Dry burning refers to the working condition where the heating tube is powered on for heating without being fully immersed in liquid medium. It is the most common abnormal working condition leading to rapid scrapping of stainless steel heating elements. Under no liquid heat dissipation, the surface temperature of the pipe wall surges sharply, which destroys the passive anti-corrosion film, aggravates material thermal fatigue, accelerates weld sensitization and causes internal heating wire fusing. This paper analyzes the damage mechanism and systematic protection design scheme.
1. Multi-Dimensional Damage Mechanism Caused by Dry Burning
(1) Rapid failure of surface chromium passive film
The natural Cr₂O₃ passive film of 316 stainless steel maintains stability within a reasonable service temperature range. Dry burning makes local surface temperature exceed 500℃ instantly, the oxide film is oxidized, cracked and even peeled off in large areas. Once the equipment is put back into liquid heating, chloride ions directly erode the bare metal matrix, and pitting corrosion will occur within a short operation period.
(2) Welding area severe thermal sensitization
The weld and heat-affected zone repeatedly stay in the 450~850℃ sensitization temperature zone for a long time during dry burning. A large amount of chromium carbide precipitates along grain boundaries, resulting in serious grain boundary chromium depletion. The heating tube is prone to intergranular embrittlement and penetration leakage under slight pressure and medium corrosion.
(3) Internal resistance wire overheating burnout
The magnesium oxide filling layer inside the heating tube relies on the outer pipe to conduct heat outward. Dry burning blocks heat dissipation, and the temperature of the electric heating wire rises far beyond its allowable limit. The resistance wire is oxidized and blown, meanwhile the magnesium powder is prone to moisture absorption and powder agglomeration, reducing insulation performance and bringing electric leakage hidden dangers.
(4) Thermal stress deformation and crack initiation
Huge temperature difference exists between the exposed dry burning section and the submerged part. Alternating thermal stress produces microcracks on the pipe wall surface. In corrosive medium, cracks expand continuously and finally evolve into penetrating cracks.
2. Common Inducements of On-Site Dry Burning Accidents
Liquid level drops below the heating tube due to tank liquid leakage and medium consumption without automatic replenishment;
Misoperation by operators starts the heating power supply before feeding liquid into the equipment;
Liquid level sensor failure or signal line disconnection leads to false liquid level feedback;
Heating equipment is started after long-term shutdown without checking the liquid immersion state.
3. Graded Anti-Dry-Burning Protection System Design
Primary protection: Liquid level interlock control
Install high and low liquid level probes inside the tank. When the liquid level is lower than the minimum safe immersion height, the PLC automatically cuts off the heating main circuit, and triggers sound and light alarm to lock the startup program. Heating can only be restarted after the liquid level returns to the standard range.
Secondary protection: Surface temperature probe interlock
Attach a temperature sensing thermocouple to the outer wall of the heating tube. Set the upper temperature threshold according to power density. Once the temperature exceeds the preset limit caused by insufficient heat dissipation, the system immediately power off to avoid sustained high-temperature damage.
Tertiary protection: Timing cumulative power-on protection
Configure working timing module to force periodic shutdown and inspection prompt for long-time uninterrupted heating, prevent abnormal unattended dry burning caused by dual failure of liquid level and temperature sensors.
Manual management specification
Formulate startup check procedure: confirm liquid level, circuit and sensor status before power transmission; add maintenance inspection item to regularly calibrate liquid level and temperature detection components.
4. Post-Dry-Burning Treatment Standard for Heating Tubes
Once obvious dry burning occurs, the heating tube shall not be put into service directly. It needs to be inspected as follows:
Test insulation resistance to eliminate internal insulation breakdown risk;
Carry out penetration inspection to check for surface microcracks;
Reconduct integral passivation to repair damaged surface passive film. If the pipe wall is discolored severely or insulation is unqualified, the heating tube must be scrapped and replaced.
表格
| Protection Mode | Working Principle | Fault Prevention Effect |
|---|---|---|
| Liquid level interlock | Cut power when medium is insufficient | Prevent root dry burning |
| Temperature interlock | Alarm and power off for abnormal overheat | Emergency backup protection |
| Timing forced inspection | Regularly interrupt heating loop | Avoid unattended long-term abnormality |
Conclusion
Dry burning damage is irreversible for 316 heating tubes. Establishing double interlock electrical protection combined with standardized operation management can eliminate over 95% of dry burning failures, effectively extend the service life of heating equipment and avoid equipment damage and safety accidents caused by abnormal temperature rise.
