Thermal Oil Boiling Dehydration & Commissioning Guide

Pre-Filling Verification and Cold Circulation Venting

In newly built or recently overhauled heat transfer systems, piping networks frequently retain welding spatter, pressurized air, and trapped moisture from hydrostatic testing or chemical cleaning. Before introducing new thermal oil, dry nitrogen must be used to perform leak checks and purge trapped moisture from system low points, instrument tapings, and dead-leg elbows, verifying that zero free water remains inside the circuit.

Thermal fluid must be pumped slowly from the low-level storage tank into the lowest point of the system using an oil-filling pump. All high-point manual vent valves and the expansion tank overflow line must remain open to let displaced atmospheric air escape. Once fluid levels reach one-third to one-half of the expansion tank sight glass and continuous liquid flows without bubbles from high-point vents, close the vents and begin cold circulation.

Start the main circulation pump and maintain unheated circulation for at least 4 hours. Operators must closely track the pump discharge pressure gauge and motor ammeter. Noticeable pointer fluctuations indicate remaining air pockets. System personnel should alternate opening high-point vent lines along main headers and reactor jackets until pressure fluctuations vanish and operating current remains stable before initiating combustion.

Initial Heating Profile: Ambient to 100°C Transition

The initial warm-up of any heat transfer fluid requires a controlled stepped curve; rushing the temperature climb can lead to mechanical failure. The stage between ambient and 100°C serves to decrease fluid kinematic viscosity, which promotes the detachment of trapped micro-bubbles from coil inner walls and guides them upward toward the expansion vessel.

A controlled heating rate between 10°C/h and 20°C/h is recommended across this range. As temperatures approach 80–90°C, air volume increases and pump suction pressure may show minor oscillations. Maintenance staff should inspect expansion tank sight levels and vent liberated air through the auxiliary vent. If pump pressure fluctuations exceed acceptable bounds, burner output must be turned down immediately while keeping circulation active until baseline stability returns.

Critical Dehydration Phase: Constant 100–130°C Boiling

The 100–130°C plateau represents the most safety-critical operating interval during initial commissioning. Water expands more than a thousand-fold by volume when vaporized at atmospheric pressure. Rapid heating within this window triggers sudden flash boiling, propelling large volumes of oil up into the expansion tank, discharging hot fluid through the vent pipe, and inducing severe pump cavitation that can rupture mechanical seals.

Upon reaching 100°C, burner controls must limit the heating rate to ≤ 5°C/h to enter the constant-temperature boiling phase. The expansion tank vent valve must stay fully open. Exhaust steam condensing along cold vent pipes reveals moisture evaporation progress. Depending on system volume and water intrusion, this stage often lasts several hours or longer. The system must remain at this temperature until water condensation stops, white vapor clears, and differential pressure gauges stay steady.

Light-End Stripping: 200–220°C Venting and System Sealing

After water is completely removed, heating can proceed at 15–20°C/h toward 200°C. Maintaining a constant temperature between 200°C and 220°C for 2 to 4 hours strips trace light boiling ends, residual cleaning agents, and entrained gases out of the fluid. This step ensures that the closed-cup flash point does not experience an abnormal drop early in operational service, preventing excessive vapor pressure at higher operating temperatures.

Once pressure readings and temperatures across all sensors stabilize at the 200°C plateau, close the auxiliary bypass vent on the expansion tank to seal the circuit. For closed-loop installations, supply dry nitrogen blanketing to maintain a positive pressure between 0.02 and 0.05 MPa inside the expansion tank vapor space. This blanket isolates the hot thermal fluid from air contact, preventing oxidative degradation and thermal sludge generation.

Technical Fluid Parameters and Thermal Resistance Comparison

Synthetic thermal fluids offer distinct chemical architectures that dictate operating temperatures, viscosity profiles, and boiling ranges. Alkylbenzene fluids deliver cost-effective performance up to 315°C, whereas pure isomeric aromatics and biphenyl/diphenyl oxide eutectics provide thermal stability up to 350°C and 400°C respectively. Matching the fluid grade to process demands ensures trouble-free commissioning and long operational life.

The engineering data table below provides key physical parameters for Chemie's high-temperature heat transfer fluids to assist process engineers during design selection or fluid replacement:

Operational Maintenance and Fluid Selection Resources

Following initial dehydration and temperature ramp-up, regular operational checks are vital to safeguard fluid life. Implement scheduled fluid sampling to monitor kinematic viscosity, closed-cup flash point, acid number, and carbon residue. If the expansion tank bulk temperature exceeds 70°C, inspect the cold-seal barrier or expansion leg isolate bypass valves to prevent fluid aeration and oxidation.

Engineering teams seeking sizing verification, film temperature analysis, or fluid replacement recommendations can utilize the Chemie Online Parameter Selector (https://calculator.szchemie.com.cn/select) or the Chemie AI Selection Assistant (https://calculator.szchemie.com.cn/ai). For on-site technical guidance, contact the Suzhou Chemie engineering support team directly.

FAQ

Sources

• 烷基苯导热油闪点异常下降机理与系统对策研究（含煮油排气规范）