Simplifying Oil Sampling

Oil Quality Management, as we all know, is critical to the overall maintenance of machines. I have spoken about the criticality of real-time oil condition monitoring, and in this post, I’d like to touch upon the basics of the first step of Oil Quality Management: Oil Sampling. Oil Analysis processes are taken for granted and it brings me to my favorite statistic related to Tribology, courtesy the ever-reliable Noria Corporation: That 54% of all machine failures are due to poor lubrication. The chain of Oil Analysis typically begins with the most common and age-old practice of oil sampling. However, it has become so common today, that we often tend to forget the very purpose behind collecting oil samples before sending them to the labs.

It is imperative to reiterate the core objectives of Oil Sampling:

  1. Oil Flowing through Turbulence: To get a realistic picture of the quality of oil flowing through your critical equipment at peak operating conditions
  2. Maximize Data Density: This is important so we can glean as much as information about the oil flowing through the system as is available and we don’t lose out any critical pieces of data because of poor sampling techniques.
  3. Minimize Data Disturbance: Ensure oil samples don’t get contaminated during the process of sampling so that we get a realistic, representative sample without any distortions.

Location of Oil Sampling

  1. Elbows/Sharp Bends: If we want to sample oil when in peak turbulence, we cannot take oil when it is flowing through a straight line. Instead, we should look at sampling oil when it is rolling, gushing and turning through the flow line. Oil samples collected in straight-flow lines are not likely to be a representative sample as a lot of the contaminants would get missed out owing to a phenomenon called as particle fly-by. The figure attached below ( Courtesy Noria Corp) is a good example of how valves located at the elbow bends of the pipe give us oil samples that have not lost any of the particulate contaminants during collection and is most representative of peak-turbulence oil flow.
  1. Away from filters: Why Oil Sampling should be upstream from filters? (Plant Reliability Web) One of the challenges with offline sampling is collecting the debris and contaminants before they are filtered out by filters and separators. The accuracy of the wear particles captured in the sample depends on when a sample is taken (see below Figure Courtseyy Noria Corp). Wear debris is lost in bursts over a period of time and is quickly filtered out by the filtration system. In a circulating system, contaminants, ingress and debris particulate matter changes the character of the oil much more frequently than typical oil sampling collections. So to ensure that particulate matter of the oil is captured, it is imperative that oil sampling collection ports are located upstream of any filters and separators.
  1. Multiple Sample Ports: In complex types of equipment, such as rotating equipment like Motors, Gearboxes, compressors, having just one sapling port is inadequate to get a holistic picture of the condition of your components or the health of your oil. Typically, it is common to have one sampling point; what people call the “primary sampling point” on the return line of a hydraulic system. However, relying only on one sampling point doesn’t allow us to pinpoint exactly at what point oil is getting contaminated. For example, if we have a circulating system of the motor, gearbox and a compressor, it would be advisable to have a couple of sampling ports for each bearing on the motor, one port on the bearing of the gearbox to monitor the oil flowing through it, and one on the return line of a compressor. Running separate analysis on each of these samples would enable us to figure out exactly which component is faulty, or at what point does the iron/wear particle content of the oil shoot up.
  2. Return Lines/Drain Lines: As much as possible, oil sampling ports should only be located downstream of wearing components, ie: ideally after it has gone through the entire system and flowing back to the tank through the return drain line. Oil collected at this point will have the most representative levels of contaminants.

Method of Oil Sampling

One of the common ways to collect oil samples is to have a drain plug from the bottom of the oil sump. Personally, I think this is not advisable for the following reasons:

  • Sediments, Wear Particles and Contaminants often tend to settle at the bottom of the tank, and the oil that is collected in this way through a drain plug will have heavy concentrations of these contaminants, such oil samples are in no way truly representative of the oil flowing through the system. The drain plug sampling method should be avoided if at all possible.
  • The draining plug method is also a slower, wasteful way of collecting the oil that often leads to unhygienic, unsafe surrounding areas with spillages and leakages. In addition, having a drain plug at the bottom of the oil sump defeats the purpose of collecting oil when from a point of system turbulence.

The most reliable way of collecting oil samples would be to have a fixed sampling valve installed at different collection points. Often, these valves come ready with the gearboxes and/or can be installed at the time of any routine plant shutdowns/when oil is drained out for oil changes, etc. This practice ensures you get good, consistent oil sampling every time, that is representative of the oil flowing through the system at operating conditions.

Summarizing:

  1. It is important to have oil samples taken when your oil is flowing through peak turbulence during normal operating conditions
  2. Oil Samples have to be collected when oil is flowing through bends and sharp edges to ensure no particulate fly-by occurs and we get a representative oil sample
  3. Multiple oil sampling ports are critical to ensure root cause analysis for oil degradation can be apportioned to each component of rotating equipment (Motor, Gearbox, Compressor).
  4. Oil Samples must be collected upstream of filters or separators so that oil collected is not devoid of the contaminants and particulate matter that are so easily lost once they go through filters and separators.
  5. Oil Samples must be collected from return/drain lines before it reaches the sump tank.
  6. Permanently fitted valves at critical sampling ports must be preferred over having wasteful drain plugs for the process of sample collection.