Purity and performance - maintaining the quality of compressed air systems

Clean, dry compressed air is essential for pneumatic systems in any manufacturing environment to run efficiently and thanks to its safety and reliability it is used by some 80 per cent of UK industrial applications.

As compressed air is usually generated on site, users are responsible for maintaining the quality of the final product and any contamination in compressed air is likely to slow down entire processes. Libby Peacock, Product Manager at Brammer, the UK’s leading supplier of maintenance, repair and overhaul (MRO) products and services, looks at the importance of maintaining high levels of purity and quality in industrial compressed air.

The presence of contaminants in compressed air used to drive machinery can have several side effects. Energy usage increases, as pressure has to be increased to cope with slowed processes. Machine sequencing can be impacted, with any machine containing more than one cylinder being more sluggish at start-up, affecting both speed  of operation and production quality. Developing an appropriate process to manage compressed air quality is therefore vital in maintaining productivity and minimising energy use.

For food manufacturers in particular, the quality of compressed air is regulated in the industry with the British Retail Consortium (BRC) working alongside the British Compressed Air Society (BCAS) to develop a compressed air quality standard and guidelines – ISO 8573.1. For food-contact compressed air, the maximum permissible number of solid dirt particles per m³ is 100,000 for particles measuring 0.1-0.5 micron; 1,000 for particles of between 0.5 and 1 micron in size; and 10 for larger particles of 1-5 micron. Oil content, either as an aerosol or vapour within compressed air, must not exceed 0.01mg/m³ while humidity (water vapour) must be -40oc PDP. For non-food contact and non-contact high-risk applications, the levels for solid particulate (dirt) and total oil content are the same, with a slight variation in the humidity permitted for non-contact low-care applications – this changes to +3oc PDP. The ISO 8573.1 guidelines also cover the use of food-grade lubricants within air compressors.

Compressed air systems are typically subject to three main forms of contamination – oil, water and rust – all of which can have a substantial negative impact on
productivity and also on overall energy consumption.

Although the majority of modern machinery does not require oil, most compressed air systems use it for lubrication and failure to provide adequate filtration to remove oil can have disastrous effects. Unwanted oil in machinery can block up silencers on machine exhaust ports, again creating sequencing issues, and also slows down operation as well as generating a back pressure which, in its worst case, can result in complete machine failure.

Water can develop in compressed air when it enters into the factory via the ring main, and is then likely to pick up contaminant particles from the internal surfaces of the pipework. As it passes over ovens and furnaces it heats up and on cooling, water droplets form. If the water is not removed before usage it will mix with any oil present and form an emulsion, which can again slow down valve and cylinder action. The presence of water in valves can also cause rubber ‘O’-rings to swell, meaning more pressure is needed to operate the valves.

Rust is also an inevitable consequence of the presence of aged iron pipes, and there are several possible solutions to rectify this before it reaches such a stage where complete replacement of the system is necessary. The options available will depend upon the severity of the rust, age and type of pipework and the size of the system affected, and MRO specialists such as Brammer can analyse the system to recommend a custom solution that treats the issues and prevents downtime. Appropriate maintenance of compressed air systems – including a well-managed filtration process – is therefore key to maximising operational efficiency and reducing energy costs.

Getting the filtration process wrong in the compressor house means the compressed air has to be cleaned at, or close to, its point of use – a far more difficult and usually less effective process. When air leaves the compressor it enters a pressure vessel known as a receiver, where it is cooled to allow droplets of oil and water to fall to the bottom. The receiver also contains a separator to allow water to be drained off from the bottom while oil floats to the top and is removed. Some systems have a manual drain which can overfill, causing the oil and water mixture to be agitated if it is not emptied regularly enough. A regular maintenance schedule for this is crucial, automatic drains can also be fitted which either operate by condensate levels, or feature a timer which can be set to empty periodically.

After leaving the receiver, the compressed air should pass through pre and post filters – these products can block if not effectively maintained so again a regular check and replacement schedule should be implemented. The final stage before entering the main production facility is drying – using either a refrigerant, which is cheaper but does not always offer optimal air quality, or a desiccant, which is more energy-intensive but delivers enhanced quality.

Once the compressed air enters the ring main, on its way to its points of use, drip leg drains collect water and remove it from the air via gravity. Once it enters the machinery, the requirement is on the filter, regulator and lubricator (FRL) (or just a filter and regulator for newer equipment not requiring oil) to remove any impurities and ensure correct pressure. A regular maintenance schedule should be in place to ensure these components are regularly checked, cleaned and, if necessary, replaced.

Consulting a specialist supplier in this area can also help ensure appropriate levels of filtration - whether this be installing a finer filter into the FRL already on the machine, or alternatively, fitting a membrane or small refrigerant dryer – and ensure optimal compressed air quality.

For further information on the products and services offered by Brammer, visit www.brammer.co.uk