Why the Edge Is the Weak Point
The PTFE on an immersion heater is applied to the cylindrical sheath and wraps around the ends. At the edge, the PTFE thickness is often less than on the flat surface. The PTFE may also have a sharp transition where it wraps around the corner, creating a stress concentration.
The bond between the PTFE and the metal is weaker at the edge. The PTFE may not adhere as well to the cut edge of the metal as it does to the cylindrical surface. The edge may also have burrs or roughness that interfere with the bond.
The edge is also exposed to the highest mechanical stress. In high-solids service, abrasive particles in the fluid impinge on the edge. The edge may also be contacted by cleaning tools or by workpieces.
| Edge Condition | PTFE Thickness | Bond Strength | Mechanical Stress | Delamination Risk |
|---|---|---|---|---|
| Rounded, smooth | Uniform | Strong | Low | Low |
| Sharp, machined | Thinner | Moderate | Moderate | Moderate |
| Burred, rough | Thin, uneven | Weak | High | High |
| Damaged, gouged | Variable | Very weak | Very high | Very high |
Edge condition and its effect on delamination risk
Why High-Solids Wastewater Accelerates Delamination
High-solids wastewater contains abrasive particles-metal hydroxides, calcium phosphate precipitates, and carryover grit. These particles impinge on the heater surface and edges. The impingement erodes the PTFE at the edge, thinning it and creating a path for fluid penetration.
The solids also deposit on the heater surface. The deposits can bridge the gap between the PTFE and the metal at the edge, creating a wick that draws fluid into the delaminated area. The fluid corrodes the metal beneath the PTFE, and the corrosion products push the PTFE further away from the metal.
The Consequences of Edge Delamination
Once the PTFE delaminates, the process fluid contacts the metal sheath. The metal corrodes, and the corrosion products occupy more volume than the original metal. The expansion pushes the PTFE further away, accelerating the delamination.
The delaminated area also creates a path for fluid to reach the internal insulation. The insulation resistance declines, and ground fault trips become more frequent. Eventually, the heater fails.
Preventing Edge Delamination
The most effective prevention is to improve the edge geometry. A rounded edge, free of burrs and sharp corners, allows the PTFE to wrap smoothly and uniformly. The rounded edge distributes the mechanical stress and reduces the stress concentration.
A thicker PTFE coating at the edge provides more material to resist abrasion and impact. The edge coating should be at least as thick as the cylindrical surface coating, and preferably thicker.
An edge protection strip-a metal or polymer strip that covers the edge-can protect the PTFE from mechanical damage. The strip is attached over the edge and shields the coating from impingement.
| Prevention Measure | Effect on Delamination | Implementation | Cost |
|---|---|---|---|
| Rounded edge | Reduces stress concentration | Fabrication | Low |
| Thicker edge coating | More abrasion resistance | Coating process | Moderate |
| Edge protection strip | Shields from impact | Assembly | Moderate |
| Regular inspection | Early detection | Maintenance | Low |
Prevention measures for edge delamination in high-solids service
When Edge Delamination Indicates a Design Problem
A heater that delaminates at the edge despite a rounded edge and a thick coating indicates that the heater design or the coating process is not suited to the application. A custom PTFE immersion heater configuration with a rounded edge, a thick edge coating, and an edge protection strip addresses the root cause of delamination. The specification process requires the solids concentration, particle characteristics, tank dimensions, and required wattage. For facilities with high-solids wastewater, this application-specific approach reduces edge delamination and extends heater service life.
