THERMAL FLUID CONDITION MONITORING
Thermal Oil Testing & Replacement Guide: TAN, Viscosity & Insolubles
Use a representative sample and laboratory trends to identify cracking, oxidation, water or contamination. Correct the cause before replacing the oil.
QUICK ANSWER
Do not replace thermal oil from color or one flash-point result alone—trend a representative laboratory sample and find the cause.
Sample safely
Use a procedure that produces a representative sample without exposing personnel to hot fluid or pressure.
Test a panel
Viscosity, acid number, moisture, insolubles and low- or high-boiling fractions describe different degradation mechanisms.
Correct the cause
Overheating, oxidation, contamination, poor venting or water ingress can damage replacement fluid again.
Decision rule: compare results with the unused-fluid baseline, prior samples and the specific fluid supplier's limits. There is no universal discard number for every heat-transfer fluid.
01 · TEST INTERVAL
Establish a baseline, then sample periodically and after abnormal events.
Keep the new-fluid certificate and a commissioning baseline. Eastman recommends annual sampling for most systems and additional tests after suspected fluid problems. Use a shorter interval where service conditions justify it.
Sample after water ingress, overheating, unexplained cavitation, deposits, poor heat transfer or mixing with an unknown fluid.
02 · SAFE SAMPLING
A poor sample can produce a precise but misleading laboratory result.
Sample circulating oil through the approved point and cooler. Avoid stagnant drains. Record fluid, date, temperature, sample location, top-ups and abnormal events.
Use the site’s approved hot-fluid sampling and isolation method. Do not draw directly from hot, pressurized piping into an open container.
03 · LABORATORY PANEL
Each test answers a different maintenance question.
| Property | What a change may indicate | Possible operating effect |
|---|---|---|
| Kinematic viscosity | Low/high boilers, oxidation, thermal stress, contamination or wrong fluid | Pumpability, pressure drop, heat transfer and cavitation |
| Total acid number (TAN) | Oxidation or acidic contamination | Corrosion, sludge and deposits |
| Moisture | Hydrotest residue, open storage, vent ingress or process leak | Cavitation, pressure instability, corrosion and vapor formation |
| Insoluble solids | Oxidation products, contamination or coke/deposit material | Fouling, blocked strainers and reduced heat transfer |
| Low boilers | Thermal cracking or contamination with volatile material | Vapor pressure, cavitation, vent loss and lower flash point |
| High boilers | Thermal degradation, oxidation or polymerization | Higher viscosity, deposits and poorer circulation |
| Flash point | Often influenced by low-boiling fractions | Useful supporting evidence, but not a stand-alone replacement rule |
Agree the test panel and acceptance limits with the supplier of the installed fluid.
04 · TREND INTERPRETATION
Read the test panel as a pattern, not seven independent pass/fail lights.
Falling viscosity with rising low boilers suggests cracking or volatile contamination. Rising viscosity, TAN and insolubles may indicate oxidation or degradation. Investigate moisture separately and compare flash point using the same test method.
Compare trends with the new-fluid baseline, temperatures, top-ups and operating events. Interpret the results against the installed fluid’s limits.
05 · ROOT CAUSE
Fluid degradation often reveals a system or operating problem.
| Finding | System checks |
|---|---|
| Thermal cracking / low boilers | Coil flow, minimum-flow protection, film temperature, burner control and local heat flux |
| Oxidation / rising TAN | Expansion-tank temperature, air exposure, inert-gas arrangement and leaks |
| Water | Hydrotest or cleaning residue, storage practice, open vents and process-user leakage |
| Insolubles | Overheating, contamination, corrosion products, filtration and dead zones |
| Wrong viscosity | Fluid mixing, process contamination, solvent residue and analytical history |
Where results suggest a system cause, check the circulation-flow calculation, pump operating point and expansion-tank arrangement.
06 · CORRECTIVE ACTION
Continue, condition, partially replace, flush or fully replace only after specialist review.
Choose the response with the fluid supplier: monitor, remove moisture or low boilers, filter, partially replace, clean or fully replace. The test pattern and deposit condition determine the action.
Fresh oil will not correct overheating, air exposure or contamination. Remove the cause and assess deposits before refilling.
Thermal-fluid lifecycle record
- Fluid trade name, batch, certificate, charged and makeup quantities;
- System volume, operating temperatures and normal pressure-drop baseline;
- Sample point, method, operating state and laboratory test method;
- Viscosity, TAN, moisture, insolubles, low boilers, high boilers and flash point;
- Overtemperature, low-flow, water-ingress, contamination and maintenance events;
- Expansion-tank temperature and inerting or cold-seal operating history;
- Filter, strainer, vent-condensate and deposit observations;
- Supplier interpretation, corrective action and follow-up sample date.
TECHNICAL REFERENCES
Primary heat-transfer-fluid resources.
- Eastman Therminol — In-service fluid analysis and maintenance
- Eastman Therminol — Optimizing heat-transfer-fluid performance
- JIELI — Thermal oil heater selection guide
- JIELI — Thermal-fluid engineering basis
Use limits and corrective actions issued for the exact installed fluid. Laboratory trends support maintenance decisions; they do not replace the fluid supplier's technical review.
FREQUENTLY ASKED QUESTIONS
Thermal oil testing and replacement FAQ
How often should thermal oil be tested?
A common supplier recommendation is annual sampling for most systems and additional sampling after suspected fluid-related problems. Severe service or critical production may justify a shorter interval.
Which laboratory tests are useful for thermal oil?
A useful panel commonly includes kinematic viscosity, total acid number, moisture, insoluble solids, low-boiling fractions and high-boiling fractions. Flash point can provide supporting information but should not be interpreted alone.
Does dark thermal oil always need replacement?
No. Appearance alone cannot distinguish normal color change from oxidation, cracking, contamination or insolubles. Use a representative laboratory sample, trend history and fluid-supplier limits.
Can fresh thermal oil be added instead of replacing the full charge?
Sometimes partial dilution is technically acceptable, but only after compatibility, test results and the root cause are reviewed. Adding fresh fluid will not correct overheating, water ingress or oxidation exposure.
JIELI THERMAL ENGINEERING
Use fluid analysis to find the system cause—not only the discard date.
Send the fluid product, sample history, operating temperatures, circulation data and abnormal events. JIELI can review heater and system conditions alongside the fluid supplier's laboratory interpretation.
Request a thermal-fluid system review