Under pressure01 December 2005

The use of compressed air systems in manufacturing and process industries is said to account for some 10-15% of energy consumption, and they are found in almost every area of economic activity. The applications and use of compressed air systems range from heavy and light manufacturing engineering industries, to schools and hospitals,. even the tyre bays of our local garages. Typical compressed air systems consist primarily of a pressure vessel of rigid construction (air reservoir), together with associated pipework and protective devices. It is, of course, possible to have a portable system, in which case many of the same elements are used. However, the pressure vessel is transportable and perhaps more common in the smaller enterprises.

With so much variety in installation and use, ensuring the safety of the whole system from its collection of components, materials and operating environment presents many challenges. Ideally, perhaps, the best way to provide the most effective solution is to design in safety from the outset. In addition to reducing the cost of wasted energy, an efficiently designed system can improve performance. It may be true to say that compressed air systems should not be designed to a capital cost, but to meet the needs of the factory, workshop or process, with a safety margin that provides security for the workforce and everyone else.

At the design and specification stage, there are now specific regulations and EU directives that cover this field. These are in addition to the existing engineering, operational and health and safety requirements, and form the starting point for selection of pressure vessels, valves, actuators, piping, meters and controls that form the whole installation. Not surprisingly, consideration must also be given to the operational demands, inspection and maintenance regimes, and the environment within which that the plant is to be operated.

All pressure equipment must meet the Pressure Equipment Directive 97/23/EC (PED) and, in the UK, the Pressure Systems Safety Regulations 2000. In fact, the PED is one of the so-called 'new-approach' directives that are product specific, and covers the design, manufacture and supply of products within the EU. The principal areas that it covers include: design and construction; installation; maintenance; safe operating limits; written scheme of examination (and examination in accordance with this); and the keeping of records.

All equipment must also meet Essential Safety Requirements (ESRs), described in appendices to the directive. It is also a requirement that pressure equipment, including compressed air systems and components, must be designed, based on a calculation method, such as design by formula, design by analysis or design by fracture mechanics. Traditionally, compressed air systems designs have used the design-by-formula approach, although alternative design-by-analysis (DBA) methodologies have been introduced as optional techniques.

There is a variety of materials for system components, with stainless steels providing the
most common applications in many industries. However, as operational and business demands grow, designers are looking at new ways of improving plant reliability and flexibility. More advanced materials can increase efficiency and reliability, and the development of new generation high-strength and duplex steels includes 9% chromium cast stainless steels.

Advances in the use of non-metallic materials has been of great benefit, too, with materials such as polyethylene, fluorocarbons and polypropylene - even for some pressure vessels. Using polypropylene as a reinforced material gives added advantages, such as reduced weight, ease of assembly and installation.

Eliminating danger

A typical installation will take the form of a network of pipes, with branches off the mains to air-operated tools and machinery. At the start of these branches, isolating valves are used to allow maintenance or repairs, or even additions to individual sections. Similarly, for pneumatically-operated process equipment, the valves, actuators and installed system must present no danger in operation, maintenance or repair.

Many systems are deployed in areas where extremes of temperature, as well as contaminating materials, are present. These contaminants may be part of the production process itself, such as in the chemical, pharmaceutical or food industries, or may be present in the atmosphere, or around machine tools, as in the engineering industry.

Compressed air quality is of great importance to the safe operation of the system, and there are nine elements of the ISO 8573 standard that identify and define the requirements to ensure that contaminants are removed from the system to safe levels. Contaminants on the materials of tubing, valve linings, seals and connectors may impact their life or operation, although there are instances where oil is introduced into the system to lubricate valves.

Reducing the impact of contaminants on the system is a key area, as the effect on equipment that is downstream of the compressor and air reservoir can lead to significant operational problems, and may pose a hazard. The presence of water and vapour in the system is dealt with by cooling and removing at the receiver, while air dryers are used to remove the water vapour contained in the pressurised air.

Solids, too, from dirt and corrosive elements taken in at the compressor inlet, along with carbon and other particles from the operation and fixings used in the installation itself, can be a problem. Typical general-purpose filters can remove particles down to 40 microns in size. Fine filtration in the region of 10-25 microns is normally required for pneumatic tools or process controls, while less than 10 microns is vital for use in pneumatic motors or air bearings. Selection and installation of filters can be a challenge, and a balance has to be struck between the filter's capability and the frequency with which filter elements need to be changed.

Controlling the pressure in the system is absolutely vital: in addition to providing a safe working pressure, failure to provide adequate pressure reduction can lead to, at best, inefficient operation and increased wear on components; and, at worst, a catastrophic system failure.

Critical measures

Matching solutions to circumstances is perhaps one of the critical measures of ensuring that the system is safe. The system must provide no danger to personnel, the general public, or the inspector or engineer surveyor tasked with assessing the status of the installation.

Among the critical measures are the loading characteristics of the system and its components. This covers a wide range of attributes: internal/ external pressure, temperature, the effects of corrosion and erosion, fatigue, attachments and fastenings, and even earthquake loading. These factors are not taken in isolation, since one or more can occur simultaneously, so the risk assessment will need to take multiple events into account, together with the likelihood that they will occur.

All pressurised systems have their Safe Working Pressure (SWP), with an over-pressure limit of 10%. The system can be designed to operate at less than the SWP, using the 10% margin as the factor of safety. However, selection of the correct pressure relief valve depends on a thorough knowledge of the entire system, its operational demands and the environment in which it is used.

The components within the installed system frequently have pressure ratings lower than that of the compressor and air receivers, and pressure regulators and relief valves are used to reduce this to safe, efficient levels. If a fault occurs, the components of the system can be subject to excessive pressures that can lead to malfunction of individual components - impacting production processes - and even component failure, which can have very serious consequences.

Another critical area is that of pressurising the system, and the valves, actuators or other components attached to branches off this network. In order to protect these components from excessive wear, or too rapid a build-up of air pressure, slow start ('soft start') valves are used, which allow air into the system gradually, with a much greater degree of control.

In summary, there are many areas that may be deemed critical, embracing the type of relief valve to control the air pressure, determining where start-up or system reset problems can occur, filtration of oil-laden air, exhaust problems and prevention of tampering or accidental misuse. So the selection of the appropriate protection device is paramount.

Inspections

Engineer surveyors undertake inspection, examination and assessments of installations. As well as the pre-commissioning and planned inspections, changing practices now include sampling of the elements of a system at regular intervals. This can include all equipment, or a range of critical components, such as control valves and actuators, or meters, and will vary according to the nature of the operation, frequency of operation, environment and materials used.

Effective inspection and testing of compressed air systems is building on research into engineering life assessment of installations, which can reduce the cost of testing, as well as improve safety and prevent damage. In turn, this is leading to increased plant availability, reducing maintenance costs and overall energy wastage. Typical of the testing methods employed today are non-destructive testing, novel sensor technologies, acoustic energy detection, acoustic emission measurements, finite element method, replica inspection, risk-based inspection programming and deterministic methods for predicting corrosion.

Examination and testing must, of course, ensure conformance to standards: the HSE guidelines and advice are based on the PSSR 2000 legislation, which requires owners and operators of systems to know the safe operating limits and to have a written scheme of examination in place before it is operated. They must also ensure the system is examined at appropriate intervals, as determined by the written scheme, which must be used at every examination.

Certain insurance companies do provide a range of assessment and certification services for owners and users of pressurised systems. The examination scheme does not have to be drawn up by the owners, but it does have to be drawn up and certified by a competent person, which may be the owner's in-house inspection department.

In the introduction to the HSE guidelines, it is a sobering thought that there are 150 dangerous occurrences involving unintentional releases from pressure systems each year. Around six of these are fatal or result in serious injury.


System upgrade is good news

The Daily Mail Group's print and distribution centre at Surrey Quays, London, has undergone a major investment programme to upgrade all printing presses and production equipment. Other upgrades were also considered, including the compressed air system. An air quality test was carried out, which revealed large volumes of condensate in the downstream system. Closer investigation revealed that several of the existing drying units were not operating correctly - with the result being lower quality air fed to production equipment.

As an interim measure, a large dessicant dryer was hired and installed to improve the air quality, which also slowly dried out the site's distribution pipework.

At the same time, a detailed report on the existing installation looked at contributory factors such as compressor sizing, air demand for the site, air flow graphs, air quality and the existing compressor control system. The report also identified other problems, including reliability. Recommendations were made to improve the whole system.

A single FD1600 refrigerant air dryer and filter set was ordered, capable of handling the whole site's air demand. At the same time, some of the pipework was increased in size from 4" and 6" to 8", which improved airflow.

With continuous liaison, careful planning and attention to detail, the installation was delivered on time and within budget - a great success for Air Solutions and Atlas Copco.

SOE

This material is protected by MA Business copyright
See Terms and Conditions.
One-off usage is permitted but bulk copying is not.
For multiple copies contact the sales team.