The Importance of Sampling For Oil Analysis Success

Drew Troyer

All of our sophisticated oil analysis tools, techniques and diagnostic processes are meaningless if the oil sample fails to effectively represent the actual condition of the oil in service in the machine. Proper sampling procedures build the foundation of an effective oil analysis program. Without good sampling procedures, time and money are wasted, and incorrect conclusions might be reached based on faulty data. Both situations undermine the perceived value of, and confidence in, the oil analysis program.

Sampling and other oil analysis program procedures add important structure necessary to assure continuity and consistency of an oil analysis program, institutionalizing it within the organization. Too often, an organization develops a world-class oil analysis program that depends upon the presence of one or two key individuals for its survival. The resignation, retirement or promotion of these key individuals places the program at risk.

Moreover, ISO 9000 compliance requires clear procedures for all aspects of an organization's business. Institutionalized oil analysis procedures ensure that the organization's investment is protected and has continuity. This eases the task of training new people, enabling the growth of your oil analysis program. Developing solid sampling procedures is a great place to start.

Establishing effective, user-friendly oil sampling procedures helps to build the kind of oil analysis program that creates value through better machine-related decisions. Here, we discuss the objectives of oil sampling at a strategic level and outline the process of developing and maintaining solid oil sampling procedures.

Goals of Sampling
In general, we want oil samples to effectively represent the body of oil about which we desire information in order to increase the effectiveness of lubrication and machine decisions. Maximizing data density, minimizing data disturbances and selecting of goal-driven sampling frequencies make up the components of our sampling objectives.

Maximizing Data Density
Maximizing data density seems like a simple objective at first glance, but it is not so simple when you look deeper into the issue. Much depends upon the nature of the data you desire. For example, when one seeks information about the effectiveness of a system filter, he or she must collect a representative sample both before and after the filter. The difference between the two samples is reflected in the differential particle count across the filter.

The analyst then recommends the filter be changed or retained in service depending upon the outcome of the analysis. In this instance, maximizing density of information requires the analyst to obtain two representative samples from specific locations to calculate the information of interest. Other oil analysis objectives require different sampling procedures. Therefore, the objective should drive the sampling procedure. More often, the lack of good sampling techniques and hardware limits the options.

The filter performance assessment example applies to measuring particles, a transient property of the oil. Transient properties, like wear debris and contamination, change depending upon the location from which the sample is collected. By contrast, homogenous properties such as viscosity and acid number (AN) tend to remain constant throughout the population of the turbulent oil. In general, transient properties pertain to equipment health and contamination, while homogenous properties pertain mostly to oil and additive health.

Simple oil condition assessment or oil change decisions can be made with any reasonable sample collection procedure from a turbulent fluid zone. It is more difficult to maximize density of transient properties (for example, particle count, wear particle concentration and moisture contamination) to make important machine condition decisions or to ensure effective contamination control. For the same reason, effective sampling of transient properties is essential if reliable and trendable data is to be expected.

Minimizing Disturbances
Data disturbances generally refer to interferences associated with gathering, preparing or analyzing the sample. Failure to sample from a machine that is running in application is a common source of disturbance. Ideally, the machine will be operating at normal load and speed in its typical environment. Otherwise, when the sample is drawn, particles and moisture settle, causing data disturbance. Likewise, using dirty sampling utensils and bottles, failing to flush the fluid pathways, and exposing open bottles and caps to the ambient environment all disturb the quality of the data.

Before analyzing the sample, it must be returned to its "like running" condition, then properly analyzed using appropriate techniques. Typically, preparing the sample for analysis requires violent agitation. Failure to effectively return the sample to "like running" condition can result in dramatic over- or understatement of the actual parameter of interest. Additionally, the analysis technique selected can disturb the quality of oil analysis data.

For instance, the selection of atomic emission spectroscopy to detect the onset of severe wear generation may yield flawed results because the target information relates to large particles while the selected technique is best suited for small particles or dissolved metals. Failure to select the right test and method for preparing the sample and running the test undermines the quality of the information produced.

Sampling at the appropriate frequency has great bearing on the overall effectiveness of the condition-monitoring program. Some oil analysis parameters shift quite rapidly, especially those that reveal machine condition, such as contamination and wear debris monitoring. Properties that reveal the lubricant's condition tend to move more gradually, but there are instances where they too can change dramatically.

Effective Sampling Procedures
The process of building effective sampling procedures should begin with an assessment of your goals for oil analysis for each machine of interest. If your objectives end with a desire to schedule condition-based oil changes, the sample location and sampling procedure can be relaxed because the properties analyzed are homogenous. However, if your objectives include controlling contamination and detecting and analyzing wear debris, the sampling location and procedure become critical.

Sampling procedures will vary in form from organization to organization. At a minimum, your sampling procedure should include the following elements:

  • sampling goal

  • sampling location

  • sampling frequency/scheduling

  • material requirements

  • sampling method

  • potential source of interference

  • safety considerations

The goals associated with sampling from a given location should be clearly and concisely stated in the procedure for two reasons. First, people do better work when they have a clear understanding of the objective and the importance of the procedure. Knowledge of the purpose for the sampling program enables the individual to act locally while thinking globally. Second, we often sample complex systems from different locations depending upon the desired information. Identification of the objective related to a specific sampling location helps ensure that the sample will be properly drawn and handled.

The sampling location is the point from which the sample is to be drawn. Ideally, this location will be graphically identified with a system schematic and/or a photograph of the sample location and valve. The use of graphics and pictures eliminates confusion about where the sample should be taken. Where multiple sample valves are used, the use of pictures and schematics is more critical.

The frequency of sampling and oil analysis depends upon a number of factors previously discussed. The desired sampling frequency should be clearly noted on the procedure. It should also be noted whether the sample procedure pertains to a routine sample taken from the primary sampling point, or an exception sample taken from a secondary sampling point for the purpose of diagnostics.

Sometimes the task of scheduling the sampling job will be performed by the organization's maintenance management system, be it computerized or manual. Alternatively, scheduling might fall under the reliability team or the oil analysis team using their own computer program or manual system. The origin from which the sampling work order is generated should be noted in the sampling procedure.

The procedure should clearly identify, in plain language, the materials required for completing a quality job of oil sampling. Sample bottle, clean tubing, vacuum sample extraction device and lint-free cloth are among the items that should be identified as required. Failure to document the material requirements may result in unnecessary trips back to the shop or, worse, force the technician to take shortcuts to complete the job in the allotted time. Either situation is frustrating and undermines the effectiveness and efficiency of your oil analysis program.

The sampling process should provide clear, step-by-step instructions for extracting a sample. Naturally, the process will vary depending upon sampling location and technique. Where possible, use graphics or photographs to visually demonstrate the process.

By communicating some potential sources of interference with a given sampling method or location, the technician can be on the lookout for these pitfalls, reducing the likelihood that oil analysis results will be compromised. This becomes especially important when the sampling point or required process is less than ideal due to the location of the machine or operation-related restrictions.

Technician safety is first and foremost in any activity. Oil sampling is no exception. Work closely with the plant's safety engineer or manager to ensure that the safety of the individual is properly considered and to ensure that the procedure complies with all of the plant's safety regulations and requirements. Some things to consider include:

System pressure: Fluid under high pressure can be deadly if mismanaged. Take special precautions when sampling from or around high-pressure hydraulic systems.

Fluid toxicity: Some lubricants and hydraulic fluids may be toxic (for example, phosphate ester). When sampling these oils, clearly state the risks associated with exposure and the recommended remedy or treatment in accordance with plant policy or manufacturer's recommendations.

Hazardous environment: Some environments are inherently hazardous due to the presence of chemicals, heat, cold, radiation, open or exposed mechanical equipment, and high-pressure systems. Proper cautionary measures when working in these areas should be noted on the sampling procedure.

A strong argument can be made that developing and deploying effective sampling procedures is the most important component to achieving oil analysis success. The sampling procedure ensures consistency of data and instills confidence in the decisions made with oil analysis information. Some closing notes to ensure you are successful in developing and maintaining procedures for oil sampling best practices include the following:

Make the development of sampling procedures a team effort.
All the individuals on the team have meaningful experience and viewpoints that should be incorporated in the procedure.

Get necessary help from an expert.
Instead of reinventing the wheel, employ someone with the necessary expertise to support your effort. Be careful not to simply farm out this important activity. Outside support should provide guidance to the team, not replace it. Ownership is too important to not maintain internal involvement in the process.

Computerize the process if possible. Routinely update the procedures.
New and better methods for sampling are created every day. Be sure your procedures are living and evolving to incorporate these improvements. It might be wise to employ an outside expert to help keep you up-to-date and make necessary revisions. Likewise, your objectives change over time. The impact of these changes on your oil analysis program should be captured in the sampling procedure.

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