Understanding Dynamic Particle Equilibrium in Closed Loop Systems
Dynamic Equilibrium Condition (DEC) is defined as a steady state condition where the normal wear rate in a machine results in no net gain or loss of particles. Knowing what that level is in any lubrication system is necessary in order to detect departures from this level as a result of an abnormality. At equilibrium operating conditions, the rate of wear particle generation is constant. Under the same conditions, the particle eparation and removal rates, although size dependent, are also constant, surprisingly, with or without a filter. Not surprisingly, the population of all but the smallest particles reaches an equilibrium level. Departure from that normal level is an indication of an abnormality in the system; perhaps the filter is bypassing or has failed, or a severe wear mode has begun. The smallest particles remain in the system, suspended and unaffected by the filter, so their population tends to increase during the life of the oil. Therefore, very fine particles are composed of the oldest particles, some of which may have been generated as larger particles, but have been reduced in size by various chemical and physical processes.
The larger particles are relatively new and are more representative of the current condition of the wearing surfaces. The particles in an oil sample tend to settle out. If they are large and dense, they settle out rapidly. The particles of wear, corrosion, oil degradation, and contamination provide valuable diagnostic information about the condition of the oil as well as the condition of the wearing surfaces of the machine. Since particles exist as a separate phase in the oil, they are not evenly distributed in the system. All the freshly made wear particles will be present immediately after a wearing mechanism such as a roller bearing, gear, sliding surface, etc. The largest metal particles, which are of such critical interest to the analyst, are soon removed by settling in the sections with slow moving oil, such as the sump, or they are filtered or otherwise separated. The very smallest particles tend to remain suspended and pass through all but the finest filters so they are generally distributed evenly throughout the oil piping system. Consequently, in order to capture a representative sample, the sampling location must be carefully considered.
Sampling bottles come in many shapes and sizes. 120 ml (4 oz) HDPE (opaque) bottles have been the most common sampling bottle for many years. Many suppliers now provide clear PET, or PS bottles for oil sampling so end users can readily see debris or water in the sample, forcing a resample before sending it over to the testing area, or outside service provider.
Super Clean bottles
Bottles that are cleaned with filtered air are sold with a designation “Super clean” and are designed for applications where particulate contamination is of utmost concern. Sometimes these bottles come with a seal wrapper between the mouth and the cap. There is no standard for what is considered “clean” so it is up to the buyer to clarify what the cleanliness level is. These usually cost more than regular sampling bottles, and are somewhat ineffective, as care is needed since the bottle is contaminated immediately upon opening the cap. Some operators minimize the ingression of debris in plant environments by sampling within a polyethylene bag, however this can get very messy as the bag makes it easy for the operator to drop the sample.
Ultra Clean Vacuum Device bottles are cleaned to an ISO code of 11/9/4 and sealed. Unlike other “super clean bottles” there is no need to open the cap. The bottle may be used in conjunction with a sampling probe or needle valve, and it also avoids the need for a sampling thief pump. The operator simply connects the tubing from the port to the bottle and opens the valve. When finished, he bottle valve is closed and the cap is replaced on the Figure 2-13: Sampling Thief UCVD (See Fig 16)
Labeling Sampling Points and Samples
Misidentified sample points and samples are a very common source of confusion. All sampling points, once identified, should be properly identified with a sign, detailing the sample point code as defined by the organization’s asset management system, and the oil brand and grade, at a minimum. (fig 17).
Use preprinted asset labels from your asset management software, LIMS or service provider.
- Immediately label samples. Misidentified samples are a very common source of confusion.
- Use preprinted asset labels from software (e.g. oilview, SpectroTrack ) where possible. The more information present, the more meaningful the sample results will be.
Machinery manufacturers will often suggest a sampling interval, but that should only be a rough guideline. The equipment asset owner is the best judge of sampling intervals. Pertinent questions in arriving at a sampling interval include:
- Safety Risk (i.e. loss of life or limb if catastrophic failure occurs)
- Criticality of equipment (or lack of redundancy)
- Environment (wet, dry etc.)
- Operating conditions (load, speed)
- What is the failure history?
- How costly is a failure? In repair cost? Lost production? Life and safety?
- Have operating conditions changed to put more stress on the machine?
In general, a quarterly or monthly sampling interval is appropriate for most important industrial machinery, whereas reciprocating engines tend to be sampled at a more frequent interval based on run time on the oil and engine. The answers to the above questions will help decide which regimen is more appropriate. In a new program, it usually makes sense to start with a monthly interval and then extend it as experience dictates. Having onsite oil analysis equipment allows the user to selfmanage and extend intervals safely as trends develop. Further guidance may be sought by consulting with the equipment manufacturer and oil supplier. The following is designed as a guide to establish proper intervals.
Sampling Routes and Use of At-line Oil Analysis Tools
Inspection Route based vibration analysis and thermography are standard methods of machinery health monitoring, and are now possible with oil condition monitoring. Machine oil condition monitoring yields useful and critical information on machinery health, and complements the data provided by vibration analysis.
Until now, machinery inspection routes have not been able to meaningfully integrate oil condition monitoring. Only handheld data recorders were used to log lubrication management metrics such as the quantity of oil added, or the lubricant level. Actual monitoring of the oil condition was not performed because there was no technology available that provided accurate, meaningful information quickly and easily. With the FluidScan™ Q1100, it is feasible to obtain critical, quantitative oil parameters right at the sampling point, in one minute, from a single drop of oil. As a result, it is now possible to implement a routebased machinery oil condition analysis work process.
The FluidScan can download all of the necessary information (Area/Equipment/Point, Oil type, and Alarm Limit sets) from the Emerson OilView program to create a route. This saves time and ensures the correct data is entered for each piece of machinery. Once the route is completed, the FluidScan measurement data for each point sampled on the route is imported into AMS so a comprehensive asset database is maintained.
Route based oil analysis provides greater flexibility for on-site oil analysis programs, and provides the potential to streamline work flow processes within an industrial setting with an easy to use instrument.