Plant life matters01 December 2008

Although the cost of components - such as rolling element bearings, rotors and fasteners - in hydraulic pumps is usually very small, compared to the list price of the pumps themselves, the cost of stopped production and any consequential losses resulting from component failure is almost invariably significant.

On some processing plants, downtime can equate to hundreds of thousands of pounds per day. Lost production in a paper mill, for example, equates to around £15,000 per hour. Yet while every manufacturing organisation has a maintenance engineering department trained to deal with mechanical failures, all too frequently, because of time and resource constraints, its activities are reactive, mainly dealing with plant problems as they occur.

Given the sheer cost of failure, the under-investment in predictive maintenance, even in some cases scheduled preventive maintenance, is astonishing. Particularly when the condition monitoring technologies capable of supporting predictive maintenance regimes - such as acoustic emissions monitoring, vibration monitoring and thermal imaging - are now relatively inexpensive and proven to protect plant and machinery of all kinds.

As Ian Taylor, business development engineer for CNES (Corus Northern Engineering Services) plant condition monitoring business unit, says: 'At Corus and for external customers, CNES' plant condition monitoring team monitors hydraulic pumps, fans, compressors and blowers, using every technique available - including patrol monitoring, and fixed and portable condition monitoring equipment. Inside Corus, for example, we monitor all pumps - some every week, but each pump at least once per month. Where the pump is critical to production, such as the hood cooling pumps on our BOS [basic oxygen steelmaking] plant, we have installed fixed condition monitoring systems that monitor the plant 24/7.'

And it's not just about preventing unplanned downtime. Condition monitoring also prevents maintenance teams replacing components unnecessarily, and thus risking introducing new and unrelated problems. As Taylor observes: 'Maintenance teams should be using condition monitoring technologies to predict when failures are likely to occur, so that they can plan for replacement during scheduled production shutdowns. In too many companies, components are simply replaced on a time basis, rather than on a condition basis, because maintenance considers this to be the safest option. The problem is that, whenever there's human intervention, problems can occur.'

Pinpoint precision
And, given that intervention could be unnecessary, it is a needless risk - not to mention wasted time and money. So what should you do? The advice is that, when it comes to monitoring the condition of pumps and other hydraulic systems, a number of techniques are available. They can be used either individually or together to protect assets deemed critical for whatever reason - ideally according to a production and business risk assessment.

'As most pumps run at a steady load and speed, patrol monitoring, using portable vibration analysis equipment, is usually the most effective condition monitoring technique,' advises Taylor. 'However, it really does depend on the pump design. Acoustic emissions monitoring may be more effective, if the pump speed is less than 80rpm, or if the technician wants to monitor the condition of plain bearings inside the pump or motor.'

Both techniques have been around for a long time. Vibration monitoring is good at identifying a number of potential pump problems, including misalignment and coupling issues, and mechanical looseness inside the pump or around the baseplate - including loose joints or fasteners. It can also successfully monitor the condition of rolling element bearings, and reveal cavitation issues and erosion of rotors, which it will see as imbalance. However, for hydraulic systems that rotate at less than 80rpm, or operate under fluctuating load conditions, or only move through part revolutions, it is more difficult to get meaningful data. So CNES recommends its own, patented acoustic online condition monitoring system, Aquilla AE Pro - although other suppliers also offer acoustic emissions monitoring tools (go to the Plant Engineer directory at www.plantengineerdirectory.org.uk vibration and fatigue testing).

Looking at typical plant problems, pumping of heavy, viscous fluids, such as foodstuffs or sludge, can cause damage to rotors, which, in turn, can result in a pump going out of balance. Similarly, rotor deterioration - caused, for example, by pumping corrosive liquids - can also lead to an out-of-balance situation. Both of these can easily be detected by vibration analysis systems, as can wear of gear teeth on gear pumps, and it's worth noting that problems with standby pumps can also be checked, using the technology.

It's an aside, but Taylor points to the classic problem caused by two identical pumps, one duty, the other standby, being operated side-by-side on a single, common bedplate. 'What happens is that vibrations from the duty pump can cause bearing problems on the standby pump, referred to as false brinelling. So, once the standby pump is switched on, it also quickly fails, resulting in two pumps out of service. To prevent this, the two pumps should be switched over regularly on an 80?20 or 70?30 duty/standby ratio,' he suggests.

Moving on to acoustic emission monitoring, key benefits of this equipment are its high sensitivity to machine faults, and immunity to audible noise and low-frequency background vibration. 'The problem is that many engineers are not fully aware of how acoustic emission monitoring systems can help them reduce plant maintenance costs and improve machine availability,' warns Taylor 'Also, many companies simply do not possess the skills inhouse to interpret the data from acoustics emissions monitoring - so they continue to use vibration or other devices.'

Best advice here is to understand that condition monitoring using acoustic emissions need not be difficult and certainly isn't new. The technique has been around since the early 1990s, having been developed at various centres, including Rolls-Royce, to detect high frequency stress waves generated by strain energy released during material crack growth, plastic deformation or phase transformation. Systems are available using surface-mounted, portable or fixed transducers that detect stress waves in the 25kHz to 1MHz frequency range, and companies such as Holroyd Instruments claim easy interpretation of results.

Refreshing the parts
Correct selection of lubricant or hydraulic fluid is important in the battle to reduce component wear and associated energy costs. Plant wear and the effectiveness of current lubricants can be empirically determined by analysing the level of degradation and debris present, which, in turn, can help determine the correct lubricant and oil change periods.

CNES' Fluid Power Technology division, which has years of experience in designing and operating hydraulic systems, often for harsh environments, is one company offering advice and guidance. The company suggests that around 75% of all hydraulic system failures are due to lubricant contamination, and says regular analysis is essential to establish fluid condition and ensure optimum performance, reliability and, ultimately, plant life and cost.

CNES also has experience on systems for water-based fluids, where there is a high risk of fire - including on servo systems.

SOE

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