ports (excluding pilot ports) and the second figure the number of distinct positions.

Non-return valves, shuttle valve, rapid exhaust valve

Non-return valve:

free (opens if the inlet pressure is higher than the outlet pressure)

spring loaded (opens if the inlet pressure is greater than the outlet pressure and the spring pressure)

pilot controlled (opens if the inlet pressure is higher than the outlet pressure but by pilot control it is possible to prevent:

closing of the valve opening of the valve)

with resistriction (allows free flow in one direction but restricted flow in the other)

Shuttle valve (the inlet port connected to the higher pressure is automatically connected to the outlet port while the other inlet port is closed)

Pressure control valves

Pressure control valve:

one throttling orifice normally closed one throttling orifice normally open two throttling orifices, normally closed

Sequence valve (when the inlet pressure overcomes the spring, the valve opens, permitting flow from the outlet port)

rd wv

Table 27.2 (continued)

Discription

Symbol

Description

Symbol

Description

Symbol

Flow control valves

Throttle valve: simplified symbol

Example: braking valve

Flow control valve (variations in inlet pressure do not affect the rate of flow):

with fixed output

♦Simplified

Flow dividing valve (divided into a fixed ratio substantially independent of pressure variations)

Energy transmission and conditioning

Sources of energy

Pressure source

Electric motor

Heat engine

Flow line and connections

Flow line:

working line, return line and feed line pilot control line drain or bleed line flexible pipe

Pipeline junction

Crossed pipelines (not connected)

Air bleed

Power take-off plugged with take-off line connected, with mechanically opened non-return valves uncoupled, with open end uncoupled, closed by free non-return valve

Rotary connection one way three way

Reservoirs

Reservoir open to atmosphere:

with inlet pipe above fluid level with inlet pipe below fluid level with a header line

Pressurized reservoir

Accumulators

The fluid is maintained under pressure by a spring, weight or compressed gas

Filters, water traps, lubricators and miscellaneous apparatus

Filter or strainer

Heat exchangers

Temperature controller (arrows indicate that heat may be either introduced or dissipated)

Cooler (arrows indicate the extraction of heat)

with representation of the flow lines of the coolant

Heater (arrows indicate the introduction of heat)

Control mechanisms

Mechanical components

Rotating shaft: in one direction in either direction

Detent (device for maintaining a given position)

Locking device (*symbol for unlocking control)

Over-centre device (prevents stopping in a dead centre position)

Pivoting devices: simple with traversing lever with fixed fulcrum

Control methods

Muscular control: general symbol by push-button by lever by pedal

Mechanical control: by plunger or tracer by spring by roller by roller, operating in one direction only

Electrical control: by solenoid (one winding)

by electric motor

Control by application or release of pressure

Direct acting control:

by application of pressure by release of pressure

Combined control:

by solenoid and pilot directional valve (pilot directional valve is actuated by the solenoid)

Measuring instruments

Pressure measurement:

pressure gauge

Other apparatus

Pressure electric switch

HI AC

i7F my

°wv pushing the piston to the instroked or 'minus' position. The other side of the piston is connected to exhaust.

To make the cylinder move to the outstroked or 'plus' position the valve has to be operated to change to its new state. This is shown in Fig. 27.27. Note that the envelope and port connections are exactly the same and it is only the connection paths inside the valve that have changed.

Piston rod in the 'plus' position

Fig. 27.27

The full symbol for a 5/2 valve (five ports, two positions) are these two diagrams drawn alongside each other. Only one half will have the ports connected.

Which half, will depend on whether the cylinder is to be drawn in the instroked or outstroked state. The method by which the valve is operated, i.e. push button, lever, foot pedal, etc. is shown against the diagram of the state that it produces.

Figure 27.28 shows a 5/2 push button operated valve with spring return. It is operating a double acting cylinder. In addition a pair of one way flow regulators are included to control the speed of piston rod movement. The symbol for this type of flow regulator consists of a restrictor, an 'arrow' which indicates it is adjustable and a non return valve in parallel, to cause restriction in one direction only. The conventional way to control the speed of a cylinder is to restrict the exhausting air. This allows full power to be developed on the driving side of the piston which can then work

Back Pressure Regulator Drawing

against the back pressure and any load presented to the piston rod.

Study Fig. 27.29 and imagine that when the push button is pressed the complete symbol moves sideways to the left, but leaves the pipe connections and port numbers behind so that they line up with the other half of the diagram. In this position the cylinder piston rod will move out to the 'plus' position. Imagine the spring pushing the symbol back again when the button is released. The numbers at the valve ends signify which output will be pressurized when the valve is operated at that end. If the button is pushed at end 12 then port 1 will be connected to port 2.

Engineering Symbol Spring

Fig. 27.29

If the button is released, the spring at end 14 becomes dominant and port 1 will be connected to port 4.

A three port valve symbol works in a similar way. Two diagrams of the valve are drawn side by side. Figure 27.29 shows the full symbol for a 3/2 valve controlling a single acting cylinder. Port 1 is the normal inlet, port 2 the outlet and port 3 the exhaust. The valve end numbers 12 and 10 indicate that port 1 will be connected either to 2 or to 0 (nothing). Since there is only one pipe supplying a single-acting cylinder, speed control of the 'plus' motion has to be obtained by restricting the air into the cylinder. Speed of the 'minus' motion is effected conventionally by restricting the exhausting air. To provide independent adjustment two one-way flow regulators are used and these are connected in the line back-to-back.

Logic functions

Designers of pneumatic circuits are not usually consciously thinking in pure logic terms, but more likely designing intuitively from experience and knowledge of the result that is to be achieved. Any circuit can be analysed however, to show that it is made up of a combination of logic functions. The four

most commonly used are illustrated in Figs. 27.30 to 27.33.

The AND function. The solenoid valve A (AND) the plunger operated valve B must both be operated before an output is given at port 2 of valve B. Fig. 27.30.

Fig. 27.30

The OR function. For this a shuttle valve is required so that either of two push-button valves A (OR) B can provide a signal that is directed to the same destination. The shuttle valve contains a sealing element that is blown by the incoming signal to block off the path back through the other valve's exhaust port. Fig. 27.31.

Valve Pneumatic Block Diagram

If a signal is given to port 10 the valve will re-set and the output exhausted. If the signal is removed the new OFF state is REMEMBERED. Fig. 27.33.

12

CO 1

—1

2

1

Fig. 27.31

Fig. 27.33

The TIME DELAY. By using a flow regulator and a 3/2 pilot operated pressure switch, a signal can be slowed down to provide a time delay. Figure 27.34 shows that when a signal is fed through the flow regulator, it will slowly build up pressure in an air reservoir (R) and on the signal port 12 of the pressure switch. This will continue until the pressure is high enough to operate the pressure switch. Then, a strong unrestricted signal will be sent to operate a control valve or other device. The delay can be adjusted by changing the setting on the flow regulator. A reservoir, of approximately 100 cc in volume, would allow a delay range of between 2 and 30 seconds. Without the reservoir, the range will be reduced to approximately 3 seconds maximum. Note that the pressure switch is like a pilot operated 3/2 valve, but uses air pressure as a return spring. The pilot signal on port 12 overcomes this, as it is working on a larger area piston.

The NOT function. This is simply a normally open valve. When it is operated by a pilot signal on port 12 it will NOT give an output. The outlet will be given when the valve re-sets to its normal state by removing the signal. Fig. 27.32.

Fig. 27.32

The MEMORY function. When a double pressure operated three port valve is given a signal at port 12, an output is obtained at port 2. If the signal is now removed the output will remain, it has REMEMBERED its ON state even when the signal that caused it has gone.

Fig. 27.34

A semi-automatic circuit is shown in Fig. 27.35. When the push button is operated and released, the 3/2 valve will send a signal to operate the 5/2 double pilot valve. This will cause the cylinder to move to the 'plus' position. A cam on the piston rod will operate the roller plunger valve and this will give a signal to re-set the 5/2 valve. The piston rod will then automatically move to the 'minus' position and wait until a further operation of the push button is given.

Bs2917 Pneumatic Circuit Diagram

Fig. 27.35

Sequential circuits

In an automatic system where two or more movements are to occur in a specific order, a sequence is formed. A typical example is a special purpose automatic machine. This may be carrying out a manufacturing, or packaging operation where air cylinders are used to power the movements in a continuously repeating sequence.

Each movement in a sequence can be produced by a pneumatic cylinder. This will either be single acting, or double acting and the choice depends on whether there is any return resistance or load requiring a powered return. Single acting cylinders are controlled by a 3/2 double pilot operated valve and double acting cylinders are controlled by a 5/2 double pilot operated valve.

For each cylinder used, a circuitry building block can be established. See Fig. 27.36. This illustrates a double acting cylinder building block for the cylinder labelled 'A'. Two command signals are required, one to move it 'plus' (a+), the other to move it 'minus' (a-). To prove that the movements have been completed,

Cam Operated Valve Symbol

two feed-back signals are required. These are provided by the two roller operated 3/2 valves. One proving the 'plus' movement (a1), the other proving the 'minus' movement (a0)

Consider a two cylinder system where the cylinders are labelled A and B. The sequence required after selecting the RUN control is A + B + A - B -, it will then repeat continuously until the operator selects the END control. The circuit is constructed from two building blocks. See Fig. 27.37. Note that flow regulators are included in the power lines to each end of the cylinders.

These provide adjustable speed control for each movement. To RUN and END the repeating cycle a 3/2 manually operated valve is included.

The two building blocks form a complete circuit by having their command and feedback lines connected together. The method of interconnection is achieved by application of this simple rule:

'The proof of position signal resulting from the completion of each movement is connected to initiate the next movement.'

The circuit can be traced as follows:

Start with the output given from the RUN/END valve when it is switched to RUN.

The a+ command is given.

Cylinder A moves+.

The a1 proof of position signal results.

This becomes the b+ command.

Cylinder B moves+.

The b1 proof of position signal results.

This becomes the a- command.

Cylinder A moves-.

The a0 proof of position signal results.

This becomes the b- command.

Cylinder B moves-.

The b0 proof of position signal results.

This becomes the supply to the RUN/END valve.

Pure Pneumatik

Sequence Run/End A+ B+ AB-Repeat

Fig. 27.37

If the RUN/END valve is still switched to RUN a repeat cycle will be started.

This simple daisy chain method of interconnection will work for any number of cylinders, provided the sequence allows their return movements to occur in the same order as their first movements. For this to be true, the first movement of a cylinder need not be plus nor is it necessary for the first half of the sequence to be in alphabetical order, e.g. the sequence B + A - D + C - B - A + D - C + conforms to these rules and can be solved with this simple daisy chain method.

If the cylinders do not return in exactly the same order as their first movements complications will arise. Take for example, the sequence A + B + B - A - and repeat. If we try to interconnect the equipment for this sequence in the same way as before, there will be two states where the 5/2 valves will have both a 'plus' and

Sequence Run/End A+ B+ AB-Repeat

'minus' command existing at the same time, therefore preventing operation. This condition is commonly known as opposed signals and can be cured in a variety of ways. For the most reliable and economical method we suggest the use of the Cascade system. See Fig. 27.38.

The cascade technique is to switch on and off the supply air to the critical trip valves in groups. The need for this will occur when a trip valve's mechanism is still held down, but the output signal has been used and requires removing. By switching off the group air that is supplying the valve, the output is also removed and achieves the desired result. After the valve's mechanism is naturally released in the sequence, the group supply is switched on again in time for its next operation. To determine the number of cascade groups for any sequence, the sequence must be split into groups starting at the beginning, so that no letter is contained

Pneumatik Diagram

more than once in any group. The group numbers are given roman numerals to avoid confusion with other numbering systems that may exist on larger systems. The placing of the RUN/END valve should be in the line that selects group I. This determines that the first task of group I is to signal the first movement of the sequence. In addition, when the circuit is at rest, inadvertent operation of an uncovered trip valve will not risk an unwanted operation of a cylinder.

By studying Fig. 27.38 it can be seen that the sequence splits into two groups. These groups are supplied from a single, double pressure operated 5/2 valve, so that only one group can exist at any time. This is known as the cascade valve.

It can also be seen that neither of the 5/2 valves controlling the cylinders can have the + and - command lines as opposed signals, since their source is from different groups.

The circuit can be traced as follows: To start, set RUN/END valve to RUN. This generates a command to select group I. Group I gives a command a+. Cylinder A moves+.

Valve a1 is operated and generates a command b+. Cylinder B moves+.

Valve b1 is operated and generates a command to select group II.

Group II gives a command b - (because group I has been switched off there is no opposing signal from a1).

Cylinder B moves -.

Valve b0 is operated and generates a command a -(no opposed signal). Cylinder A moves -.

Valve a0 is operated and generates a command to start the sequence again.

If at any time the RUN/END valve is switched to END, the current cycle will be completed, but the final signal will be blocked and no further operation will occur.

The rules for interconnection are as follows:

1 The first function in each group is signalled directly by that group supply.

2 The last trip valve to become operated in each group will be supplied with main air and cause the next group to be selected.

3 The remaining trip valves that become operated in each group are supplied with air from their respective groups and will initiate the next function.

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