Pneumatic systems for industrial and manufacturing
PNEUMATIC SYSTEMS are widely used in industrial and manufacturing plants. However, knowledge and skills of many engineers and operators on pneumatic systems and devices are often limited. A pneumatic system is a combination of pneumatic circuit and devices which use pressurized gas, usually compressed air, to produce mechanical motion; the motion can be linear or rotary, depending on the type of actuator. A basic and widely used actuator is a pneumatic cylinder, with maximum force on the piston rod being determined by air pressure and cross-sectional area of the piston. Examples of other pneumatic actuators include: rotary actuators, pneumatic motors, grippers, various types of pneumatic rod-less actuators, etc.
Pneumatic systems are commonly operated with compressed air or compressed inert gases. These systems can be driven by a wide range of compressed gases such as carbon dioxide (CO2), dry nitrogen (N2), etc; they also can be driven by compressed gases stored in cylinders allowing for portability or sometimes applications in standalone emergency systems. Pneumatic systems are used in many plants, machineries and facilities because a centrally-located and electrically powered air compressor system, that powers pneumatic devices, can usually provide motive power in a better, safer, more flexible, and more reliable way than a large number of other actuators such as electric actuators, electric motors, etc.
Most industrial pneumatic applications use pressures of about 5.5 Bars to 9.5 Barg; although specially designed pneumatic systems can be designed for higher pressures than the abovementioned such as 10 Barg, 11 Barg or more. Gas under high pressure can cause an explosion if its storage tank/vessel is damaged; therefore, operating pressures of pneumatic systems are usually limited to below 11 Barg or 12 Barg. Most pneumatic circuits and devices run at low power ratings usually around 2 to 30 kW; although, in specific applications, higher power ratings, say 90 kW or more, have been used. Two main advantages of pneumatic circuits are their low initial cost and simplicity. Because pneumatic systems operate at relatively low pressure, the components can be made of relatively inexpensive materials often by mass production processes such as plastic injection moulding, aluminium die- casting, or similar. These processes cut secondary machining operations and cost. Pneumatic systems are cost effective, versatile and simple systems with standard low cost components which are widely used in different applications and machines. This article discusses pneumatic systems for industrial and manufacturing facilities.
Pneumatic systems are power systems using compressed air (or other compressed gases) as the working medium for the power transmission. An air compressor system converts the mechanical energy of the prime mover (often electric motor) into pressure energy of the compressed air. This transformation facilitates the transmission, storage, and control of energy. This also can provide a safer alternative for areas where electrical power is risky to be employed. After compression, the compressed air should be prepared and cleaned using different filtration and drying stages. There have been many failures or damages in pneumatic systems due to contaminations or dirt in compressed air; therefore these steps of filtration, drying, etc. need great care.
Pneumatic systems have numerous desirable features. These systems are simple, low cost, effective, high performance and reliable. There have been many advantages for such a system; for instance, air is available everywhere and it can be vented to the atmosphere; all these are reducing the complexity, cost, and weight of the overall system. Pneumatic systems have been used for some considerable time for carrying out simple or complex mechanical tasks in a wide range of applications. In recent decades, such systems have played an important role in the development of automation systems through different arrangements and configurations such as mechatronic systems, etc. As a rule, many modern pneumatic components are designed for a maximum operating pressure of 9 – 10 Barg. However, it is recommended to operate the same between 5 and 8 Barg for safer and more reliable use.
AIR SUPPLY SYSTEM
One of notable benefits of pneumatic systems arises out of the simple fact that air is free. The air supply for a particular pneumatic application should be sufficient and of adequate pressure, temperature and quality. Air is drawn from the atmosphere via an air filter and raised to required pressure by an air compressor set. The air is compressed to around 1/ 7th or 1/8th of its volume with the help of a properly selected air compressor set; the air compressor system has usually “n+1” configuration (one standby). The air temperature is raised considerably by compression. Before the air can be used it should be cooled. Air also contains a significant amount of water vapour; all these result in the formation of condensation. To ensure proper quality of the air, air treatment equipment is utilized to prepare the air before it is applied to a pneumatic system and actuators. In other words, the air compressor should be followed by a cooler set and air treatment unit.
An air treatment unit is usually a combination of compressed-air filter, dryer, and regulator. The compressed-air filter and dryer package performs the job of filtering all contaminations from the compressed air flowing through it as well as condensates, traces of water and water vapour. The purpose of the regulator is to keep the operating pressure virtually constant regardless of any fluctuation. Regulators are fitted with proper components that react to changes in the downstream air pressure. In other words, a regulator attempts to automatically maintain a constant (pre-set) pressure within a pneumatic circuit as long as the supply (reservoir) pressure is greater than the required circuit pressure.
It is always recommended to use at least two measurement/control sources, for instance, a regulator and a pressure gauge, to monitor and control the performance of a pneumatic system. Every pneumatic system should have a pressure relief valve to prevent over pressure conditions that can develop.
An air receiver is fitted to act as a buffer and reduce pressure fluctuations. Compressibility of air/gas makes it necessary to store a large volume of pressurised air in an air receiver (air reservoir/ vessel), to be drawn on by the load. The air reservoir stores the pressurised air used to operate the pneumatic circuit components; it acts like a pneumatic battery. Significant amounts of energy can be stored in a pressurised air receiver. For this reason, such reservoir should be strong and robust; safety provisions and precautions should be fully considered for such a vessel. This system is delivered the air to an air distribution system. Many plants, factories and manufacturing units produce compressed air at one central station and distribute an air ring main to all places on the site in a similar way to other utility services such as electricity, water, gas, etc.
Valves control the flow (“on/off”) and the speed of air flow. Pneumatic valves can be operated by hand, mechanically, electrically (solenoid) or air ( piloted). Solenoid valves are electrically operated valves that control the direction and flow of pressurized air to and from pneumatic actuators or circuits. They are widely used in plants and units. These valves can be either “mono-stable” or “bi-stable”. Mono-stable valves are spring return to a default condition. Bi-stable valves have no preferred or default condition thus remaining where it was last positioned. There have been many different types of pneumatic valves. For example, a widely used valve is three- port, two- position solenoid valve. This means the valve has three ports and two possible conditions (passing or not passing) and it is electrically operated (solenoid).
A pneumatic actuator or tool is a type of power device driven by compressed air. These devices and actuators are usually more compact, cost effective and safer to run and maintain than their electric power devices equivalents, and have a higher power-to-weight ratio, allowing a smaller, lighter, more cost effective device to accomplish the same task. In general, pneumatic devices, actuators and tools are cheaper than the equivalent electric- powered devices or tools. General grade (standard) pneumatic actuators are commonly cheaper than any other devices. Pneumatic actuators are becoming increasingly popular, and have always been ubiquitous in industrial and manufacturing facilities.
A pneumatic actuator is often a cylinder type linear actuator; it mainly consists of a piston, a cylinder, and some valves or ports. The piston is equipped with a diaphragm or seal set, which keeps the air in the cylinder, allowing air pressure to force and move the piston. The larger the size of the piston, the larger the output force can be. Having a larger piston can also be good if air supply is relatively low or might be variable and dropped to a low value, allowing the same forces with less pressure input. Linear pneumatic actuators are available in many different configurations. These cylinders are fitted with pistons of various types, diameters, strokes or lengths. They are specified as single acting (powered in one direction) or double acting (powered in both directions). Single acting spring return cylinders are more economical with respect to air consumption. However, their applications are limited to some services where such a spring return system is acceptable; double acting cylinders are commonly used in many services.
A pneumatic motor or compressed air engine is a type of motor which does mechanical work in form of rotary motion by expanding compressed air. Rotary motion is supplied by different types and models such as a vane type air motor, piston air motor, etc. In other words, some types rely on pistons and cylinders; others use turbines or similar. Pneumatic motors have been offered in many forms and shapes over the past 150 years, ranging in size from hand-held turbines to air engines of up 500 kW (or even more).
SPEED OF RESPONSE
Pneumatic systems use compressible air/gas; for this reason, a pneumatic system is slower in responding to loads, especially sudden output loads, than other systems such as a hydraulic system. Similarly, torque or force requires time and output motion to build-up. Responses to sudden output loads often show initial overshoot. Much more complex networks or other damping means are usually required to develop stable response in closed-loop systems. On the other hand, there are no harmful shock waves analogous to the transients that can occur in hydraulic systems, and pneumatic system components last comparatively longer.
The low stiffness of pneumatic systems is another indicator of relatively long response time. Resonances can occur between the compressible gas and equivalent system inertias at lower frequencies. Even the relatively low speed of sound in connecting lines contributes to response delay, adding to the difficulty of closed-loop stabilisation. Fortunately, it is possible to construct sophisticated pneumatic systems to achieve stabilisation at a point. Such pneumatic stabilising means are commercially available and are important elements of closed-loop pneumatic control systems. The control of pneumatic system is almost exclusively by valves, which control the flow from a pneumatic pressure source. Low- pressure outlet ports should be large enough to accommodate the high volume of the expanded air/gas.
One of reported problems in some pneumatic systems is caused by sluggish pneumatic actuators, which presents some difficulties such as it can limit the control bandwidth of actuated systems. Too often, a relatively low speed of response in pneumatic systems is a problem and should be considered and verified particularly for circuits and applications which require fast responses. Gas/air compressibility makes pneumatic systems 1 or 2 orders of magnitude slower than hydraulic systems. There have been some applications where hydraulic systems were used instead of pneumatic systems only because of the speed of response.
PNEUMATICS VS HYDRAULICS
Both pneumatics and hydraulics are applications of fluid power. Pneumatics uses an easily compressible gas (air, etc.), while hydraulics uses relatively incompressible liquid media such as hydraulic oil. As an indication, hydraulics applications commonly use pressures from 50 to 350 Barg and specialized hydraulics systems may exceed 500 Barg. Pneumatic systems use operating pressures far lower than ones of hydraulic systems, as noted, somewhere between 6 to 10 Barg. Considering operating pressure range, hydraulic systems are more compact with lower power-to-weight ratio; pneumatic systems therefore require larger actuators than hydraulic systems for the same load; although pneumatic actuators are still smaller than electrical ones.
Pneumatic systems offer specific benefits which make them better options for certain applications. There have been some advantages for pneumatic systems over hydraulic ones. Simplicity of design and control is one of advantages of pneumatic systems. Machines and mechanisms are easily designed using standard pneumatic cylinders, pneumatic actuators and other components, and operate via simple control (often very simple on- off logic). Although there have been some standardization on hydraulic systems, hydraulic units are usually designed and installed for each application. Pneumatic systems usually have long operating lives and require little maintenance. Because air/gas is compressible, equipment is less subject to shock damage. Gas absorbs excessive force, whereas oil in hydraulics directly transfers force. Compressed gas can be stored, so machines and facilities still run for a while if electrical power is lost. Although accumulators and other types of storage vessels have been used in hydraulic systems; the stored energy is very limited. There is a very low chance of fire compared to hydraulic oil. Fluid viscosity and its temperature variations are virtually negligible with pneumatic systems.
First cost of an air circuit is less than a hydraulic circuit but operating cost can be higher. Compressing atmospheric air to a nominal working pressure requires a lot of power. Pneumatic actuators are usually expensive to operate; specifically air motors are one of the most costly components in plants to operate. On the other hand, air- driven machines are usually quieter and safer than their hydraulic counterparts. Lower noise is mainly because the power source (air compressor system) is installed remotely from the machine in an enclosure that helps contain its noise.
Because air is compressible, a pneumatic actuator cannot hold a load rigidly in place like a hydraulic actuator does. An air- driven device can use a combination of air for power and oil as the driving medium to overcome this problem, but the combination adds cost to the circuit. This is only used in very special applications. Pneumatic systems are usually cleaner than hydraulic systems. Leaks in an air circuit do not cause housekeeping problems, but they are expensive.
BIO: AMIN Almasi is a lead mechanical engineer in Australia. He is chartered professional engineer of Engineers Australia ( MIEAust CPEng – Mechanical) and IMechE ( CEng MIMechE) in addition to a M. Sc. and B. Sc. in mechanical engineering and RPEQ ( Registered Professional Engineer in Queensland). He specialises in mechanical equipment and machineries including centrifugal, screw and reciprocating compressors, gas turbines, steam turbines, engines, pumps, condition monitoring, reliability, as well as fire protection, power generation, water treatment, material handling and others. Almasi is an active member of Engineers Australia, IMechE, ASME, and SPE. He has authored more than 150 papers and articles dealing with rotating equipment, condition monitoring, fire protection, power generation, water treatment, material handling and reliability.