|
Since June 2002, the RULER (voltammetric method) has been approved as an official new test method and has its own standard: ASTM D 6810-02 - Standard test method for detection of phenolic antioxidants in non-zinc containing turbine oils. This method is a new ASTM method and will be helpful as a guideline and reference for all users of Voltammetry.
The ASTM D 6971-04 method has been approved (Standard Test Method for Measurement of Hindered Phenolic and Aromatic Amine Antioxidant Content In non-zinc Turbine Oils by Linear Sweep Voltammetry This method allows RULER users to more focus on the high performance, modern industrial lubricants which apply these synergistic mixtures of amines and phenols as primary antioxidants).
For monitoring lubricant oxidative life, this test method can be performed quickly and reliably on a wide range of lubricants and greases, containing antioxidant(s). The information resulting from the test can be compared to RPVOT test information (ASTM D-2272), whereas RULER will differentiate antioxidants.
Below a typical graph from turbine oils, with differentiation between phenolic and aminic antioxidants.
Meanwhile, as part of the development efforts at D02.C00 on turbine oils, the RULER is also part as a method in 2 other existing methods:
- D 6224: "Standard Practice for In-Service Monitoring of Lubricating Oil for Auxiliary Power Plant Equipment"
This method has been revised and balloted, and is now available as a new method at ASTM, D 6224-02.
- D 4378:" "Standard Practice for In-Service Monitoring of Mineral Turbine Oils for Steam and Gas Turbines"
Visit the ASTM International Website for more information. www.astm.org
Procedures for Grease Testing
The RULER can be successfully and easily used for grease life testing, by monitoring the remaining antioxidant concentration present in the greases.
The RULER test can be performed on any type of grease (mineral or synthetic base oil, with Li-OH, polyurea or other type of thickener). RULER Grease testing procedures are not much different from the Lube Oil testing procedures, except that 1 additional step will have to be performed: spread 250 mg of grease on the inside of an empty test vial (make a thin film of grease), and add after that the 5 ml of test solution. Shake the mixed solution for 30 seconds, and leave the vial for 15 to 30 minutes, before you can perform the RULER test. If the RULER reading (RULER Number) is less than 100 for the fresh oil, we advise increase the sample volume to 400 or 500µl of grease sample, which shall logically result in a higher RULER response.
As this test does not require additional grease handling procedures e.g. to separate the oil from the grease thickener structure - the RULER test represents a quick and unique field test method, resulting in life estimation data for used and new greases.
Previous research paper have shown that when lack of antioxidants occur on mid to high temperature ranges greases, that these greases will start to harden. Under this rheologically changed conditions abnormal wear can start to occur quickly and wear out the internal bearing components.
RULER Standard Comparison
Every RULER test will need a comparison with its appropriate standard, in order to start making the correct conclusions. Fluitec advises RULER users to establish a database of Standards or reference oils; this can be done both on the RULER CE320 instrument or on the RDMS database. At regular times these standards needs to be updated, as new oil batches may differ (as part of the oil companies' product improvement process) . Always consult your oil supplier or representative to ask for the latest and correct oil sample, which shall be part of your RULER oil standards database.
RULER Test Data - Combine or Not Combine
Significant cost savings can be made by using RULER analysis as part of Proactive Maintenance Strategies, but it can not be emphasized enough how important it is to combine RULER test data with other analytical data. Which parameters will be important?
This will be depending much from the application, but the table below gives you an indication of which tests are complementary to the RULER test data. By combining the RULER with the different test methods, this will result in a maximum value from the test.
| Application
|
Additional Tests
|
|
|
| Gas and steam turbines*
|
Water
|
Contamination (cleanliness between 1 and 5µ)
Varnish potential (gas turbines)
|
Foaming inhibition
|
| Compressors
|
cleanliness
|
Spectrometric wear metal analysis
|
|
Hydraulics
Gear Oils
Industrial Circulating Oils
|
Water
|
Cleanliness
|
Viscosity
|
| Jet Engine Oils
|
Spectrometric Wear Metal Analysis
|
Acid Number
|
|
* In some cases RPVOT testing can be a necessary test for Remaining Oxidative Life estimation when the RUL% is approaching the 25%, as RPVOT will also have the ability to monitor the oxidative life contributed by the baseoil.
RULER Oil Analysis Frequency
The frequency of testing oil samples with the RULER is an important parameter, to ensure that the defined targets or additive limits are not exceeded. The additive targets are part of the Proactive Maintenance Strategies, more specifically the Root Cause Failure Analysis. As part of this strategy the control of the lubricant will be done at the right frequency, in order to define timely the corrective actions, and keep the oil in its good oxidative health conditions.
The successive oil samples will result in a Trending graph, which will help operators to detect variations from a normal trend, and identify a need for appropriate actions (such as additional testing, or maintenance actions).
Basically what we say is:
VARYING TRENDING GRAPH = VARYING OPERATING CONDITIONS
The oil analysis frequency has to be adapted in function of the RUL%, and of course also the application. A jet engine from a helicopter will have a different antioxidant depletion trend, than an industrial gearoil at a paper plant.
The table below will try to give you the guidelines, by dividing the RUL% in different zones:
- Adapt oil analysis frequency in function of RUL%
- Green zone: 50 - 100% RUL
- Orange zone: 25 - 50% RUL
- Red zone: < 25% RUL
- Quality Control of incoming batches is very important to assure that the correct oil with the appropriate amount of antioxidants will be added to the lubricant reservoir.
50-100% RUL 25-50% RUL < 25% RUL
| Gas turbine lubricants
|
2 Months
|
1 Month
|
2 Weeks
|
| Steam Turbine Central Lube
|
3 Months
|
2 Months
|
1 Month
|
| Pump Bearing Fluids
|
1 Month
|
2 Weeks
|
1 Week
|
| Gear Olis
|
2-3 Months
|
1 Month
|
2 Weeks
|
| Backup diesels
|
3 Months
|
2 Months
|
1 Month
|
| Hydraulics
|
2-3 Months
|
1 Month
|
2 Weeks
|
With Low RUL% values => perform RPVOT test to measure oxidative stability if necessary
With Low RUL% values => perform RPVOT test to measure oxidative stability if necessary
As antioxidants and base-oil have a defined interaction, RULER Voltammogram offers additional information beside the antioxidant voltammetric response. Literature refers often to "Lube Oxidative Health", as being the total capacity of the oil against oxidative stress. This oxidative health is generated by the antioxidants, its intermediates as well some of the base-oil (natural) antioxidant capacity.
No need to explain the function of antioxidants, as well the detection capabilities of RULER. For what concerns the intermediates, these are chemical compounds which are created from the reaction between the oxidation product and the functional group of the antioxidant. As antioxidants often consist of several functional groups (usually between 2 and 4), it is obvious that antioxidants will make intermediate compounds, which are still active as antioxidants. The RULER method is perfectly able to detect these type of intermediates, and usually appear as voltammetric peaks at 1-3 seconds earlier than the original antioxidants. The graph below shows an original antioxidant (green upper graph) with its depleted antioxidant, as well new generated intermediate (red curve).
As you can see from the graph the RULER test method will include the RUL% by these intermediates, by using the area between the curve and the baseline.
But once the original antioxidants as well intermediates have depleted, the base-oil is left without any protection. The oxidation reaction, as being a reaction between oxygen and the hydrocarbon base-stock will create a lot of polar oxidation products. These polar compounds are still soluble, and will logically increase the conductivity or the voltammetric response in the GREEN test solutions.
The graph below shows this increase of voltammetric response between 15 protect the oil against oxidation products buildup.
In most of the cases, this increase will start as a small tail and which will become, with highly oxidized oils, more important till this curve will exceed the Ruler Number 1000.
At that time it will be vital for any oil analysis program to start checking the particle counts (finest particulates < 5 micron), as well varnish potential and acid number, as a result from polymerization/polycondensation products. These products will results in varnish, sludge and deposits on metallic surface.
|
