Thermal Oil Heater Tube Overheat Alarm Causes and FAQ

Tube Wall Alarm vs Bulk Overheat: Why Is Film Temperature Critical?

In thermal oil heater operations, technicians often confuse bulk outlet temperature with tube wall skin temperature. A tube overheat alarm is typically triggered by thermocouples attached to the external surface of coils in radiation or convection sections. This measurement reflects the tube metal skin temperature, which directly governs the liquid film temperature of the heat transfer fluid in contact with the inner tube wall. Due to thermal conduction across the metal boundary, a steady temperature gradient exists between the tube wall and the turbulent fluid core. Under normal operating conditions, the laminar boundary layer film temperature is approximately 20°C to 40°C higher than the average bulk fluid temperature.

The ultimate thermal stability limit of any heat transfer fluid is determined by its maximum allowable film temperature, not its bulk operating limit. When internal velocity drops or localized heat flux surges, the laminar boundary layer thickens, causing the film temperature to exceed molecular thermal cracking thresholds. This degradation produces volatile light ends (inducing vapor lock) and heavy polymers that deposit onto the inner tube wall as insulating carbon coke. Thus, tube overheat alarms act as an indispensable operational safeguard against coking, localized overheating, and eventual tube failure.

Initial Troubleshooting: How to Identify False Instrument Signals?

When an overheat alarm trips, operators should not immediately assume severe combustion runaway before verifying the sensor loop. Skin thermocouples installed on furnace coils operate continuously in flue gas environments reaching 500°C to 900°C. Over time, thermal protection sleeves oxidize, and mounting brackets loosen or degrade. If a probe detaches and becomes directly exposed to open flame radiation, it will record false values several hundred degrees above actual tube metal temperatures.

A practical cross-verification method involves checking the thermal balance across the entire system. Observe whether bulk inlet/outlet fluid temperatures, differential pressure, and stack gas temperatures remain steady. If skin temperature spikes instantaneously while bulk temperature and flue gas trends show no deviation, the cause is typically an open thermocouple circuit, loose extension wire, or faulty transmitter. Verification using a portable infrared pyrometer or dual-color optical thermometer through inspection ports helps rule out electrical interference before planning hardware interventions.

Hydraulic Anomalies: How Does Inadequate Flow Induce Film Overheating?

Once instrument errors are ruled out, restricted hydraulic circulation is the primary mechanism causing convective heat transfer coefficients to drop inside the heater tubes. Forced convection heat transfer correlates directly with fluid velocity. Liquid-phase thermal fluid heaters are typically designed to maintain fluid velocities between 1.5 m/s and 2.5 m/s within fired tubes. Any drop in velocity thickens the boundary film layer, impairing thermal transport and causing rapid localized tube wall temperature escalation.

Key hydraulic components require immediate inspection, starting with circulation pump cavitation. Incomplete degassing after top-ups or the accumulation of thermal cracking light ends at the pump impeller suction lowers available net positive suction head (NPSH), creating vibration, noise, and severe discharge pressure fluctuations. Furthermore, clogged Y-strainers, fouled side-stream filter baskets, unintended bypass valve opening, or stuck check valves restrict effective mass flow across radiant coils, triggering high skin temperature alarms.

Internal Coking: The Thermal Resistance Feedback Loop

Internal tube coking represents a persistent mechanical hazard in high-temperature systems. Solid carbonaceous coke exhibits a thermal conductivity often below 0.15 W/(m·K), whereas carbon steel heater tubes typically possess thermal conductivities around 40 to 50 W/(m·K). Consequently, a compact carbon layer just 1 mm thick presents heat transfer resistance equivalent to dozens of millimeters of steel pipe, blocking furnace heat from entering the moving fluid stream.

To maintain targeted bulk thermal transfer, the external tube metal temperature increases under fixed firing rates. This higher wall temperature intensifies thermal cracking of fluid passing through the boundary layer, accelerating coke deposition. This destructive feedback loop leads to thermal fatigue, tube swelling, bulging, and eventual rupture. A progressive rise in tube wall temperature accompanied by a narrowing bulk temperature differential under identical firing duties is a clear diagnostic indicator of internal coking resistance.

Combustion Malfunctions: Localized Heat Flux from Flame Impingement

Improper burner air-to-fuel ratios and nozzle atomization defects are frequent physical causes of localized thermal hot spots. When burner tips suffer from coke deposits, wear, or misaligned swirl diffusers, the combustion flame can distort, expand, or deflect off-center. This results in direct flame contact or intense gas impingement onto radiation coils and convection banks.

Direct flame impingement can elevate localized heat flux to more than double original engineering design ratings, far exceeding convective heat dissipation capacity at standard fluid velocities. Even if total circulation flow and bulk outlet temperature remain normal, skin sensors on impinged tubes will promptly trip high-temperature thresholds. Routine visual inspections through burner sight glasses are essential to confirm flame centering, shape stability, and absence of tube washing.

Emergency Response Protocol: Standard Steps for Real Overheat Alarms

When an overheat alarm reflects actual physical conditions, site operators must execute standard emergency shutdown procedures immediately. The fundamental safety rule is: cut off fuel or electric power instantly, but never shut down the main thermal fluid circulation pumps. Refractory linings and flue structural components retain substantial heat; terminating circulation causes stagnant oil inside the coils to crack and boil within minutes, generating hard carbon and severe tube damage.

Operators should open stack dampers or emergency exhaust bypasses to enhance natural airflow cooling through the radiant chamber. Maintain continuous circulation pump operation to convey residual heat away from the coils. If an emergency cold oil bypass circuit is installed, introduce cold seal fluid gradually according to system design guidelines. Circulation pumps must continue running until the chamber cools below 200°C and tube skin temperatures fall safely within allowable parameters.

Fluid Selection Strategy: Expanding Thermal Safety Margins

Preventing excessive film temperatures starts with appropriate fluid thermal stability design. Engineering specifications require maintaining a safety buffer of at least 20°C to 30°C between the maximum predicted film temperature and the fluid maximum allowable film temperature. Conventional mineral-based oils frequently experience rapid thermal degradation when approaching 300°C, whereas synthetic aromatic and biphenyl/diphenyl oxide chemistries maintain stability at elevated temperatures.

Suzhou Chemie supplies engineered synthetic heat transfer fluids engineered for high-temperature demands from 315°C to 400°C. With high flash points, elevated boiling ranges, and generous film temperature allowances, these chemistries absorb transient thermal shocks during load swings while minimizing carbon residue buildup. Engineers can verify thermal margins and fluid compatibility via the online Chemie Selection Tool (https://calculator.szchemie.com.cn/select).

FAQ

Sources

• 导热油常见事故原因分析与超温结焦预防
• 温控机设备温度异常升高与超温报警故障树排查