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PNEUMATICS CONTROL

1. Introduction

In automation work where pneumatics is concerned, there is a need to ensure that there is regular preventive maintenance work carried out. In the following sections, we will be covering the need for clean compressed air, the maintenance of air distribution equipment and the maintenance requirements for the components.

Block diagram 1: Summary of air supply system to the control plant.

Air composition:

78% Nitrogen
21% Oxygen
1% Other Gas (CO2, H. Ne, Krypton, Xenon, Water, etc)

Properties of Air:

a. Air is compressible.
b. Air is expandable.
c. Air when compressed produces a maximum temperature of 200 to 300OC.
d. Air contains 40ppm of solid particles and increases to 9 times when compressed.

Advantages of Compressed air:

1. Amount-
   Air is available practically everywhere for compression in unlimited quantities.
2. Transport-
   Air can easily be transported in pipelines, even larger distances. It is not necessary to return compressed air.
3. Storable-
   A compressor need not be in continuous operation. Compressed air can be stored in and removed from a reservoir. In addition, transportation in the reservoir is possible.
4. Explosion proof-
   Compressed air offers no risk of explosion or fire, hence no expensive protection against explosion is required.
5. Cleanliness-
   Compressed air is clean since any air which escapes through leaking pipes or elements does not cause contamination. This cleanliness is necessary, for example, in the food, wood, textile and leather industries.
6. Construction-
   The operating components are of simple construction, and are therefore inexpensive.
7. Speed-
   Compressed air is very fast working medium. This enables high working speeds
   to be attained. (Pneumatic cylinders have a working speed of 1 to 2 m/sec)
8. Adjustable-
   With compressed air components, speeds and forces are infinitely variable.
9. Overload Safe-
   Pneumatic tools and operating components can be loaded to the point of stopping and they are therefore overloading safe.

Disadvantages of Compressed Air

1. Preparation-
   Compressed air needs good preparation. Dirt and humidity must not be present. (Wear of pneumatic components).
2. Compressible-
   It is not possible to achieve uniform and constant speeds with compressed air.
3. Force Requirement-
   Compressed air is economical only up to a certain force requirement under the normally prevailing working pressure of 700kPa (7 bar/101.5 psi) and dependent on the travel and speed. The limit is between 20,000 and 30,000N (2,000 and 3,000kgf).
4. Exhaust air-
   The exhaust air is loud. This problem has now however been largely solve due to the development of sound absorption material.
5. Cost-
   Compressed air is relatively an expensive means of conveying power. The high energy costs are partially compensated by inexpensive components and higher performance. (Number of cycles)

2. Need for Clean Compressed Air

Unconditioned compressed air contains a number of undesirable, interfering substances. Water oxidised compressor lubricating oil and various solids, all of which can be removed, or can be widely eliminated during the correct installation of the compressor plant.

2.1 The less desirable constituents of compressed air

The less desirable constituents of compressed air:
• Dust and gases drawn in with the intake air
• Wear and rust particles from the compressor or pipeline system
• Lubricating oil carried from the compressor
• Airborne water vapour and water condensed in the compressor or pipeline system

Since the compressed air comes into contact with a wide variety of working, actuating, control and signal elements, provision must be made for removing contaminants from the compressed air. The correct preparation of the compressed air causes the life of elements to be increased, and for breakdown time of the controls following disturbances and the repair of elements to be kept to a minimum.

2.2 Indication of poor compressed air preparation

The following is an indication of poorly prepared air preparation:
• Rapid wear of moving parts in cylinders and valves
• Water drops forming in the pipeline
• Water contained in the lubricator
• Slow speed of working elements
• Silencers on valves become dirty (black or yellow)
• Working elements operate at different speeds

3.Compressed Air Preparation

To understand the need for good air preparation, we will go through the compressed air distribution flow chart shown below.

Block Diagram 2: Production of pneumatic energy

3.1 Air compressors

The compressor is the first part of the flow chain; it is used to compress the air at atmospheric pressure of about 1 bar to working pressure of 6 to 10 bar. There are a variety of compressors available in the market.

Block Diagram 3: Types of compressors

Positive Displacement Compressors:
(Air is compressed by containing it in a chamber and then reducing the volume of this chamber)

Figure 1: Screw Compressor

Figure 2: Sliding vane compressor

Figure 3: Piston compressor

Flow Compressors:
(Air is compressed by mass acceleration)

Figure 4: Radial flow compressor

Figure 5: Axial flow compressor

Chart 1: Chart showing air intake and pressure ranges for every type of compressor

3.2 Air compressor maintenance

An interfering substance in the unconditioned compressed air is the lubricating oil ejected by the compressor. Each cylinder lubricated in the compression chamber delivers oil into the pneumatic system. This can be prevented only by dry running the machines.

The amount of oil delivered into the system depends on the design of the piston space, the lubricating oil and the compression temperatures.

Often it is wrongly assumed that this oil can be used to lubricate compressed air cylinders or compressed air engines and similar equipment. But the opposite is true, this oil which is oxidised, causes considerable damage to the system. The oil would mix with moisture and would form sludge, varnish (or resin) and amino acids, all of which have detrimental effects on the equipment to which it is applied.

Another detrimental constituent of unconditioned compressed air are the various solids contained within the atmosphere. These can be removed to some degree at the input stage where a filter can be used to remove a considerable percentage, but there are other solids generated in the compressor space, eg. oil carbon. Rust particles and scale are also added to the air inside the pipeline system.

Having looked at how foreign particles can enter the compressed air, there are some ways of removing these unwanted properties. To prevent the possibility of damage to the pneumatic equipment resulting in contamination of the compressed air, the following equipment should be provided when designing a compressed air system:
• Suction filter
• Intercooler and Aftercooler

A suction filter at the compressor inlet provides for separation of dust particles. Intercoolers and Aftercoolers are required to separate the condensate. The purpose of the aftercooler is not only used to ensure clean conditions in the air pipeline (separation of foreign particles and water), nut also to avoid accidents that could be caused by an explosion of oil mist/air mixture. This device should separate as much lubricant as possible.

3.3 Compressed air receiver

After the compressor has compressed the air to working pressure it is directed to the air receiver. The receiver has several functions:
• Providing constant air pressure in a pneumatic system regardless of fluctuating consumption
• Emergency supply to the system in cases of emergency
• The large surface area cools the air
• Moisture can be separated from the air

Figure 6: Compressed air reciever

3.4 Moisture (water) in compressed air

Air had the physical characteristic of absorbing a given quantity of water vapour that depends exclusively upon the temperature of the air, the pressure has no influence whatsoever. The amount of moisture drawn in is primarily a consequence of the relative humidity and this in turn depends on the air temperature, volume and weather conditions. If the saturation point of compressed air is exceeded, the moisture takes the form of drops of water.

If moisture is allowed to enter into pneumatic system, the following consequences result
• Corrosion in pipes, cylinders and other components. These increase wear and higher maintenance costs.
• The basic lubricants in the cylinders are washed away.
• The switching function of valves is impaired.
• Contamination and even damages are caused if the compressed air is directly in contact with certain materials e.g. food, paints, textile materials, etc.

3.5 Air drying

As can be seen, moisture in the system causes considerable damage. If the intercooler and after cooler do not have the capacity to produce absolute dry compressed air, the air must be put through a further drying process. The three commonly used drying methods are:

• Low Temperature Drying
• Absorption Drying
• Adsorption Drying

3.5.1 Low temperature drying

If compressed air is cooled down below dew point, condensation occurs and water is precipitated. The compressed air to be cooled flows into the low temperature dryer via the air to air heat exchanger in the first part of the equipment. Here the warm compressed air to be dried is pre cooled by the cold and dry air flowing out. This causes water and oil to be separated, and thus the refrigerating machinery is required to operate at a capacity of only about 40%. The pre cooled compressed air enters the refrigerating unit only in the second station. Compressed air is then cooled to a temperature of 1.7°C.

To ensure that the function of the air to air heat exchanger and the refrigerating unit is not affected by oil, a pre filter is provided to ensure that the oil and dirt particles are separated prior to their entry into the drying unit.

Graph 1: The Dew point curve –graph showing water content of air at 50g/m3 @ 40OC

Figure 7: Air cooler

3.5.2 Absorption drying

It is a purely chemical process. The air is guided over a bed containing salt tablets, the tablets will absorb the water vapour from the air and become a liquid which will drop to the bottom of the drying tank.

Unfortunately, most of the drying agents used are strongly corrosive and filtering is necessary to ensure that the drying agent is not carried along with the dried air. Pre filtering is also necessary to prevent the oil from damaging the drying agent. A further problem with this type of drying agent is that at temperatures exceeding 30°C the drying agent will soften and bake, which will lead to increased pressure drop through the dryer.

The special features of the absorption process are:
• Simple installation of the equipment
• Low mechanical wear as there are no moving parts
• No external energy requirements

Figure 8: Absorption Drier

3.5. 3 Adsorption drying

Moisture, gases or dissolved materials from the air deposits on the porous surface of the desiccant. It is a physical process and regeneration is possible through hot air flow.

Figure 9. Adsorption drier

4. Distribution of Compressed Air

Owing to the increased rationalization and automation of manufacturing equipment, the air requirement in factories continues to rise. Each machine and each appliance needs a certain volume of air, and is provided with air from the compressor via a pipe system. The pipe diameter should therefore be selected such that the pressure drop between the receiver and user does not exceed 10kPa (0.1 bar). A higher pressure drop endangers the economics of the system and considerably reduces the performance.

When planning a new installation, allowance should be made for a possible later enlargement to the compressor plant, i.e. higher air consumption, and the pipelines should therefore be generously dimensioned. Later installation of a larger pipe system is often very expensive.

4.1 Installation of pipelines

Not only are the correct design dimensions of pipelines important, but also their installation. Compressed air pipelines require regular maintenance and inspection. Because of this, they should not be installed in brickwork or in narrow pipe shafts. This only makes checking of leaks in the pipelines more difficult. Even small leaks cause noticeable pressure losses.

Pipelines should be installed with a downward grade of 1 to 2%, in the direction of flow, particularly in branch lines, because of condensation. In horizontal runs of pipe, branches should always be made from the top of the pipe.

Figure 10: air distribution

Figure 11: Branch line

This design prevents any condensate from the main line appearing at the point of usage through the branch line. Special pipelines are fitted to the underside of the main line to trap and drain the condensate.

Figure 12: Ring circuit

Ring circuits are installed most frequently as the main line. Branch connections are made from this main line. Using this type of compressed air line, a uniform supply of air can be maintained where there is heavy use. The compressed air can flow from two directions.

Figure 13: Interconnected system

The interconnected system is also a ring circuit which, because of the length and cross connections of the system, makes it possible to work with air at any point. It is possible to use shut-off valves to shut off certain compressed air lines if they are not required or if they have to be closed for repair and maintenance.

5. Air Service Unit

At the plant level, the service unit is the last protection before the air is used in the system.

The service unit consists of:
• Air filter
• Pressure regulator
• Air lubricator
• Pressure gauge

Figure 14: Air filter unit

5.1 Air Filter

The Air Filter is used for
• Preventing dirt particles from entering into the system
• Separate condensate out of the compressed air.

5.1.1 Installation

• Direction of air flow (marked by arrow)
• Vertically
• Visible and easily accessible

5.1.2 Trouble-Shooting

• Minimal air flow – Filter cartridge is clogged up.
• Audible exhausting at the drainage – Drain screw is leaking.
• Excessive pressure drop (> 0.5 bar) (Inlet pressure – Outlet pressure >0.5 bar) – Filter cartridge is blocked.

5.1.3 Maintenance

• Condensate level must be checked regularly (It should not exceed the mark).
• Condensate to be drained after each shift.
• Filter cartridge must be replaced or cleaned at regular intervals when it is dirtied or blocked.

Depending on the type of filter, the filter elements must be replaced or cleaned at regular intervals. When cleaning is carried out, the manufacturer’s specifications concerning the cleaning agents to be used must be observed. Many agents (e.g. Trichloroethylene) produce tension cracks in the filter bowls, which may then burst when, re-subjected to pressure. As a rule, it is sufficient to use lukewarm soapy water, applied with a brush or paintbrush. The filter elements should then be blown out, working against the flow direction.

It is advisable to fit an automatic condensate drain in cases where there is continuous incidence of condensate. A drain can be added to any filter bowl by screwing it in. When a particular level of condensate is reached, a valve opens automatically and discharges the water.

5.2 Air Regulator

The Air Regulator is used for
• To Provide a Constant Supply Pressure Irrespective of the Pressure Fluctuations in the Main Line

All pneumatic systems have an optimum pressure (6 bars) and are lower than working pressure in the pipeline (10 – 15 bar). Pressure is always fluctuating due to the air being compressible.

Air pressure too high
• Energy losses
• Increased wear
• Slow switching of valves

Air pressure too low
• Low output force for working elements
• Poor efficiencies
• Uneconomical operations

Figure 15: Pressure Regulator

Changes in the pressure level in the pipe system affect the switching characteristics of valves, the running times of cylinders and the timing characteristics of flow control and memory valves.

In order to provide constant pressure conditions, the pressure reducer or pressure regulator is fitted downstream of the compressed air filter and has the task of keeping the operating pressure constant, independently of pressure fluctuations in the system of air consumption. The system pressure which has proved in practice to be the best economic and technical compromise between compressed air generation and the efficiency of the components is approximately 6 bars in the power section (Optimum pressure) and 4 bars in the control.

A higher operating pressure would lead to unfavorable energy utilization and increased wear, whereas a lower operating pressure would lead to poor efficiency particularly in the power section.

5.3 Air lubricator

Lubrication of the compressed air by means of mist lubricators may be necessary in certain cases:
• In cases where extremely rapid oscillating motions are required.
• With cylinders of large diameter, from approximately 125 mm upwards.

As far as possible, lubricators should be installed only directly upstream of the consuming cylinders.

Figure 16: Air lubricator

In recent years, increasingly critical options have been expressed concerning the use of a lubricator and, in certain cases; the question has been raised as to whether a lubricator should be used at all. This reflects problems originating in experience and knowledge gained in the past:
• Malfunctions occur due to excessively lubricated components.
• Lubricated air pollutes the air in working rooms.
• Gumming effects occur after lengthy plant standstills.
• Difficulties are experienced in adjusting the lubricator correctly.

The sliding parts in working elements (cylinders, motors, etc) require lubrication. A certain amount of oil is added the compressed air by means of a lubricator. The compressed air then supplies the oil particles to the working elements.

Things to note are:
• Direction of air flow (observe strictly).
• Type of oil (light mineral oil, eg. SAE 10)
• Rate of lubrication.
• Air flow rate.
• Fitted vertically.
• Installed directly upstream of the working elements (cylinder) only.
• Never mix oil with petrol since it is a grease solvent and promotes corrosion.
• Do not install beyond 5 metres from working elements.

Advantages of Lubrication:
• Reduce wear and tear.
• Reduction of frictional losses.
• Protection from corrosion.

The amount of lubrication depends on the air consumption in the system.

A piece of white paper should be held at a distance of about 200 mm from the exhaust port of the final control element. The system is then allowed to operate for some time. Observe the following:
• A film of yellow colour should be visible on the paper (that is a good indication).
• Dripping oil is a clear sign of over lubrication.
• Over lubrication causes gumming (sticky) when it solidifies.

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