Filter facts09 December 2010

Pneumatic systems need to operate reliably, even in extreme conditions. John Hill examines the requirements, in terms of components, filters and maintenance

However extreme the conditions, pneumatic systems today are expected to function reliably – and that is as true of mobile plant as it is of fixed installations in, for example, the oil and gas industry, where corrosive, wet, dirty and high temperature environments are commonplace.

To that end, pneumatic equipment designers have developed components capable of continuous operation, with previously unattainable levels of performance. Extreme environment designs – such as the Parker VikingXtreme range of valves – are constructed using tough, corrosion resistant aluminium alloys, stainless steels and techno-polymers. They have also largely been simplified, with fewer moving parts, while sensitivity to dirt and contamination has been reduced by introducing carefully mapped flow paths. And maintenance, too, is part of the equation, with parts designed to be easy to change.

But no matter how tolerant valves and cylinders may be of corrosion, metal, dirt, moisture and even microbial growths in the air supply downstream of the compressor, allowing these to persist will eventually affect both the reliability and longevity of pneumatic circuits. Removal technologies remain essential at key points throughout each installation.

Airline filters or water separators are often used as general purpose pre-filters, with, for example, water separators removing in excess of 92% of all bulk liquid contamination, including oils and corrosive chemicals from the compressor. Then coalescing filters are probably the single most important purification equipment type, removing not only fine droplets of oil and water, but also stopping solid particulates, potentially down to 0.01micron.

But these don't stop water vapour and, even at moderate ambient temperatures, unless moisture is removed from the air flow, it may condense as liquid in air lines, and wash away essential cylinder and valve lubricants. Further, if the ambient temperature drops below freezing, ice can form in air lines and on the internal surface of components. Hence the additional requirement for air drying systems, such as refrigerant driers, which condense water vapour in general plant applications, and desiccant or adsorption dryers, which pass air over material that absorbs liquid from the compressed air system.

The latter are normally the best choice for low temperature operation and are available in compact designs for transport applications or where space is limited. Some make use of conventional steel pressure vessels, while others use extruded aluminium pressure vessels. They can also be installed in single- or twin-tower configurations – and there are important differences, in terms of cost and maintenance, that determine which is appropriate for an applications.

Both operate on the same principle of cyclical adsorption and desorption of water vapour. However, a twin tower dryer is capable of providing a continuous stream of dry air, because as the air is dried in one tower, desiccant is regenerated in the other – with a bleed of dry air being expanded across a purge orifice, and the cycle switching every few minutes.

In a single tower design, dry air to the application is stored in a receiver downstream of the dryer. This tank must be sized to maintain the air system with an additional volume for purge air. Then, when the desiccant bed is ready for regeneration, the compressor is unloaded and the desiccant bed depressurised. Purge air is then expanded across a purge orifice and counter-flowed through the desiccant bed in a short cycle, with purges approximately every 90 seconds.

Purge air requirements for twin tower driers are typically 15% that of rated inlet flow, while those for single tower driers can be 15—25 % of rated flow. Single tower dryers are best where space is at a premium. They are also around half the price of equivalent twin tower models, although operating life expectancy is much shorter.

Either way, dew point suppression is the key measure of performance – more important that the air temperature rating. For every 7°C increase in temperature, air's ability to contain water vapour doubles, so when choosing a dryer, you need to establish the dew point for the application and then reduce it by at least 5—7°C below the lowest ambient temperature.

So much for choice of filtration systems; the final part of the equation is maintenance. The schedule for a single tower dryer typically calls for the desiccant to be replaced every 30—45 days. Twin tower systems require less frequent attention to the desiccant, which is another important factor in applications where frequent maintenance interventions are undesirable or impossible. Likewise, filter elements in coalescing filters degrade over time, so must be replaced at intervals before they are compromised.

John Hill

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