⛽ Foaming characteristics of oils
Assessment of lubricating oil tendency to foam and its stability by air bubbling at controlled temperatures (24°C and 93.5°C).
Overview
Foaming of lubricating oils is a serious operational problem, leading to loss of lubricity, pump cavitation, insufficient oil pressure, overheating and accelerated oxidation. Foam forms when air is mixed with oil in lubrication systems — for example in gearboxes, hydraulic systems, compressors and turbines. Control of foaming tendency is mandatory in specifications of all industrial, turbine, hydraulic and transmission oils.
The method according to ISO 6247 (equivalent to ASTM D892) evaluates two properties: foaming tendency (foam volume immediately after air bubbling ends) and foam stability (foam volume after 10 minutes rest). The test is conducted in three sequences:
Sequence I: Air bubbling (94 mL/min) through 1000 mL oil at 24°C for 5 minutes. Foam measurement immediately and after 10 min rest. Sequence II: Repetition at 93.5°C (simulating operating conditions in engine/turbine). Sequence III: Repetition at 24°C after cooling from 93.5°C (foam assessment after thermal cycles).
The result is expressed as a pair of values: tendency/stability (e.g., 50/0 mL means: 50 mL foam after bubbling, 0 mL after 10 min). Oil specifications specify maximum permissible values for each sequence, e.g., turbine oils according to ISO 8068 require ≤150/0 for Sequence I.
Method principle
An oil sample (190 mL or 180 mL depending on sequence) is placed in a 1000 mL graduated cylinder immersed in a thermostatic bath. Dry, clean air is bubbled through a sintered glass or porous stone diffuser (porosity 40–100 µm) at a flow rate of 94 ± 5 mL/min for 5 minutes. Air entering contact with oil creates bubbles which accumulate above the oil surface as foam. After 5 minutes of bubbling, foam volume is read (tendency). After another 10 minutes of rest (without bubbling), foam volume is read again (stability). Lower stability means faster foam collapse.
Applications
- Turbine oil quality control (ISO 8068, ISO 32/46/68)
- Hydraulic oil testing (ISO 6743-4, HLP, HVLP)
- Transmission oil assessment (API GL-4/GL-5, SAE 75W-90)
- Engine oil control (ACEA, API SN/SP)
- Compressor and circulation oil testing
- Antifoam additive (silicone, polyacrylate) effectiveness assessment
- Oil condition monitoring in service — additive degradation
- Machinery manufacturer specifications (Siemens, GE, Flender, ZF)
Key parameters
| Parameter | Value |
|---|---|
| Temperature Sequence I and III | 24°C ± 0.5°C |
| Temperature Sequence II | 93.5°C ± 0.5°C |
| Air flow | 94 ± 5 mL/min |
| Bubbling time | 5 min |
| Rest time (stability) | 10 min |
| Sample volume | 190 mL (Seq. I), 180 mL (Seq. II, III) |
Standard
- Standard number
- PN-EN ISO 6247:2008
- Title (PL)
- Przetwory naftowe — Oznaczanie skłonności do pienienia olejów smarowych
- Title (EN)
- Petroleum products — Determination of foaming characteristics of lubricating oils
Step-by-step procedure
1. Sample preparation
Measure 190 mL of oil into clean, dry 1000 mL cylinder. Ensure oil is at room temperature and does not contain air bubbles.
2. Thermostating — Sequence I
Place cylinder with oil in thermostatic bath at 24°C ± 0.5°C. Wait for temperature equilibration (min. 30 min).
3. Diffuser mounting
Immerse diffuser in oil, connect air line. Set flow to 94 ± 5 mL/min on rotameter.
4. Bubbling — Sequence I
Turn on air flow. Bubble for exactly 5 minutes. Observe foam formation.
5. Tendency reading (Sequence I)
Immediately after turning off air, read foam volume above oil level to 10 mL accuracy. Record as "tendency Seq. I".
6. Stability reading (Sequence I)
Wait 10 minutes without bubbling. Read remaining foam volume. Record as "stability Seq. I". Typical result: e.g., 50/0.
7. Sequence II — heating to 93.5°C
Transfer cylinder (or use new portion 180 mL) to bath at 93.5°C. Thermostat min. 30 min. Repeat bubbling, tendency and stability reading.
8. Sequence III — cooling to 24°C
Cool sample from 93.5°C to 24°C (or use new portion 180 mL cooled from 93.5°C). Thermostat at 24°C. Repeat bubbling, tendency and stability reading.
9. Result reporting
Record results for all three sequences as tendency/stability [mL/mL], e.g.: Seq. I: 20/0, Seq. II: 50/0, Seq. III: 20/0.
10. Equipment cleaning
Rinse cylinders with petroleum ether, then toluene and acetone. Wash diffusers with acetone and dry in oven (110°C, 30 min). Check diffuser patency.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Oil foaming test apparatus | Koehler K17400 (Dual Bath), Tamson TV 4000 FCS, Stanhope-Seta 17000-0 | 40 000–90 000 PLN |
| Graduated cylinders 1000 mL | Borosilicate cylinders 1000 mL with 10 mL graduation, set of 4 pcs. | 200–500 PLN/pc. |
| Diffusers (porous stones) | Sintered glass diffusers, porosity 40–100 µm, certified | 500–1 500 PLN/pc. |
| Rotameter (air flowmeter) | Glass rotameter 0–150 mL/min, calibrated | 500–1 500 PLN |
| Thermostatic bath | Water/oil bath with 20–100°C control, 4 positions | 10 000–25 000 PLN |
| Air compressor with filter and dryer | Oil-free compressor + coalescence filter + membrane dryer | 3 000–8 000 PLN |
| Laboratory timer | Digital timer with ±0.1 s accuracy | 50–200 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Acetone | 67-64-1 | Analytical grade, for washing cylinders and diffusers after test |
| Toluene | 108-88-3 | Analytical grade, for preliminary washing of vessels from oil |
| Petroleum ether | 8032-32-4 | For preliminary rinsing of cylinders from heavy oil |
| Reference oil (CRM) | — | Certified oil with known foaming values for method validation |
Health and safety (OHS)
- Bath 93.5°C — burn risk from hot oil and bath, use heat-resistant gloves
- Lubricating oils — avoid prolonged skin contact (potential irritating effect)
- Acetone, toluene, petroleum ether — flammable, work under fume hood
- Compressed air — do not direct stream at persons
- Safety glasses, nitrile gloves and coat mandatory