Key Takeaways
- Track viscosity, acid number, moisture, low-boil content and flash point together rather than in isolation.
- Rising low-boil content points to thermal cracking and affects pump cavitation risk.
- Topping up with fresh fluid does not reverse degradation and can mask a developing problem.
Thermal fluids degrade gradually, and the first sign is usually a change in one or two laboratory indicators rather than a sudden failure. A simple sampling programme makes replacement decisions predictable.
Why thermal fluids degrade
Heat transfer fluids operate at the hottest point in the system, usually the heater surface, which is well above the bulk fluid temperature. Thermal cracking at that surface produces lower-boiling fractions, while oxidation produces acids and higher-viscosity products. Both processes accelerate as the fluid ages, and both change how the fluid behaves in the loop.
The five indicators worth tracking
A standard condition-monitoring panel normally reports viscosity at 40 degrees Celsius, total acid number, water content, low-boil content by distillation, and flash point. Trends across successive samples are more informative than a single result against a limit.
- Viscosity: a rise of roughly 10 percent against the baseline suggests oxidation or contamination; a fall may indicate light-fraction ingress.
- Acid number: an increasing trend signals oxidation and warrants investigation of venting and air ingress.
- Moisture: even modest water content can cause pressure fluctuation and corrosion; keep the expansion tank dry and the vent properly designed.
- Low-boil content: a growing fraction indicates cracking and raises vapour pressure at the pump suction.
- Flash point: a falling flash point is a safety indicator and should trigger an immediate review.
Sampling discipline
Samples should be taken from the same point, at operating temperature where the procedure allows, into clean dry containers. Sampling from the expansion tank is common but can be unrepresentative if the tank is stratified. Record the operating hours, recent top-up volume and any process changes with each sample so the laboratory data can be interpreted in context.
Annual sampling is a reasonable baseline for a stable system; systems running near the upper temperature limit, or with frequent top-ups, benefit from more frequent checks.
When to replace instead of top up
Topping up corrects the level but not the chemistry. When low-boil content rises persistently, when acid number climbs despite venting improvements, or when viscosity moves outside the supplier's recommended window, plan a fluid replacement rather than continuing to add fresh product. Replacement also requires flushing to remove sludge from low points and dead legs.
Safety and system checks alongside the fluid
Fluid condition is only part of the picture. Check pump seals, expansion tank level control and nitrogen blanketing where fitted. A leaking seal admits air, and air ingress accelerates oxidation faster than most operators expect.
Published October 31, 2026 by Metalub Application Engineering Team, Lubricant Application Engineering. Reviewed and updated October 31, 2026.
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