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Part 1 – Understanding the consistency of oil Sampling

Oil analysis has been one of the most widely used Condition Monitoring (CM) techniques to observe any changes occurring internally in the equipment. Typically, it is one of the most basic techniques utilized within the industry and is viewed as the first line of defense to detect any abnormalities. However, is there a need for oil analysis if the team employs other CM techniques which can also detect changes within the equipment?

In this trilogy, we will explore these main concepts:

  • Understanding the consistency of oil sampling (Part 1)
  • Reading the oil analysis report (Part 2)
  • Selecting the best CM technique for your equipment (Part 3)

Oil Sampling – Consistency is key!

Let’s think about a turbine system and taking a sample from one of these systems. A gas turbine may be connected to compressors, gearboxes and other small assets. Thus, if we’re trying to monitor the internal condition of the turbine, we need to ensure that the sample we obtain is representative of the system. As such, it is critical that a sample is taken from a turbulent area (or an area which is not a dead zone). Samples should also be taken upstream of the filters (this ensures that any debris or contaminants are not filtered out and can be examined) and downstream of components.

If we are sampling from a closed loop system, we need to remember to label the sampling points and ensure that the samples are taken with consistency. Data trending is one of the main aspects of oil analysis. When reading oil analysis reports, it is usually compared to the last report/s to determine if there was any progression of contaminants, wear metals or even additive packages. Thus, if someone samples upstream of the filter and component for one sample but then samples downstream of the filter and component for the follow up sample, there can be a big difference in the results leading to misconceptions about the condition of the equipment.

How often do you sample?

While consistent sampling is important, determining the frequency of sampling is also critical. There are a couple of factors to consider when calculating this frequency of sampling, namely;

  1. Aim of the program
  2. Criticality of the asset
  3. Type of test / oil
Factors to consider when determining sampling frequency
Figure 1: Factors to consider when determining sampling frequency

 

The most important factor to consider is the aim of the test. What are we trying to establish, trend or prevent? When we fully understand this, then this provides us with the purpose of the oil analysis program. This allows us to then tailor a program suited for the assets within our plant or facility and to determine whether continuous monitoring is needed or if sampling should be done at a specified frequency.

Criticality of your assets

The criticality of the asset helps us to determine the frequency of sampling and the types of tests which need to be carried out. Assets can be classified into critical, semi-critical and non-critical. Critical assets are those which can cause significant impact on the plant or facility. The type of impact can vary depending on the factors which are important to the plant. Some plants may hold production as an important factor while others may hold the availability of their equipment for a service as their important factor, others can hold both or many others. Sometimes, these can also be tied to the KPIs (Key Performance Indicators) of the plant.

For critical assets, samples should be taken monthly or even bi-weekly depending on what the user is trying to find out. Let’s take a gas turbine as an example. On start-up, if we wanted to monitor the rate of antioxidant depletion or the rate of degradation of the oil, we could test the sample on a bi-weekly schedule for the first two months and then move the sampling to a monthly, quarterly or even semi-annual schedule once it goes into operation. This is also dependent on the results we get from the sample.

Semi-critical assets are those which do not significantly impact the plant. For instance, if these go down, the plant can still function but at a lower capacity. This does not mean that they are of lesser importance. It simply suggests that the frequency of sampling for these semi-critical assets would not be the same as critical assets. On the other hand, the non-critical assets are those which do not impact the plant if they shut down. Typically, these are the secondary pumps or equipment. These assets will have a lower sampling frequency than the semi-critical assets. The criticality of the asset should be assessed before the determining the frequency of testing.

The types of tests also have a role in determining the sampling frequency as there are some tests which require 24-48 hours to be processed before a result is given. Typically, these tests are slightly more expensive and designed to measure specific properties which may not degrade at a very rapid rate. These speciality tests are usually done quarterly, semi-annually or even annually such as RPVOT, RULER, MPC. They also require a larger volume of oil sample which makes it difficult to obtain from those smaller equipment sumps.

When determining the oil sampling frequency, a matrix can be used to help rank assets as per criticality. These assets can also be classified according to the type of oil test they require and if any speciality tests may be needed. Determining the frequency of sampling is just one aspect of having a successful oil analysis program. The next factor is actually reading the results and making maintenance recommendations for the assets. Stay tuned for part 2 when we take a deeper dive into reading the oil analysis report.

Thank you Sanya Mathura with Strategic Reliability Solutions Ltd for sharing this informative and educational article with us!

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, by Diana Pereda