Machinery Lubrication magazine published by Noria Corporation
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www . machinerylubrication.com | January - February 2020 | 23 and membrane patch colorimetry (MPC) are among the most effec- tive test methods. Acid Titration Curves e acid number test (ASTM D974) is commonly used to measure the acid number of in‐service fluids. In this colorimetric test, a color change determines the endpoint, making the measurement diff i- cult for darker samples. Also, this method only measures stronger acids and not carboxylic acids (organic varnish) or weaker acids (alkylphe- nols). e ASTM D664 test, which measures a broader range of acids and provides a total acid number (TAN), is recommended instead. In addition to measuring all types of acids, this test method is not influenced by fluid color, reducing the number of errors. e D664 apparatus is also automatic, which eradicates any bias due to the oper- ator as compared to the D974 test. An acid titration curve, as generated by the D664 test method, is shown in Figure 4. Linear Sweep Voltammetry Liner sweep voltammetry is widely used to monitor antioxidants for turbine oils, as per ASTM D6971. A linear sweep is applied to fluid samples in an electrolytic cell, resulting in a voltammogram. Similarly, this test method can be utilized for monitoring alkylphe- nols generated during hydrolysis a nd t herma l degradation. For turbine oils, this method shows the consumption of antioxidants, but for phosphate esters, it can reveal an increase in contaminants gener- ated by fluid degradation. Linear sweep voltammetry is very sensi- tive to alkylphenols and measures below 100 ppm. erefore, it is the preferred method for checking the hydrolysis and condensation reac- tions in phosphate ester fluid. Membrane Patch Colorimetry e power generation industry routinely uses membrane patch colorimetry (MPC) for measuring varnish potential. A similar method is applied to phosphate ester fluid as well. e method involves mixing 50 milliliters of sample with 50 milliliters of solvent. After mixing, the fluid mixture passes through the cellulose-ester patch. Measuring the color on the patch surface yields a Table 1. Recommended condition monitoring test methods and their condemning limits for phosphate ester fluids Figure 5. Different technologies recommended for removing specific contaminants NO. TEST METHOD ASTM TEST TEST UNITS MEASURES FREQUENCY ACTION LIMITS 1 Acid Number (Potentiometric) D664 Mg KOH/g Stronger and weaker acids Monthly 0.2 2 Acid Number (Colorimetric) D974 Mg KOH/g Stronger acids (partial) Monthly 0.1 3 Membrane Patch Colorimetry (MPC) D7843 WK66361 Delta E Varnish/soot Monthly 30 4 Water Content D6304 ppm Moisture Monthly 500 5 Elemental Analysis D5185 ppm Metal soaps/ leached metals Quarterly 5 6 Linear Sweep Voltammetry D6971 WK61352 Peak area Alkylphenols/ alcohols Monthly 8,500 7 Resistivity D6611 G-ohm-cm Resistivity Quarterly 5 • Soot • Pyrophosphates • Metal Soaps • Gels • Dirt/Dust Moisture Removal • Acids • Varnish • Leached Metals • Metal Soaps • Gels Acid Scavenging Media Electrostatic Filtration Membrane Air Drying Particulate Filtration "delta E" value. e higher the color, the higher the varnish potential. Observing the color on an MPC patch can provide even more information about the fluid. For example, a black patch may be indicative of soot formation in the system. A dark brown patch may reveal incomplete combustion products formed at very high temperatures, while a green patch may indicate leaching of copper. More information can be gathered from an MPC patch by merely performing the test with a low sample volume and rinsing the patch with a polar solvent. e presence of soot or organic varnish may then be identified under a microscope. ML