Acid number analysis is one of the most critical tests for evaluating the oxidation condition and remaining service life of steam turbine oils. This technical article explains the principles behind acid number (acid value) determination, the international and national test standards used by accredited third-party laboratories, sampling and testing procedures, result interpretation, and recommended corrective actions for power generation and industrial turbine lubrication systems.
Technical Overview
The acid number (also called acid value or total acid number, TAN) of steam turbine oil is a fundamental quality and condition-monitoring parameter. It quantifies the mass of potassium hydroxide, in milligrams, required to neutralize the acidic constituents present in one gram of oil, and is expressed in mg KOH/g. In freshly formulated turbine oils, a small acid number is normal and largely originates from weakly acidic antioxidant additives. As the oil ages in service, oxidation of the base stock and depletion of oxidation inhibitors generate carboxylic acids, peroxide decomposition products, and other acidic species, causing the acid number to rise.
A rising acid number is therefore an early and reliable indicator of lubricant degradation. Elevated acidity accelerates the formation of sludge, varnish, and insoluble deposits that can foul servo valves, bearings, and oil system strainers; it also promotes corrosion of babbitt and steel surfaces, reduces demulsibility, and shortens the life of seals and filters. Because turbine oils are expected to remain in service for many years—often ten years or more in large steam units—periodic acid number analysis is a cornerstone of predictive maintenance programs for turbine lubrication and hydraulic control systems.
Typical acceptance guidance: new turbine oils generally have an acid number below approximately 0.10–0.20 mg KOH/g depending on formulation; for in-service oils, standards such as GB/T 7596 and the monitoring guidance of ASTM D4378 suggest that a significant increase above the new-oil baseline (commonly 0.1–0.2 mg KOH/g or an absolute limit around 0.2–0.3 mg KOH/g) signals advanced oxidation and the need for evaluation, reclamation, or oil replacement.
Testing Samples
New (Unused) Turbine Oils
Delivery verification of new lubricants, including L-TSA turbine oils conforming to GB 11120 and turbine oils meeting ISO 8068 specifications, to confirm that the acid number of the product as supplied matches the manufacturer's certificate of analysis and the relevant specification limit.
In-Service Steam Turbine Oils
Routinely sampled oil from the main oil tank, bearing drain lines, or oil return lines of steam turbine-generator sets in fossil-fired, nuclear, and combined-cycle power plants, used to track oxidation trends over time.
Gas Turbine and Hydraulic Control Oils
Oils from gas turbines and the separate electro-hydraulic control (EHC) systems governing steam valve actuation, where high bulk-oil temperatures make acidity monitoring especially important.
Reclaimed, Treated, and Stored Oils
Oil samples taken after purification, vacuum dehydration, Fuller's earth or ion-exchange treatment, or after long-term storage, to verify the effectiveness of conditioning and to confirm stability before the oil is returned to service.
Testing Items
Acid number analysis is usually performed as part of a broader turbine oil condition assessment. Core and commonly associated test items include:
- Total acid number (TAN), expressed in mg KOH/g
- Strong acid number (SAN), indicating the presence of strong inorganic acids from contamination
- Water content (Karl Fischer, ASTM D6304 / GB/T 7600)
- Viscosity at 40 °C (ASTM D445 / GB/T 265)
- Oxidation stability (RBOT/TOST, ASTM D2272 / ASTM D943; SH/T 0193)
- Remaining antioxidant content (RULER, ASTM D6971 / ASTM D6810)
- Air release and foam characteristics (ASTM D3427, ASTM D892)
- Demulsibility (ASTM D1401)
- Corrosion performance (ASTM D665 / GB/T 5096)
- Cleanliness class (ISO 4406 / NAS 1638 particle counting)
- Color and appearance (ASTM D1500 / GB/T 6540)
Testing Methods
Potentiometric Titration
The preferred instrumental method for dark, contaminated, or additive-containing in-service turbine oils. The sample is dissolved in a toluene–isopropanol solvent mixture with a small amount of water and titrated with standardized alcoholic KOH solution while the endpoint is detected through the potential change of a glass electrode reference cell. This approach eliminates subjective endpoint judgment and is specified in ASTM D664 and its Chinese equivalent GB/T 7304. Repeatability and reproducibility are typically on the order of 0.03–0.05 mg KOH/g at typical turbine oil acidity levels.
Color-Indicator Titration
A manual visual method in which a p-naphtholbenzein indicator is added to the sample solution; the titration endpoint is the color transition from orange to green or blue-green. It is quick and inexpensive but requires a light-colored, transparent sample, making it better suited to new oils than to dark in-service oils. Representative standards include ASTM D974, GB/T 264, and GB/T 7599 (the latter specifically addressing turbine oils in service).
Sampling and Handling Requirements
Representative sampling is essential for meaningful results. Samples should be drawn in accordance with ASTM D4057, ISO 3170, or GB/T 4756 using clean, dry glass bottles, preferably from a live zone of the oil tank or a sample valve while the circulation system is running. Hot samples should be cooled, sealed, and protected from light and air before testing; free water and gross particulates should be noted, and samples should be warmed and homogenized before aliquots are taken for titration.
Applications
Acid number analysis supports turbine oil management across a wide range of industries:
- Condition-based maintenance of steam turbine-generator sets in fossil, nuclear, and biomass power stations, where long oil life and varnish prevention are critical
- Gas turbine lubrication programs in combined-cycle and peaking plants, including peaking units subject to frequent start-stop cycling
- Electro-hydraulic control (EHC) system fluid monitoring, where acidity-driven deposits can directly threaten valve responsiveness and unit trip reliability
- Petrochemical, refinery, and chemical plant turbomachinery, including compressors and expander trains sharing lubrication principles with steam turbines
- Marine steam plants and LNG carrier cargo-handling turbomachinery
- Acceptance inspection of new oil deliveries, verification of reclamation and purification effectiveness, and root-cause investigation following contamination events or unexpected oil darkening
FAQ
What acid number is considered acceptable for steam turbine oil?
New turbine oil typically reports an acid number of about 0.05–0.20 mg KOH/g depending on the additive system. For oil in service, most utility guidelines treat an increase of roughly 0.1–0.2 mg KOH/g above the new-oil baseline, or an absolute value approaching 0.2–0.3 mg KOH/g, as an action threshold requiring investigation. Values should always be interpreted alongside new-oil reference data, antioxidant content, and rotating bomb oxidation test results rather than in isolation.
Why does the acid number of turbine oil increase?
The dominant cause is oxidation of the hydrocarbon base stock at elevated temperatures in the presence of air, water, and metal catalysts, which generates organic acids. Other contributors include depletion of phenolic or aminic antioxidants, contamination by combustion products or acidic process fluids, mixing of incompatible oils, and localized overheating in bearings.
How often should acid number testing be performed?
For continuously operating steam turbines, testing every three to six months is a common routine, with increased frequency when upward trends appear or after contamination incidents. Quarterly analysis combined with annual or semi-annual antioxidant (RULER) measurement provides an effective early-warning strategy, consistent with the monitoring philosophy of ASTM D4378.
What should be done if the acid number is too high?
First confirm the result with a fresh sample and review operating conditions, such as oil temperature, water ingress, and air removal performance. Moderate increases can often be managed by vacuum dehydration, fine filtration, and adsorbent (Fuller's earth or ion-exchange) treatment, which removes acidic polar species. Substantially degraded oil with depleted antioxidants and failing oxidation stability generally requires replacement, followed by system flushing to remove varnish precursors.
Which test method is best for dark or heavily used turbine oil?
Potentiometric titration (ASTM D664 / GB/T 7304) is recommended for dark or contaminated in-service oils because the endpoint is detected electrochemically rather than visually, avoiding the color interference that limits color-indicator methods on aged samples.
