Parker Hannifin's European Hydraulic Filter Division offers the following advice on condition monitoring in hydraulic systems.
Although the scale of the problem caused by the contamination of hydraulic oils and lubricants by moisture and particles is yet to be fully appreciated, it is widely accepted that it is a significant problem in many applications. Indeed, it has been estimated that over 85 per cent of hydraulic system failures can be attributed to contaminated fluid. Such failures can be catastrophic in the extreme or, at the very least, costly in terms of downtime and equipment repair.
Naturally, the quality of hydraulic oil can gradually degrade over time due to the operating environment and demanding conditions. However, it is the identification of contamination before it reaches the point where machine performance starts to decline that is crucial in preventing complete machine failure. And gradual nature of this problem often makes it difficult to detect.
Additionally, contamination of the hydraulic system can stem, paradoxically, from the very areas that the oils are intended to protect; surface wear from rotating shafts and bearings is almost inevitable in machinery, no matter how efficient the lubrication. As the metallic wear particles gradually build up in the system, the oil starts to lose its lubricating properties, so the machinery becomes more susceptible to wear, the oil becomes more contaminated and less able to lubricate components, and a breakdown can result.
Similarly, the problem of moisture ingress into hydraulic systems can gradually occur. While particle contamination is normally generated by wear within the system, moisture generally enters the system through worn or perished seals and gaskets. The main effect of moisture build-up in the system is the growth of bacteria and fungal deposits. These can cause deterioration in oil quality and lead directly to acid corrosion and oxidation of bearing surfaces and rubber seals, ironically exacerbating the original problem still further.
Despite these factors, the monitoring of hydraulic oil is rarely carried out effectively, with occasional oil changes being seen as the easiest - but not necessarily the cheapest - solution to the problem of degrading hydraulic fluid. However, if end users are to minimise their operating margins and comply with the latest health and safety legislation, this approach needs to be replaced by a simple but effective maintenance strategy in which condition monitoring forming the backbone.
Contamination in hydraulic oils has traditionally been measured using methods such as pure and bright and gravimetric Millipore, both of which require samples to be extracted from the operating system for laboratory analysis. Each of these methods is time-consuming, has the potential for external contamination to corrupt the sample, and produces only historical rather than real-time data; problems within a process can often remain undetected while laboratory results are being collated.
Now, however, to provide an alternative to conventional condition monitoring practices, a range of innovative, low-cost portable analysers is available, ranging from simple handheld devices through to online instrumentation with communications to other control and instrumentation networks. Both offline and online systems can analyse oil content either dynamically in situ or via static sampling points. Data is analysed at the point of recording, giving an instant reading of oil quality. It can also be downloaded for subsequent analysis - such as trend analysis – on a PDA or laptop. Generally, data generated by these systems can be downloaded for trend analysis to detect subtle changes in oil condition before any abnormal wear becomes a problem.
How they work
Most of these devices use laser-based technologies for particle detection. Typically as an oil sample is drawn into the instrument it passes through a precisely targeted light beam aimed at integrated photo diode sensing cells. As particles pass the photo diode sampling point, the reduction in light intensity transmitted is proportional to particle size. Instruments such as Parker's icountLCM20 portable particle counting device can measure particles down to a few microns in size, while recording both quantitative and qualitative data on the degree of contamination. A complete test cycle can normally be completed in just two minutes.
Also exploiting recent advances in laser technology is the latest generation of compact online detectors, such as Parker's icountPD, which have been designed to monitor the cleanliness of hydraulic oil continuously. Once installed, these devices provide warnings of low, medium or high contamination levels through simple onboard LED indicators or via links to remote control systems.
The icountPD, for example, displays data that is updated every second, with readings conforming to the ISO 4406 method for coding the level of solid particle contamination of hydraulic fluids. The moisture sensor reports the saturation levels of the fluid passing through the instrument's sensing cell, giving a linear output between 5 per cent and 100 per cent saturation. In normal operation, instruments like the icountPD are essentially monitors but, with a full range of outputs including 4-20mA, 0-5V and RS232 capability, they can easily be fully integrated into PC- or PLC-based control systems that can automatically activate oil filtration units, for example.
Extended operating life
Furthermore, these devices are compact, robust and simple to fit during routine maintenance. Most importantly, they allow plant operators to predict the exact point in time when hydraulic fluids will need replacing; often this is long after the recommended service interval, making it possible to prolong the operating life of oils. Additionally, it enables engineers to ensure that the health, and therefore the operating efficiency, of hydraulically operated systems is maintained at optimum levels; similarly, by detecting increasing levels of contamination at an early stage it is possible to investigate the causes and take remedial action long before a problem or possible system failure occurs.
The latest generation of handheld devices are a far cry from traditional equipment of this type, which was generally expensive, bulky and often inaccurate. The new high-performance instruments have been designed to detect and measure the dielectric constant of oil, highlighting condition changes resulting from the ingress of water, particle contamination, metallic content or oxidation. In addition, while few of the earlier models had the necessary certification to allow them to be used effectively in hazardous or explosive areas, the latest equipment from the leading manufacturers is ATEX-approved.
Recent developments in condition monitoring technologies mean that engineers can receive an early warning of an impending system failure and take remedial action, thereby extending the operating life of machinery. Unlike traditional laboratory analysis, these new methods enable condition monitoring to be performed quickly, easily and accurately at the point of use, resulting in a positive impact on a company's profitability, reducing operating costs and optimising equipment performance.