A plating shop pulled a PTFE immersion heater from a zinc plating tank after nine months of service. The heater showed a horizontal crack running along the liquid line-the boundary between the submerged and exposed sections. The crack had penetrated the PTFE and exposed the metal sheath. The tank was equipped with a float switch that controlled the liquid level, but the switch had a dead band of approximately 40 mm, and the level cycled up and down across the top of the heater.
Float switches are common level control devices in plating and chemical processing tanks. They are simple, reliable, and inexpensive. But they have a dead band-a range between the switch-on and switch-off levels-that allows the liquid level to fluctuate. If the heater is positioned so that the liquid line falls within the dead band, the heater cycles between submerged and exposed states, and the PTFE cracks along the liquid line.
Why the Liquid Line Is a Stress Concentration Zone
The liquid line is the boundary between the submerged and exposed portions of the heater. The submerged portion is cooled by the process fluid. The exposed portion is cooled by air, which is much less efficient. The temperature difference between the two portions can be 50–100°C.
The submerged portion expands less than the exposed portion. The differential expansion creates shear stress at the liquid line. The stress concentrates at the PTFE surface, where the polymer is bonded to the metal sheath. Repeated cycling causes fatigue cracking.
| Liquid Level Cycle | Temperature Difference at Liquid Line | Stress Amplitude | Time to Crack |
|---|---|---|---|
| <10 mm | 40–60°C | Low | >36 months |
| 10–25 mm | 50–80°C | Moderate | 18–30 months |
| 25–40 mm | 60–100°C | High | 8–16 months |
| 40–60 mm | 80–120°C | Very high | 4–10 months |
| >60 mm | >120°C | Severe | 2–6 months |
Liquid level cycling and its effect on PTFE cracking at the liquid line
Why Float Switches Create Liquid Line Cycling
A float switch operates by opening or closing a circuit when the liquid level reaches a set point. The switch has a dead band-the level must drop a certain amount before the switch closes the fill valve. The dead band is typically 20–50 mm, depending on the switch design.
If the heater is positioned so that the liquid line falls within the dead band, the heater cycles between submerged and exposed states with each fill cycle. The number of cycles depends on the evaporation and drag-out rate. In a tank with high evaporation, the level may cycle several times per day.
Why the Crack Forms at the Liquid Line
The crack forms at the liquid line because that is where the stress is highest. The PTFE at the liquid line is subjected to the full temperature difference between the wet and dry sections. The shear stress is concentrated at the PTFE-metal interface.
The crack propagates horizontally along the liquid line. It may remain a surface crack for months, or it may penetrate the PTFE and expose the metal. Once the metal is exposed, corrosion begins, and the heater fails.
Preventing Liquid Line Cracking
The most effective prevention is to position the heater so that the liquid line is well below the float switch dead band. The top of the heated section should be at least 75 mm below the minimum liquid level. This ensures that the heater remains fully submerged even when the level is at the low point of the dead band.
A low-level interlock provides additional protection. The interlock cuts power to the heater if the level drops below the safe threshold. The interlock should be independent of the float switch and should require manual reset.
A heater with a shorter heated section that stays well below the normal liquid line provides a margin against level fluctuation. If the tank is deep enough, positioning the heated section in the lower half of the tank keeps it submerged even during significant level drops.
| Prevention Measure | Effect on Liquid Line Cracking | Implementation |
|---|---|---|
| Lower heater position | Avoids dead band | Heater mounting |
| Low-level interlock | Cuts power on low level | Level sensor and relay |
| Shorter heated section | Keeps heater submerged | Heater design |
| Reduced dead band | Less level cycling | Float switch upgrade |
Prevention measures for liquid line cracking in tanks with float switches
When Liquid Line Cracking Indicates a Design Problem
A heater that cracks at the liquid line despite a low mounting position and a low-level interlock indicates that the float switch dead band is too large for the application. A custom PTFE immersion heater configuration with a shorter heated section, a lower mounting position, and a low-level interlock addresses the root cause of liquid line cracking. The specification process requires the tank dimensions, the float switch dead band, the normal and minimum liquid levels, and the required wattage. For facilities with float switch level control, this application-specific approach reduces liquid line cracking and extends heater service life.
