In-Use Thermal Oil Testing & Sampling Guide

How to Perform Hot Closed Sampling in Closed-Loop Systems?

Analytical accuracy for in-use heat transfer fluids depends entirely on proper sampling techniques. Following GB/T 24747 and GB/T 23971 testing specifications, sampling taps should be placed along turbulent straight-pipe sections, such as the main circulation pump discharge or the central delivery manifold. Drawing samples directly from the cold expansion tank produces misleading results, as stagnant, low-temperature fluid in the expansion vessel accumulates condensed volatile light fractions and moisture.

Sampling lines must incorporate a closed-loop spiral tube cooler. High-temperature thermal oil must pass through the cooling coil to reduce its temperature to between 30°C and 50°C before entering a clean, dry borosilicate glass bottle or stainless steel vessel. Cooling the fluid under closed conditions prevents personnel burn injuries, stops immediate oxidative degradation upon atmospheric contact, and preserves volatile light components essential for accurate closed-cup flash point and boiling point measurements.

Prior to sample collection, open the sampling valve to flush 1 to 2 liters of fluid through the line to clear out accumulated line debris, then stabilize the cooling water flow. Collect approximately 1.0 to 1.5 liters of fluid, filling the container to 80%–90% capacity to leave thermal expansion headspace. Seal the container immediately, label it with sampling date, system operating temperature, pressure, and sample point location, and store it away from direct sunlight.

Core Physical & Chemical Parameters and Scrap Limits Explained

Under continuous high-temperature circulation, thermal fluids undergo thermal cracking and oxidative polymerization over time. Determining fluid serviceability relies primarily on five indicators: kinematic viscosity change rate, acid value, carbon residue, closed-cup flash point, and moisture content. Breaching critical limits compromises thermal efficiency and operating margins.

The table below outlines standard in-use evaluation parameters and scrap criteria for synthetic heat transfer fluids in accordance with GB/T 24747:

Impact of Viscosity, Acid Value, and Carbon Residue Surges

Kinematic viscosity shifts indicate the direction of fluid degradation. When viscosity increases beyond +15%, polycondensation reactions dominate, generating high-molecular-weight polymers that increase fluid drag, raise pump power demand, and thicken boundary films on heat exchanger tubes, elevating thermal film temperatures. Conversely, a viscosity reduction exceeding -15% points to thermal cracking into light fractions, which can trigger cavitation in circulation pumps if left unmanaged.

Acid value directly tracks fluid oxidation. If nitrogen blanketing fails or the expansion tank operates without adequate seals, fluid contact with oxygen at elevated temperatures produces low-molecular-weight aliphatic acids and polycarboxylic acids. When the acid value exceeds 0.5 mg KOH/g, corrosive by-products damage internal tube surfaces and catalyze secondary polymerization into insoluble lacquers, which rapidly carbonize into hard scale along heated walls.

When carbon residue climbs above 1.5% alongside a flash point decline greater than 20%, substantial volatile fractions and heavy polycyclic aromatics have accumulated. Carbon coke deposits have a thermal conductivity a fraction of steel, degrading heat transfer rates, causing localized tube wall hot spots, and increasing thermal stress on system metallurgy.

Engineering Actions from Minor Anomalies to Severe Fouling

When lab reports indicate low flash points and light-ends buildup while viscosity and carbon residue remain within operational boundaries, run a controlled degassing routine. Operators should open the deaerator or secondary venting bypass and circulate fluid at 110°C to 130°C. This allows low-boiling volatile fractions to vaporize into the expansion tank vent for condensation and removal until flash point values return to acceptable ranges.

For intermediate degradation where acid values range from 0.3 to 0.5 mg KOH/g and carbon residue reaches 1.0% to 1.5%, integrate an offline bypass filtration or clay-treatment skid. Media such as activated bleaching earth or synthetic aluminosilicate adsorbents capture organic acids and polar gums, while sub-5-micron high-temperature metallic filter elements remove circulating carbon particles. Partial top-ups of 10% to 20% with virgin aromatic fluid help rebalance thermodynamic stability.

If viscosity shifts exceed ±20%, carbon residue surpasses 2.0%, and acid value exceeds 0.5 mg KOH/g, fluid degradation is irreversible. The system must be shut down and drained. Circulation cleaning using dedicated chelating or dispersant flushing agents is required to strip coke deposits from tube walls before drying, nitrogen purging, and charging fresh synthetic aromatic fluid.

Synthetic Heat Transfer Fluid Selection and Maintenance Advice

Preventing rapid fluid breakdown begins with matching fluid chemistry to actual operating temperatures. For operating regimes above 300°C, mineral-based oils exhibit lower thermal stability limits, accelerating acid generation and coke formation. Suzhou Chemie provides synthetic aromatic chemistries engineered for thermal stability and resistance to polymerization under high heat flux.

For liquid-phase operations between 300°C and 350°C, Eurotherm 350 (a pure single-component aromatic fluid; operating range -30°C to 350°C, maximum film temperature 380°C, boiling point > 390°C) and Eurotherm THT (hydrogenated terphenyl; operating range -10°C to 350°C, maximum film temperature 380°C, kinematic viscosity 28 mm²/s at 40°C) provide dependable thermal resistance, degrading primarily via clean homogeneous cleavage rather than heavy coking. For processes requiring broad temperature coverage and sub-zero pumpability, Eurotherm 330 operates from -50°C to 330°C with a pour point of -60°C and maximum film temperature of 350°C. For operations prioritizing high open-cup flash performance, Eurotherm HFT-M operates from -30°C to 330°C with an open-cup flash point of 205°C and pour point of -47°C.

Annual comprehensive testing through accredited laboratories is recommended for active thermal systems. Plant engineers can use the Suzhou Chemie online selector (https://calculator.szchemie.com.cn/select) or the AI selection tool (https://calculator.szchemie.com.cn/ai) to evaluate temperature ranges, film temperature margins, and system requirements, or consult Suzhou Chemie engineers for custom testing and flushing recommendations.

FAQ

Sources

• GB/T 24747-2009与GB/T 23971在用有机热载体检测与取样规范
• 导热油关键检测项目与在役报废判定标准分析