Assessment starts with a sample
Regardless of the details of any contamination-monitoring program, its usefulness will depend on fluid sampling. Fluid samples must accurately represent the condition of the fluid within the hydraulic system. This means that the sampling technique and devices, as well as the container, must not contaminate the fluid sample.
The point from which samples are extracted should be determined by the information desired from the sample. For example, sampling fluid from a system’s pump discharge line will likely produce different results from a sample taken from a return line. Fluid from the pump discharge line is more likely to contain pump wear debris than fluid from a return line because filters would have captured pump wear debris before it would reach a return line.
Experts advise, however, that samples taken from the reservoir usually are the most unreliable. First, because a reservoir acts as a storage device, its contents have accumulated over a relatively long interval, whereas fluid from a hydraulic line is more representative of conditions at the time the sample is taken. Second, most reservoirs are designed to minimize turbulent flow so contaminants can settle to the bottom and air can rise to the top. This makes it difficult to obtain a sample with a representative concentration of water and other contaminants.
Fluid analysis techniques
Once fluid samples have been obtained, any of several methods can be used to analyze the size, concentration, and nature of the contaminants. The most common analysis techniques for hydraulic systems are particle distribution, gravimetric, ferrographic wear debris, proton induced X-ray, and water content.
Each of these tests produces different results according to the type of information desired. Therefore, they should not be viewed as competing technologies. Rather, the more tests that are conducted on a sample, the more knowledge that can be gained. But no matter which technique is employed, obtaining a pure and representative sample is essential to achieving accurate results.
Particle distribution summarizes the number of contaminant particles classified by size for a sample. Automatic particle counters have gained wide acceptance for this previously time-consuming, laborious task that produced inconsistent results. The widespread use of particle counters is a testament to their ease of use and consistent reliability. Technicians often use them at manufacturing and maintenance facilities.
Gravimetric analysis summarizes the total mass of solid particles above a given size for a specific volume of fluid. Results are reported as mass density, usually mg/l. Unlike particle counting, gravimetric analysis quantifies only solid particles, not water.
But gravimetric analysis does not indicate size distribution, so a sample may contain 25 mg/l of solid particles greater than, say, 5 µm. It does not indicate what percentage of particles is greater than 10 µm and how many are greater than 15 or even 25 µm. As with particle counters, technicians often use gravimetric analysis instruments to monitor contaminants in hydraulic systems.
Ferrographic wear debris analysis quantifies wear debris (primarily metals) in a fluid sample. Because the most highly stressed wearing parts of machine components are made of steel, wear debris usually are influenced by magnetic fields. Ferrographic analysis can be used to evaluate a system’s wear mechanisms, assess the severity of wear, and identify the predominant materials being worn away.
Proton-induced X-ray emission (PIXE) summarizes the elemental composition of solid contaminants and wear debris in a fluid. After the fluid sample is exposed to a proton beam, a computer interprets the results of the test by producing data on the entire spectrum of elements in the target, not just a single element. Neither particle counting nor gravimetric techniques can differentiate between foreign contaminants and wear debris, which makes PIXE useful for gaining insight into the nature of particles found in a fluid.
Water-content analysis determines how much water is present in a base fluid. Next to particulate matter, water, by far, is the most damaging contaminant in a hydraulic system—or any oil-lubricated system, for that matter. Higher concentrations of water in hydraulic oil accelerate wear, fluid degradation, corrosion, and reduction in service life. Therefore, once the amount of water present in a hydraulic fluid has been found, the challenge becomes determining how much can be tolerated.
The test itself uses a solution that conducts electrical current based, in part, on the amount of water contained in a sample. Measuring the current and its duration indicates the water content in the base fluid. The test can be accurate to within 10 ppm, but additive packages common to hydraulic fluid tend to produce less detailed results.