Proper machinery lubrication requires a clean, effective supply of lubricating oil. Oil can become full of wear products, become contaminated by water or chemicals, or otherwise lose the properties it was chosen for in the first place. Preventing this from happening, and thereby helping to ensure a healthy lubrication program, requires sampling and analyzing of a system’s oil supply.
One of the most important factors in obtaining a representative oil sample is the sample location. There are several important factors to consider when determining a good sample point.
Turbulence – The most representative samples are drawn from areas of turbulent flow, where the oil is churning and mixing in the piping. This can be best accomplished by locating the sampling point at sharp bends or 90 degree angles in the pipes.
Downstream of Components – To get a clear picture of component wear, sampling points should be located downstream of the components. Once the oil reaches the tank or sump, the concentration of these particles will become diluted.
Unfiltered – Filters and strainers remove contaminants and wear products from the oil, so obtaining a sample after the oil has passed through a filter, separator, or settling tank will remove important data from the sample. Sampling upstream of these components will yield a more informative sample.
Drain Lines – Drain lines are useful sample locations, as they are downstream of components and usually upstream of any filters. Depending on the system, though, drain piping may not be completely full of oil. To minimize the amount of air contamination, the sample can be drawn from the underside of horizontal piping. Oil traps, such as goosenecks, can also be installed to ensure oil is concentrated at the sample location.
When sampling lube oil, there are two primary goals: acquiring a sample containing the maximum amount of information possible and that the sample gives a clear, uncontaminated, and uniform picture of the actual state of the oil. There are several sampling methods used to meet these goals, the choice of which will depend on the type and design of the system being sampled.
Pressurized Line Sampling – Sampling a pressurized and working system, often referred to as live sampling, can yield a highly representative sample in a quick, clean manner. Pressurized samples are usually drawn from small valves or quick-connect fittings and collected into sealed or capable sample bottles.
Vacuum Sampling – Sampling unpressurized system piping, such as a drain line, can be accomplished by the use of a vacuum pump. These pumps have adapters that attach to valves or fittings, allowing the sample to be drawn into a sample bottle.
Non-Circulating Systems – Non-circulating systems, like many gearboxes, have no supply or return piping to use for sampling. In this case, the sample must be drawn from the sump or casing.
Drain Plug Sampling – Sampling can be done by removing a drain plug and filling a sample bottle, but this method is generally avoided because it can lead to unrepresented results from sediment settled at the bottom of the casing. A more accurate method of drain plug sampling involves inserting a tube through the port to the center of the tank, where the fluid is continuously mixing.
Drop Tube Sampling – Drop tube sampling involves inserting a tube into a fill or dipstick hole and pumping the oil into a collecting bottle. This method can yield good results, but also poses several challenges and risks, such as difficulty controlling the movement and location of the tube and contamination of the tube itself.
Consistency – In order for oil samples to produce any meaningful information, they must be taken using the same methods and equipment every time. Sampling and analysis procedures should be well documented and followed by all personnel.
Testing of oil systems has become a vital concern in industry. The amount of contaminants in oil directly relates to the life span of a piece of equipment. Wear particle analysis is the preferred method used to mark the trends of lubricating oil system contamination. Wear particle analysis and oil testing will extend the life of machinery well beyond the limits imposed by preventive maintenance.
Wear Particle Analysis – Wear particle analysis is performed by studying the shape, composition, size, and quantity of particles separated from lubricating oils. This technique provides usable information regarding the wear condition of equipment. There are two methods used in wear particle analysis; the first is the regular monitoring of total solid content in the lubricating oil, and the second is the actual analysis of particles in each oil sample.
Sampling Locations – A typical lubricating oil system contains the following components: a machine, drain line, sump, lube oil pump, and a filter. Samples should be taken from the drain line, sump, or prior to the filter to ensure the oil has reached equilibrium with particles of all sizes.
Significance of Wear Particles – The concentration and size distribution of wear particles reveal considerable information about the condition of lubricated wearing surfaces within a machine. As particle concentration and size increase, the wear process progresses from a normal operating condition to incipient failure, and finally to catastrophic failure.
Sampling Considerations – If sample lines and valves are not flushed properly, large numbers of old particles will find their way into the oil sample. The results of wear particle analysis are no better than the oil sample itself, since the lubricant must contain a representative selection of wear particles.
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