Proper sampling of hydraulic oil for preventive and proactive maintenance

The analysis of hydraulic oil is key to achieving oil cleanliness and a pivotal part of proper preventative and proactive maintenance programs. And the obvious first step of oil analysis is the retrieval of an oil sample. How one goes about this retrieval can affect the results of the analysis, potentially skewing the data to such a degree that the entire analysis is useless. So, before you take the first step of, you need to know how to get the right sample.

Sampling equipment

Sampling oil properly requires certain tools and knowledge. Without the right equipment, it may be difficult, or even impossible, to get consistent samples that can be trended to uncover actionable insights into the condition of the machine and the oil itself.

Sample extraction tube

If you are bringing oil from the valve or sump to the sample bottle, you should be using a sample extraction tube. If it is used with a valve, the tube will have a probe or adapter fitting on the end. Sample tubing is often made of low-density polyethylene. You can purchase tubing in bulk and cut to length as needed. Tubing comes in various sizes, most commonly 3/16, 1/4 and 5/16 inches. Microbore tubing with a diameter close to 2 millimeters may also be used in certain precision or hard to reach applications.

Vacuum pumps

Extracting oil without contaminating the sample or introducing contamination into the rest of the machine can be difficult without the right accessories. Vacuum pumps are used to extract oil samples from pressurized systems not equipped with sampling valves.

To set up the vacuum pump assembly, cut a piece of extraction tubing long enough to reach halfway down into the vertical oil level height in the compartment you intend to sample. If you are sampling from a valve, the tube must be long enough to reach the valve. On the other end, insert the tubing about 25 millimeters through the knurled knob on the vacuum pump. This is the pump location where you'll screw on the sample bottle below where you tighten the knurled knob to grip and seal the tube (do not overtighten).

Make sure your vacuum pump accepts the size of your tubing. The bottle should be threaded tightly onto the pump to achieve a vacuum-tight seal. It's best practice to place each bottle in a zip-lock sandwich bag (see the previous link) in advance to restrict particle ingression from the ambient air and dirty hands during sampling. Once the pump is assembled, follow the proper method for drop-tube vacuum pump sampling or valve and tube-adapter sampling. It's important to note that you should change the tubing each time you draw an oil sample to prevent cross-contamination.

Oil sample bottles

Choosing the correct oil sample bottle depends on the application and planned oil tests. Before selecting a sample bottle, you'll need to consider features like bottle size, cleanliness, and material. Bottles are typically made of materials in three categories:

Opaque plastic

Opaque plastic, like high-density polyethylene (HDPE), is one of the most common bottle materials on the market. You should try to avoid this material because it's difficult to visually inspect the sample due to its opaqueness (similar to a plastic milk jug). A less opaque polypropylene is also sold.

 Polyethylene terephthalate (PET)

This type of plastic is completely clear and compatible with most types of lubricating oil and hydraulic fluids, including synthetics. Other clear plastics, including polyvinyl chloride (PVC), are sometimes used.

Glass

Glass bottles are more expensive, heavier and come with the risk of breaking. Glass bottles can be cleaned and reused multiple times, and their cleanliness usually exceeds that of plastic bottles.

Speak with the lab to ensure you're using the correct bottle size for your sample. Bottle size is based on the type of fluid and the types of tests the lab will run. Most standard oil tests require the sample to be taken in a 100- or 120-milliliter bottle. Sometimes the test requires a 200-milliliter or larger bottle.

Finally, you'll want to confirm that your bottle meets ISO cleanliness standards to ensure the bottle doesn't add a reportable amount of contamination to the sample. Again, cleanliness depends on the type of test to be conducted and the objectives. Generally, the sample bottle should have a specific cleanliness level of two ISO codes cleaner than the target cleanliness objective. ISO 3722 provides a guideline for bottle cleanliness testing. The following cleanliness categories are frequently applied according to their contribution to the particle count:

• A clean bottle requires fewer than 100 particles greater than 10 microns per milliliter of bottle volume.
• A superclean bottle requires fewer than 10 particles greater than 10 microns per milliliter of bottle volume.
• An ultraclean bottle requires fewer than one particle greater than 10 microns per milliliter of bottle volume.

Location

While one may assume that, since the oil inside a machine is all part of one body of oil, the sampling of oil, regardless of sample location, should provide consistent data. This is not the case, however. The location from which an oil sample is collected can heavily influence the results of oil analysis.

A sample collected from an area in the machine where build-up and settling occur (the hydraulic reservoir’s drain port for example) will yield a higher concentration of debris than a sample pulled from elsewhere like the middle of the reservoir (which will provide a diluted sample).

Ideally, in hydraulic systems, an oil sample is pulled from the return line before the oil reaches the return filter. This location provides the best representative level of wear debris; the concentration of contamination becomes diluted when the oil reaches the tank.  Secondary samples, taken from oil that has passed through the filter or other major components, provide a thorough look at the system, allowing debris sources to be accurately pinpointed.

Turbulence and fly-by

Highly turbulent areas make for good sampling locations. Sharp bends and elbows in piping create turbulent oil flow; sample valves located in these areas will provide data-rich samples. Sampling valves placed on long, straight sections of piping may not provide accurate samples. Samples from such valves may be the victim of particle fly-by, reducing the particle concentration in the collected oil.

When circumstance allows, sample valves should be placed downstream from wear-prone components and some distance away from ingression areas.

Testing

Oil analysis can include additional testing aimed at determining the mode of wear. Analytical ferrography, for example, magnetically separates particles, allowing for the analysis of metal particle contamination. Such analysis can provide clues to the nature and cause of the wear.

Testing is also useful to determine the cleanliness of a regenerated oil. Oil that is regenerated using SKF RecondOil’s Double Separation Technology is tested to ensure cleanliness. Results of this testing often reveal the oil is cleaner after the process than when it was new.

Conclusion

Improper sampling techniques can lead to inaccurate data. Unless oil is sampled correctly, it is impossible to take an efficient, proactive approach to lubrication. Without the ability to trend data, root causes of machine failure will go undetected until a failure actually occurs. Once sampling procedures are aligned with best practice guidelines, the resulting data will improve decision quality across your entire preventative or proactive maintenance program.