ASTM D6224 Standard Practice for In-Service Monitoring of Lubricating Oil
ASTM D6224 Standard Practice for In-Service Monitoring of Lubricating Oil for Auxiliary Power Plant Equipment
5. Factors Affecting the Service Life of Oils
5.1 New Oil Quality and Suitability for Intended Use - Use of high-quality oils that meet recognized standards (such as manufacturer or military specifications) is the best assurance of potentially long service life.
5.1.1 Viscosity is the most important characteristic of an oil. An oil's load bearing and lubricating properties are related directly to its viscosity. The use of oil with incorrect viscosity can increase wear rates, heat build-up, and lube degradation. In extreme cases, the use of oils with incorrect viscosities can result in rapid catastrophic failures.
5.1.2 Oils that meet the equipment manufacturers' requirements should be used. For situations where the manufacturer simply offers a generic viscosity classification without specific performance criteria, the user should consult the equipment manufacturer, lubricant suppliers, and experts in the field of lubrication.
5.1.3 When fresh, unused lubricants are received, a representative sample of oil may be taken and tested (see Table 1) to ensure that general specifications are met. This test data should be compared to a reference baseline from the lubricant supplier and then used for future condition monitoring.
5.1.4 Manufacturer shelf life recommendations should be observed. Oils should be stored to preserve their original quality and prevent contamination. Stored oils may be tested to ensure and document their quality, cleanliness, and continued suitability for their intended use.
5.1.5 Make-up oils should normally be of the same type, quality, and manufacturer. Available formulations may change over a period of time. Lubricant incompatibility can arise from mixing differing base stocks and additive packages and should be avoided. When oils must be mixed, testing should be performed in an attempt to determine compatibility. Consideration should be given to consulting the lubricant supplier(s) and equipment manufacturer prior to mixing oils.
5.2 Deterioration of Oils in Service - Air (oxygen), elevated temperatures, metals, and water are present to some extent in oil systems. These factors promote oil degradation. Deterioration occurs by one or more of the following processes:
5.2.1 Oxidation Degradation - Chemical changes are brought about by oxygen in the atmosphere forming oxidation by-products which degrade the performance of the oil. These changes can adversely affect the oils viscosity and acidity.
5.2.2 Thermal/Oxidation Degradation - At elevated temperatures, hydrocarbons are subject to thermal cracking which forms unstable compounds. Performance additives in the oil may also degrade at high temperatures. The unstable compounds are easily oxidized and also tend to polymerize to form resins, waxes, and sludge. Thermal oxidation can occur at local hot spots within a system and as a result of high bulk oil temperatures.
5.2.3 Loss of Additives - Additives are used to protect the oil and enhance its performance abilities. When these additives are depleted with service, oil oxidation, foaming, excessive wear, or premature rusting may result.
5.2.4 New Oil Make-up - Addition of new oil is required in nearly every system to make up for losses due to leakage, filter changes, or other maintenance. The amount and frequency of added make-up oil sometimes plays a very significant part in determining the life of a system oil charge. Make-up can vary from less than 5 % per year to greater than 30 % in extreme cases. In equipment where the make-up is very low (below 5 %), oil oxidation and additive depletion are the primary determinants of service life. In-service oil should be tested at sufficient intervals to detect contamination, oxidation, and additive depletion. In equipment where make-up is relatively high, the degree of degradation is minimal because the additives are being replenished.
5.3 Contamination - Contamination of lubricating oils occurs both from outside and from within the system. Common types of contamination are: debris introduced at initial startup or after an overhaul, lube degradation byproducts, component wear debris, airborne particulates, and water. Contamination is often the most significant factor affecting oil service life. Contamination of oil is a valid reason to change oil and flush to restore system cleanliness.
5.3.1 Condition of Equipment on Start-up - Oil system contamination prior to start-up usually consists of preservatives, paint, moisture, rust particles, and construction debris such as dust, dirt, or welding spatter. Extreme care must be taken to avoid the introduction of any contamination into a machine during manufacture, assembly, installation, servicing, or repair. Whenever practical, flushing the system before starting operation is recommended. Fluid cleanliness should be brought to a level of one to two ISO 4406 classes below warning levels before beginning operation. If flushing is not performed, oils should be tested soon after startup or repair to verify their cleanliness.
5.3.2 External Contamination-Solids - Solid contamination consists of any material small enough to pass through bearing seals and vents or which can be introduced with make-up oil. Consideration may be given to prefiltering make-up oil to prevent introducing contaminants into an otherwise clean system.
5.3.3 External Contamination-Liquid - Coolant leaks, moisture or steam condensation, or introduction of improper lubricating oils can compromise the oil. Accumulated water promotes oil degradation as well as interfering with lubrication. Contamination with an improper lubricant is not easily corrected without a complete oil change. An oil monitoring program may be used to monitor and identify contaminants likely to be encountered in service.
5.3.4 Internal Contamination - Contaminants include wear debris and oil degradation products. The types of internal contaminants will vary by equipment type and oil type; the rate of generation will be highly dependent on the equipment operating conditions. The analysis methods employed must be able to identify expected wear debris and degradation products. Testing frequencies should be sufficient to account for operating conditions.
