Event Tree Sequencing
The most critical first step of the investigation is gathering the facts about the case. The more information that is available initially increases the speed and depth of the analysis of the sample. Recall that ideally, the sample provided is a representation of the fluid in the system. Indicating the machine condition, lubricant condition, and contaminant condition. Our experiences solving lubricant related equipment problems over the years for both national and international clients led us to develop an investigative approach to ensure we have consistency when tackling non consistent problems. Every problem has some uniqueness to it- type of equipment, or working conditions, lubricant condition etc. that must be taken into account. Add to this the fact that the person observing the symptoms or who is trying to evaluate the failure may not know what questions need to be asked in order to arrive at the root cause.
Figures 2 through 6 are event trees for analysis of an oil sample. They are a graphical representation of some steps and choices the analyst must make before arriving at a recommendation. Each choice will lead to a different level of analysis. Each tree must be followed before a recommendation or next step may be given.
The obvious starting point of the investigation is the first question. An experienced analyst must ensure the tests are appropriate for the equipment and fluid type.
Are customer specific requirements or instructions present?
This is a key question for an oil analyst. If there is specific requirements as deemed by an equipment provider, or the problem is straightforward, analysis of the oil sample is easy, because you have a target to aim for. If no formal requirements are present, or the requestor is unsure of what approach to take, some good old fashioned gumshoe detective dialog needs to take place to find answers to questions such as:
"What is the problem and/or what tests should be performed?"
Find out what failed, or what is not performing well. Get a top line summary of what the client is observing.
Follow this up with determining:
Equipment Identification and Sampling Location
"Where in the equipment is this sample coming from?"
The next piece of critical information is the type of equipment and the sampling location of the sample. This could range from an oil sample taken from a 55-gallon drum to a steam turbine. The application typically defines the appropriate tests and test alarm levels for the sample. Different pieces of equipment with the same lubricant will have different alarm levels.
"Where was the sample taken?"
Many pieces of equipment could have many different sample points. One may have seven samples from the same piece of equipment from various points in the fluid stream, such as before filter, after filter, and sump. Some fluid properties should not change with location such as viscosity. Others may have some slight variation such as TAN, TBN and Particle Count.
"How old is this equipment?"
"How long has the oil been in the system?"
"When was the sample taken?"
These pieces of information could help to answer questions to the lubricant condition, equipment life, and the levels of materials if present in the sample.Equipment Location
"What are the operating conditions?"
"What do you do at this plant?"
"How are the drums stored?"
This information could help to explain several conditions such as: For the high amounts of water in the drum sample if the drums are stored incorrectly, high zinc levels in a compressor in a galvanizing plant, and high silicon in a dusty environment.
Is the Original Equipment Manufacturer (OEM) Data Sheet available?
This document should state the typical properties and operating parameters of the lubricant in the equipment. Some OEM's are involved in oil analysis and have detailed information while others are minimalist. This information is helpful in setting alarm levels, and may be very useful when warranty claims are at stake. Use the internet nowadays to gain ready access to these.
Do you have knowledge of the metallurgy of lubricated parts in this equipment?
This information is important for an equipment owner when faced with wear debris data. This information may be available in the equipment instruction manuals, or in consultation with the equipment supplier. Take an example where high chromium levels found in an engine sample. A source of chrome could be the cylinder ring liner wearing excessively. This helps to point the end-user to the trouble areas and proper maintenance procedures.
Do you know the Lube Type and grade?
Identifying the lubricant type is quite important. As with OEM specifications, a set of expectations or target is important. In some cases that lube type will not be identified because it is not available. In this case the lab may not comment on the physical properties of the lubricant (additives, some contaminants, TAN, TBN, and viscosity). One may comment on machine wear, such as a high level of iron. However if the type of equipment is known there may be in service fluid guidelines to compare to. If the fluid and the equipment are not known there is insufficient information provided to make any sort of valuable analysis.
Do you have the latest revision of the Lubricant Data Sheet?
As with the OEM data sheet, this details the physical properties of the lubricant such as viscosity, TAN, TBN, and other parameters. This is a valuable guide to setting up alarm limits for both equipment and lubricant. It is easy to get the latest revision from the web. Take note- formulations change rapidly nowadays.
Do you have a Reference Fluid?
A reference fluid is defined as taken from a properly stored location and positively identified as the lubricant that is in the equipment. It is imperative that the fluid is from a known source because this lubricant will be the baseline for all future samples of that lubricant type. New oil is defined as a sample sent to the lab from an end-user that has already sent a reference oil to be analyzed. Inspection of new oils is helpful to evaluate storage practices. This is also helpful in detecting possible reformulation of the lubricant. Once the fluid has entered a piece of equipment it is now in-service oil. An analyst will compare the reference fluid values to the in-service values. If the end-user has not sent in a reference the analyst will check to see if any customer has sent in a reference sample for that fluid.
Is there an Alarm Matrix?
An alarm matrix is the set of alarm limits that an analyst has created for a type of fluid, equipment, or combination of the two. Always look over the matrix to make sure it is applied correctly. This matrix should have the reference fluid values. The three types of alarms are absolute, rate of change, and statistical. The lab will use a combination of all three along with dialogue with the end-user in setting alarms. The end-user will have the knowledge of lubricants in use, equipment, historical problems, and the reliability goals of the piece of equipment. The combination of input from all sides will make the most logical alarms. If the end-users goal is run to failure and replace, warning the end-user about marginal iron counts is not effective. The goal is to set up a matrix that will work with all parties involved.
Samples taken from the same piece of equipment, at the same sample location over a period of time are entered in the same Unit ID. This allows the analyst to look at a time history of the piece of equipment and watch for trends or increases or decrease of a particular property. Having a history on a piece of equipment allows the analyst to create statistical alarms and rate of change alarms if the sample population is large enough, generally 30 samples, under nominal operating conditions. Trend analysis and rate of change alarms are helpful, since absolute alarms are based on a large population of many machines. Some units may be more sensitive than others, thus prone to failure.
As one may see there are numerous steps that the analyst must go through before a recommendation may be given. At this point retests or additional tests may be requested to confirm data. The most complete recommendation occurs when complete information is provided in every step. The analyst may comment on trends, refer to OEM/lube specs, and refer to the reference lubricant supplied to by the customer. Incomplete steps will prevent the analyst from selecting the best recommendation. The most critical pieces of information are the type of equipment and lube type. If both of these pieces of information are not known the analyst may not make a recommendation unless a customer specific requirement is made, such as Viscosity should be between 28 to 36 cSt.
As stated above, it is important to remain objective during sample analysis. Each sample offers unique insight into the recent operating conditions for that particular piece of equipment, and, as such, requires unbiased analysis. Compare test data to prior test results, history of unit, manufacturer's specifications, applied matrices, and reference (virgin) sample. Any anomalies must be investigated, as they could indicate an abnormal condition with the equipment, lubricant, or test data. Unusual conditions are based on unit history (if known), customer comments or queries, lubricant specifications, equipment type, operating conditions, and analyst experience.
Induced Draft Fan Motor Bearing:
Oil samples were taken from the motor inboard and out board bearing at a power plant. The motor is 500 HP rated and used a babbitt lined sleeve bearing.
Goal: It was a routine analysis to determine if any problems were present prior to an general maintenance outage, at which point it would be less disruptive to work on the motor. Following the approach outlined above, the client was asked, and provided detail on the equipment metallurgy, the oil type,
The oil analysis test package included the following tests
• Fine spectrometric metals analysis
• Rotrode filter spectrometric analysis
• FTIR Fourier Transform Infra Red analysis
• Analytical Ferrography
The fine spectrometric analysis result indicated 6 ppm iron and 3ppm lead but the RFS (Rotrode filter spectrometric) analysis indicated 82 ppm iron and 20 ppm lead.
The rotrode filter spectroscopy technique detects large and coarse wear metal particles and coarse contaminants in used oil. Coarse particles include particles larger than 6μm. These particles are very important as indicator of the onset of abnormal wear situations. Rotrode filter spectroscopy provides a low cost efficient screen for ferrography, and it is superior to DR(direct read) ferrography because of its ability to measure nonferrous wear particles quite effectively. Due to the soft babbitt metal in the sleeve bearings the wear particles generated in case of abnormal wear are coarse and greater than 10μm. This result justified the use of ferrography. The ferrogram indicated light amount of fine ferrous wear particles and moderate amount of severe sliding wear particles at the entry point of the ferrogram. The severe ferrous wear particles have a particle diameter in the range of 15 to 35μm. The ferrogram also indicated nonferrous wear particles all along the ferrogram. Most of the nonferrous wear particles are babbitt metal particle and have a particle diameter in the range of 20 to 45μm. Some of these babbitt metals have oxidized surface due to the fact that Pb/Sn alloys are extremely susceptible to oxidation at temperatures that are considered low in ferrous metallurgy.
Fig.7:Photomicrograph of a babbitt metal.
Particle size 45μm Magnification 500X
Fig.8: Photomicrograph indicating an isolated copper particle
Light amount of crystalline nonmetallic (sand/dirt) contaminants were also present. The presence of babbitt wear particles on the ferrogram and the high level of iron and lead detected by the RFS indicated the criticality of the wear mode in the bearing. A recommendation was given to the customer to inspect the motor bearing, because there was a concern about wiping.
Inspection of the bearing indicated wiping of the lining as expected. The babbitt lining was clearly smeared by wiping and also removed from the surface of the bearing. The extent of wiping damage as indicated in the picture is higher at the end side of the bearing.
A proper approach to investigation of oil samples can lead to meaningful information that is integral to reliability decisions. Formalizing the approach to investigations, minimizes mistakes, avoids delays, and enhances what the testing technology, instrumentation and experience can provide.
By Dan Walsh
PdM 2008 and Lubrication World, September 15-18 2008, Omaha Nebraska
PdM-2008 Predictive Maintenance Technology Conference on the web at http://www.maintenanceconference.com