Breath of fresh fluid 09 December 2014

What bothers engineers concerned with pneumatics can be very different to those working solely with hydraulics.

Hydraulics and pneumatics: two branches of motive power and control engineering often lumped together for their superficial similarities – in terms of technology and application – yet quite dissimilar when it comes to some of the detail. Fundamentally, whereas the former relies on the properties of incompressible fluids, the latter thrives on its very compressibility. So, while many of the principles underlying, for example, valve and accumulator designs, may be comparable, their execution – in terms of materials, tolerances, pressures, control mechanisms, etc – is likely to be quite different.

That's why the selection of one technique over the other is generally (although not always) a clear cut engineering function-, scale- and/or cost-based decision. It's also why some of the issues that exercise practitioners in each of the disciplines can be so diverse.

Take maintenance: in hydraulics it's increasingly accepted that around 80% of failures on machinery can be traced back to contaminated oil, while in pneumatics, air cylinder problems might be caused by contamination, but equally side-load issues, lubrication, poorly synchronised cycling or operation beyond component limits. Then move over to air compressor systems, and plant engineers know that primary failure modes here concern the mechanical problems associated with any mid-large rotating equipment, as well as those of the usual ancillaries.

So it is that among those charged with inspecting and maintaining hydraulic systems, one of the ongoing debates concerns many practitioners' stubborn reliance on nothing more than component monitoring. Chris Gray, filtration product manager at Bosch Rexroth, is one that wonders why, if figures suggest that only 20% of unplanned downtime can be identified through component monitoring, so many maintenance engineers still put their faith in this fundamentally limited approach.

Oil analysis
"The truth is that oil analysis has the capability to prevent the need for expensive repairs, minimise downtime, detect early damage to components, optimise filtration systems and even provide environmental relief through the minimisation of wasted oil," he argues. "Surely then, oil analysis is a more reliable option as part of a modern day programme?" And, given the range of contaminants and their potentially devastating effects if left undetected, you can appreciate his thinking.

Gray points to three contaminant types, all identified by oil analysis. First are solids, such as dirt and dust particles, which can cause jamming, influence control behaviour
and prematurely wear components and/or clog filters. Second are insoluble liquids that can again lead to wear and corrosion, but also viscosity problems and potentially chemical reactions in the hydraulic fluid itself. And third are gases, such as air, which can result in foaming in the oil sequences, imprecise valve responses, energy losses, pump damage and, again, chemical reactions.

All have the potential to impact machine availability, and result in hefty maintenance and replacement bills. And for those wondering about the provenance of some of these contaminants, it's not just about those left over from the component manufacturing process and not cleaned out prior to installation in the system or machine. Once the hydraulics are operational, air sources in the manufacturing environment can entrain contaminants via nothing more complicated than piston rods, labyrinth seals and vents. Equally, wearing metal (caused by abrasion or erosion of components) can cause seal damage, chemical corrosion and oxidation residues.

Contamination speads
"A typical contaminant would be silica, which can get between the working surfaces or clearances on machinery components, such as a pump or cylinder," explains Gray. "Once contamination has occurred it will circulate between the two components, gradually wearing away the surfaces. As the surface abrasions enlarge, leakage occurs, leading to component failure, such as valve stiction." And he adds that, once in place, contamination spreads, setting up a "chain reaction" of particles via components such as cylinder rods. "Two particles become four, four become eight – yet, often, the solution is as simple as installing filter units."

That said, he acknowledges that the problem for many maintenance teams is time and resource. "While oil analysis can be incredibly effective as part of a preventive maintenance programme, it does take time and that takes engineers away from [other] problems they encounter every day." Hence, oil analysis schedules fall by the wayside as more critical issues take priority – although some of those would probably have been prevented had a rigorous oil analysis schedule been in place.

For Gray, those schedules would see machines being sampled at least quarterly, with contamination trends faithfully recorded, using the ISO 4406 standard cleanliness codes, which centre on particulates per 100 millilitres of oil. "Oil sampling does come with a small cost," he concedes, "but if a company has 100 machines working intensely, a simple oil analysis, costing as little as £15, has the potential to prevent a costly component fault or stoppage, which far outstrips the sampling cost."

Enough on that. Moving on to air, and specifically compressors, perhaps the most important development concerns the Ecodesign Directive 2009/125/EC, Lot 31. Back in March 2012, Netherlands-based consultancy VHK was tasked with carrying out a preparatory study to guide the implementation of energy-saving regulations, with a requirement to report in June this year. BCAS (the British Compressed Air Society) executive director Chris Dee explains that, to make this feasible, initial consultation resulted in splitting compressors into five distinct application groups, with the initial focus to be centred on 'standard' air compressors, comprising oil-injected screws/vanes and oil-lubricated pistons – the workhorses of industry.

"The statistical approach revealed a substantial 20–30% spread in the isentropic efficiency of these compressors, depending on capacity," says Dee, adding that, even so, their energy-saving potential was found to be "moderate". That said, despite Pneurop (the European association of compressors, vacuum pumps, pneumatic tools and allied equipment manufacturers) arguing that, if a regulation was introduced, energy benefits would be far lower than those available from a whole system approach, the European Commission disagreed. On 23 October, the EC Consultation Forum ruled that – like electric motors before them – 'standard' compressor manufacturers will be subject to EU energy efficiency regulations.

Says Dee: "The EU wants the regulation to be published by the end of 2015 and the first of the two stages to be effective from January 2018, with the second stage coming into force in January 2020. The expected result by January 2020 will be the removal of about 40% of the compressors that do not then meet the required efficiency levels within the scope."

And he adds: "Instead of focusing on effective measures to improve complete compressed air systems, manufacturers will be compelled to devote their engineering resources primarily to substitute the products banned from the EU. This will enable little or no progress towards achieving the BAT [best available technology] level and will inevitably impair their longer-term global competitiveness."

It remains to be seen what will happen to non-standard compressors and particularly those regarded by Penurop and BCAS (for 'low pressure' and 'oil-free air') as requiring further study.

You have been warned.

Brian Tinham

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