THERMAL FLUID CONDITION MONITORING
Thermal Oil Testing & Replacement Guide: TAN, Viscosity & Insolubles
Use safe representative sampling and laboratory trends to distinguish thermal cracking, oxidation, water ingress and contamination before selecting corrective action.
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.
Retain the new-fluid certificate and, where practical, an initial system sample after commissioning. Trend the same laboratory properties over time. Eastman states that most systems should be sampled annually and also when a fluid-related problem is suspected; the appropriate interval can be shorter for severe temperatures, contamination risk, frequent startups or critical production.
Additional triggers include persistent pump cavitation, unstable pressure, reduced heat transfer, unexpected viscosity, dark deposits, heater fouling, a process-side leak, water ingress, overtemperature, extended air exposure or mixing with an unknown fluid.
02 · SAFE SAMPLING
A poor sample can produce a precise but misleading laboratory result.
Use the fluid supplier or site specialist's approved sampling point, cooler and personal-protection method. The sample should represent circulating fluid, not stagnant material from a dead leg or drain pot. Identify the system, fluid, date, operating temperature, sampling location, recent additions and any abnormal event.
Never improvise sampling from hot, pressurized piping. Allow for thermal expansion, hot surfaces, vapor and the possibility of water or low boilers. Consult the safety data sheet and the site's isolation and permit procedures.
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 |
Eastman’s in-service analysis includes acid number, kinematic viscosity, insoluble solids, low boilers, high boilers and moisture. Exact tests and acceptance limits should come from the supplier of the fluid actually installed.
04 · TREND INTERPRETATION
Read the test panel as a pattern, not seven independent pass/fail lights.
Falling viscosity together with increasing low boilers suggests a different problem from rising viscosity, acidity and insolubles. Water with otherwise normal organic-fluid properties points toward ingress or incomplete dehydration. An isolated flash-point change should be reviewed with low boilers, sampling method and the relevant test method.
Compare the result with the virgin-fluid baseline, previous samples, actual bulk and film temperatures, makeup history and fluid-supplier guidance. A one-time reading near a generic internet limit is weaker evidence than a consistent trend supported by operating changes.
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 |
Review the circulation-flow calculation, pump operating point and expansion-tank arrangement when the laboratory pattern indicates a system cause.
06 · CORRECTIVE ACTION
Continue, condition, partially replace, flush or fully replace only after specialist review.
Possible responses range from continued monitoring to removal of water or low boilers through an approved procedure, filtration, correction of the operating cause, partial dilution with compatible fresh fluid, system cleaning or complete replacement. The fluid supplier and system specialist should decide which option is technically and commercially justified.
Adding fresh fluid without correcting overheating, oxygen exposure or contamination can hide a trend temporarily while the damage continues. Likewise, replacing all fluid without determining whether deposits require cleaning can leave fouled surfaces and poor heat transfer in place.
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