Hydraulics & Pneumatics January/February 2022

HYDRAULICS 18 HYDRAULICS & PNEUMATICS January/February 2022 www.hpmag.co.uk Is my hydraulic fluid really that clean? Clean hydraulic fluids are essential if operators of hydraulic equipment want to achieve trouble-free operation, long component and system service lives and the resulting reduction in operating costs. The good news is that these fluids have got progressively cleaner over the years – and are now some 200 times cleaner than they were just 35 years ago. The effects of contamination on lubricated systems have been publicised for decades. So, every professional should now be aware of these effects and hopefully be operating cleanliness management programmes. The sharp increase in cleanliness levels Clean hydraulic fluid – the non-negotiable It is essential to maintain high levels of fluid cleanliness in hydraulic systems if equipment is to achieve its projected lifespan and reliability levels, writes Mike Day of CMS Consultants. is underlined in Figure 1, which shows the results of surveys, with cleanliness represented by the ISO 4406 system [ 2 ]. (Note that decimal ISO codes have been generated to give better resolution). As the systems’ life and reliability are proportional to the cleanliness level, these aspects should have improved too. The data from the late 1990s approached stabilisation, but this was inconsistent as progressively more practitioners implemented some form of cleanliness management. The problem lay with the use of sample bottles in the analysis – creating errors and effectively limiting the minimum contamination level that could be measured, The cause is the amount of extraneous contamination generated in the process. Although the use of proper procedures can minimise this input, it can never eliminate it. Clearly, the cleaner the fluid the more significant the error is – which makes the interpretation of trend data almost impossible. The use of on-line instruments – such as automatic particle counters (APCs) that are connected directly to the system being monitored – can eliminate these errors and measure the ‘true’ cleanliness level. The integration of Cloud-based data acquisition systems, such as Reynolds Contamination Control’s (RCC) The Link, provides immediate notification of any departure from the control value of any of the measured parameters so that, if required, corrective action can be taken, and the risk of potential damage minimised. The ‘purity through science’ mantra of RCC’s technical director Wayne Hubball is a formula that others would do well to emulate. Message in a bottle – but how many are unaware of errors? While knowledge of errors resulting from using sample bottles is not new – with some practitioners aware of the problems for at least 40 years. Pioneering work was performed by Tampere University of Technology (TUT) in the 1980s when they made on-line measurements on some 133 systems and took simultaneous bottle samples for analysis in the laboratory [ 3 ]. The results of this project have been used to examine the error (ie the difference between the on-line and off-line data) and this is seen in Figure 2. For clarity, only the data at 6µm(c) is considered. It is seen that the error increases as the fluid cleanliness gets better and at the cleaner levels the dirt added in the sampling and analysis process completely ‘swamps’ the data, The TUT data showed that off-line data stabilized at around ISO 16/14/11. Thus, at ISO 6, for instance, the generation rate of the system could increase by eight times (3 ISO classes) and this will not be seen if sample bottles are used. If data in these areas of cleanliness is used for predictive maintenance, then on-line analysis is mandatory. The cause of this error, as stated earlier, is the number of stages in the sampling and analysis process and this is seen diagrammatically in Figure 3. Although these errors can be minimised with approved procedures, they can never be eliminated. Furthermore, these errors are random, and it’s impossible to evaluate them so that they can be subtracted from the result. With a permanently mounted on- line APC there are no stages and with portable units there are only two – Figure 1: Development of Hydraulic system Cleanliness.