A question of quality

Published:  05 August, 2010

Jeremy Salisbury of Brammer, examines the issues surrounding maintaining the purity and performance of compressed air systems.

Clean, dry compressed air is essential for pneumatic systems in any manufacturing environment to run efficiently. In many factories, the compressor provider will supply filtration systems as part of their contract which will deal with contamination in the generation process. However, the story is often very different at usage stage. The presence of contaminants in compressed air used to drive machinery is likely to slow down the process, with pressure having to be increased to try to overcome these issues, with a corresponding increase in energy usage.

Machine sequencing can also be impacted - any machine containing more than one cylinder will be 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 to maintaining productivity and minimising energy use.

Types of contamination

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. Oil which gets into machinery can block up silencers on machine exhaust ports, again creating sequencing issues, slowing down operation, and 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, where it can pick up contaminant particles from the internal surfaces of the pipework. As it passes over ovens and furnaces it heats up but, 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 again potentially slows down valve and cylinder action. The presence of water can also cause rubber 'O'-rings in valves to swell, meaning more pressure is needed to operate the valves, again using more energy and potentially effecting sequencing.

Rust is also an inevitable consequence of the presence of aged iron pipes and ultimately the only way to avoid rust is to replace the entire pipework system with an alternative such as aluminium – however, for production facilities operating around the clock, the downtime this would entail means this route is simply not a viable option.

Getting it right at the source

Appropriate maintenance of compressed air systems – in particular a well-managed filtration process and regime - 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 receiver – a pressure vessel where it is cooled, with oil and water droplets falling 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/ water mixture to be agitated if it is not emptied sufficiently regularly. A regular maintenance schedule for this is crucial, while companies can also consider fitting an automatic drain operated by condensate levels or alternatively a drain with a timer which can be set to empty periodically.

After leaving the receiver, the compressed air should pass through pre and after filters– these products can block if not effectively maintained so again a regular check and replacement regime should be established. 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 does deliver enhanced quality.

Once the compressed air enters the ring main, on its way to is 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. Once again, a regular maintenance regime 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 please visit: www.brammer.co.uk

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