LUBRICATION GUIDE
Thermic Fluid Maintenance: Replacement Interval and Cracking
By the PCG Technical Desk · Updated 27 September 2026
Quick answer: There is no fixed replacement interval. Change a thermic (heat-transfer) fluid when laboratory tests show it has degraded. Test at least once a year, and more often in systems running near their temperature limit. The warning signs are a falling flash point (cracking), rising viscosity and acid number (oxidation), and rising carbon residue or insolubles (deposits).
Two ways a thermic fluid degrades
Thermal cracking happens when oil is overheated, typically at the heater tube film. Molecules break into light ends (which lower the flash point) and heavy residues (which form carbon). Oxidation happens when hot oil contacts air, usually through an open or poorly blanketed expansion tank. It raises viscosity and acidity and forms sludge.
What to test
| Test | What a change means |
|---|---|
| Kinematic viscosity | Rising: oxidation / polymerisation. Falling: cracking. |
| Flash point | Falling: light ends from cracking (fire risk). |
| Total acid number | Rising: oxidation. |
| Carbon residue / insolubles | Rising: deposits and coking risk. |
Good practice
- Never exceed the fluid's rated bulk temperature. IOCL rates SERVOtherm Medium to 300 °C and Special/Super to 315 °C.
- Keep circulation above the heater's minimum flow so film temperatures stay controlled.
- Keep the expansion tank cool and, where possible, blanketed. Vent moisture slowly on start-up.
- After replacing degraded fluid, flush if deposits are present.
Outside Karnataka? The technical information on this page applies anywhere in India. PCG Business Solutions supplies SERVO in seven Karnataka districts. If you are in another state, send us your requirement and we will point you to IndianOil's authorised channel for your area, or answer any technical question about the grade.
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FAQ
FAQs
Overheating, usually at the heater tube wall because of low flow, flame impingement or operating above the fluid's rated temperature.
