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Publisher Summary Hydraulic or pneumatic system is concerned with moving, gripping, or applying force to an object. Devices that actually achieve this objective are called actuators, and can be split into three basic types. This chapter discusses linear and rotary actuators. Linear actuators are used to move an object or apply a force in a straight line. Rotary actuators are the hydraulic and pneumatic equivalent of an electric motor. The third type of actuator is used to operate flow control valves for process control of gases, liquids or steam. These actuators are generally pneumatically operated and are described with process control pneumatics. The operational speed of an actuator is determined by the fluid flow rate and the actuator area (for a cylinder) or the displacement (for a motor).

Pneumatic process control has by no means been superseded by electronic and microprocessor technology, so it is worth looking at the reasons for its popularity. First and foremost is safety. Much process control is done in chemical or petrochemical plants, where explosive atmospheres are common. If electrical signals are used, great care must be taken to ensure no possible fault can cause a spark, which could ignite an explosive atmosphere. While this can be achieved, the result is complex and maintenance may be difficult. A pneumatic system contains only air, so it presents no hazard under these conditions. No particular care needs to be taken with installation, and maintenance work can be carried out “live” with simple nonelectrical test instruments. A great deal of design and application experience has evolved over the years, and this base of knowledge is another major reason for the continuing popularity of pneumatic control. Companies with a significant investment in pneumatic control and a high level of staff competency are unlikely to change. Many devices in the loop are, in any case, best provided by pneumatic techniques. Although electrical actuators are available, most valves are driven by pneumatic signals—even when transducer and controller are electronic.

Safety legislation differs from country to country, although harmonization is underway in Europe. This chapter describes safety from a British viewpoint, although the general principles apply throughout the European community and are applicable in principle throughout the world. Most safety legislations have a common theme. Employers and employees are deemed to have a duty of care to ensure the Health, Safety, and Welfare of the employees, visitors, and the public. Failure in this duty of care is called negligence. Most industrial plants have the capacity to maim or kill. It is therefore the responsibility of all people, both employers and employees, to ensure that no harm comes to any person as a result of activities on an industrial site. In pneumatic or hydraulic systems, pressure measurement is equivalent to electrical voltage measurement, while flow measurement is equivalent to current measurement. There is no direct simple measurement equivalent to electrical resistance. Pressure tests, and (to a lesser extent), flow tests thus form the bases of fault finding in pneumatic or hydraulic systems. Approved Codes of Practice (ACOPs) are documents written to define safe working methods and procedures by organizations, such as CENELEC and British Standards Institute. They are approved by the Health and Safety Commission. While they are not mandatory, failure to follow ACOPs may be viewed as a contributory factor in investigations of an incident.

Publisher Summary A hydraulic system is closed, and the oil used is stored in a tank or reservoir to which it is returned after use. Although, probably the most mundane part of the system, the design and maintenance of the reservoir are of paramount importance for reliable operation. The reliability of a hydraulic system is strongly influenced by the state of fluid. Contamination from dirt or the products of oxidation and deterioration of a fluid's lubrication ability will lead to rapid wear and failure. The various end devices in a pneumatic system are linked to the air receiver by pipes, tubes, or hoses. In many schemes the air supply is installed as a fixed service, similar in principle, to an electrical ring main allowing future devices to be added as required. Generally, distribution is arranged as a manifold or as a ring main. With strategically placed isolation valves, a ring main has the advantage that parts of the ring can be isolated for maintenance, modification, or repair without affecting the rest of the system. Pneumatic systems are vulnerable to moisture and, to provide drainage, the piping should be installed with a slope of about 1% (1 in 100) down from the reservoir. A water trap fitted at the lowest point of the system allows condensation to be run off, and all tap-offs are taken from the top of the pipe to prevent water collecting in branch lines.

Pneumatic systems use a compressible gas, hydraulic systems an incompressible liquid, which leads to some significant differences. The pressure of a liquid may be raised to a high level almost instantaneously, whereas pressure rise in a gas can be distinctly leisurely. The vast majority of pneumatic systems use compressed atmospheric air as the operating medium. Unlike hydraulic systems, a pneumatic system is “open;” the fluid is obtained free, used, and then vented back to atmosphere. Pressure in a hydraulic system can be quickly and easily controlled by devices such as unloading and pressure regulating valves. Fluid is thus stored at atmospheric pressure and compressed to the required pressure as needed. The slow response of an air compressor, however, precludes such an approach in a pneumatic system and necessitates storage of compressed air at the required pressure in a receiver vessel. The volume of this vessel is chosen so there are minimal deviations in pressure arising from flow changes in loads and the compressor is then employed to replace the air used, averaged over an extended period of time. Often, air in the receiver is held at a slightly higher pressure than needed to allow for pressure drops in the pipe lines. A local pressure regulation unit is then employed with the secondary air treatment close to the device using air. Composite devices called service units comprising water separation, lubricator, and pressure regulation are available for direct line monitoring close to the valves and actuators of a pneumatic system.

Publisher Summary There are two types of pump (for fluids) or compressor (for gases) illustrated in this chapter. Typical of the first type is the centrifugal pump. Fluid is drawn into the axis of the pump, and flung out to the periphery by centrifugal force. Flow of fluid into the load maintains pressure at the pump exit. Should the pump stop, there is a direct route from outlet back to inlet and the pressure rapidly decays away. Fluid leakage will also occur past the vanes, so pump delivery will vary according to outlet pressure. These devices are known as hydrodynamic pumps, and are primarily used to shift fluid from one location to another at relatively low pressures. Water pumps are a typical application. A hydraulic pump takes oil from a tank and delivers it to the rest of the hydraulic circuit. In doing so it raises oil pressure to the required level. The operation of such a pump is also illustrated in the chapter. On hydraulic circuit diagrams a pump is represented by a symbol with an arrowhead showing the direction of flow. Hydraulic pumps are generally driven at constant speed by a three phase AC induction motor rotating at 1500 rpm in the UK (with a 50 Hz supply) and at 1200 or 1800 rpm in the USA (with a 60 Hz supply). Often pump and motor are supplied as one combined unit. As an AC motor requires some form of starter, the complete arrangement is also illustrated in the chapter.