Why Should You Avoid Using Wire Rope Slings Directly on a PFA Heater During Installation to Prevent Indentation Cracking?

Large PFA heaters, in particular long and heavy heaters (2–3 m long, 10–20 kg weight) are commonly lifted and positioned into the tank with wire rope slings or chain hoists by the installers. Direct contact between the steel wire rope and PFA sheath causes local high compressive stress. The contact area of the wire rope (single strands 1-2 mm diameter) is minimal and generates pressures of 10-50 MPa under the weight of the heater. The compressive yield strength of PFA at room temperature is around 12-15 MPa. The wire rope makes indentations in the PFA surface creating permanent deformation evident as grooves or markings. These indentations provide stress concentrations that act as the initiation sites for cracks during later temperature cycling or chemical exposure. The cracks grow from the depression point through the PFA wall causing premature collapse. The safe method is to utilise fabric slings (nylon or polyester webbing, minimum 50 mm width) or padded lifting beams. Never allow any contact of the wire rope slings with the PFA. Field findings indicate that wire rope lifted heaters had 3-5x higher crack initiation rates at the indentation sites than fabric sling lifted heaters.

Indentation Behaviour and the Development of Cracks
When a wire rope (6 mm diameter, 6×19 structure) carries the weight of a 15 kg heater, the force is distributed across the contact points. The contact area of one wrap is typically 50-200 mm2. The compressive stress is 0.5–3.0 MPa below yield point. But the heater does not remain static during the lift. Swinging, bumping and uneven load distribution generate peak localised stresses 3-10x average. A transient impact (lifting jolt) of 50 kg on a 1 mm 2 strand contact point produces 500 MPa-far over PFA's compressive strength. The PFA is deformed plastically and leaves a permanent depression of depth 0.1-0.5 mm. Fast deformation induced work-hardening of the polymer at the bottom of the indentation and microcracks. The wall is locally thinned by the depression . A 0.3 mm dent in a 2.0 mm wall reduces the remaining thickness to 1.7 mm .

The PFA swells and compresses on consecutive thermal cycling (heater off/on). The indentation is a stress concentrator with a theoretical stress concentration factor (Kt) of 3–10, depending on the sharpness of the dent. A smooth dent (Kt ≈ 3) increases the nominal thermal stress (2–4 MPa) to 6–12 MPa, getting close to the yield strength. A sharp indentation (Kt≈8) increases the stress to 16–32 MPa, which is higher than the yield strength and leads to the onset of cracks. The crack propagates through the wall thickness from the dent bottom in 100–1000 thermal cycles usually. In chemical environment, the fracture is promoted by environmental stress cracking (ESC). A heater that might last 10,000 cycles without indentation could fail in 500 cycles if it had a wire rope dent.

Correct ways of lifting
Lifting Method Contact pressure (typical, MPa) Risk of indentation Crack initiation rate (compared to cloth sling)Recommended For You
Wire rope sling (6 mm) straight 10-50 (immediate), >100 on contactVery high 3-5× higher Never
Wire rope sling with rubber hose (10 mm wall) 2–5 Medium (hose compresses, but wire can cut through) 2–3× higherTemporary only
Direct chain sling 20–100Very high 5–8 times higherNever Nylon web sling (25 mm width) 0.5 – 1.0 Low 1.0× (baseline)little light heating
Nylon web sling (50 mm wide) 0.2–0.5Very Low 0.8-1.0xStandard for most heaters
Special padded lifting beam <0.1 Negligible <0.5×Heavy, long, or crucial heaters
Vacuum lifting pad No. (not in contact)None 0× Only straight smooth sections
Fabric sling, PTFE or PFA coated 0.2–0.5Very low 0.8–1.0× Chemical plants (avoid contamination)
Inspection & Remediation
Check the sheath carefully before installation if the PFA heater has been lifted with wire rope slings. Use 5-10 x magnification and bright light. Search for:

Indentations or grooves visible (move fingertips over surface, dents are tangible)

Stress whitening (white or hazy patches around the indentation caused by microcracking)

Raised burrs or displaced material at the edge of the dent

Any damage over 0.2 mm or stress whitening is a reason to reject the heater. A heater with minimal surface markings (0.1 mm deep, no whitening) might be appropriate for low stress service (<80°C, no thermal cycling), but should be monitored. Any heater showing any visible indentation should be rejected for critical service. The cost of a replacement heater (500-2,000) is far less than the cost of a failure due to a wire rope dent (5,000-50,000 in downtime and product loss).

Two remediation methods mitigate risk in the absence of rejection: (1) Polish the indent region with fine sandpaper (400-600 grit) to remove stress whitening and smooth rough edges. Polishing shallow (depth of cut < 0.1 mm). (2) Put a piece of PFA or PTFE tape on the dent to serve as a secondary barrier. Neither remediation returns full strength. Heater is at increased risk.

Field Research
Six identical PFA heaters (2.5 m long, 12 kg, 2.0 mm wall) were put in six plating tanks in a chemical company. Three heaters were raised using nylon web slings (50 mm width). Due to request of the installation, three heaters were lifted by wire rope slings (10 mm diameter). All heaters were run at 85°C in 15% H2SO4 with daily thermal cycling (25°C to 85°C). After 18 months, all three heaters raised by wire rope showed cracks at the indentation areas as confirmed by insulation resistance drop (<10 MΩ) and ocular inspection. Two failed in a big way (ground fault). The three heaters, raised using nylon slings, were free of fractures and had an insulation resistance >200 MΩ. The facility changed its installation method to prevent the use of wire rope slings and enforced the use of fabric slings for all PFA heater installations. Failure rate fell to almost zero.

Conclusion: PFA Wire Rope Slings Not Permitted
Wire rope slings shall not be in direct contact with a PFA heater during installation. The high localised compressive stress (10-100+ MPa) causes permanent indentations that act as stress concentrators that reduce heat cycle life by 70-90% and initiate cracks that propagate through the wall. Never use fabric slings (nylon or polyester webbing, minimum width 50 mm) or padded lifting beams. Where space restrictions necessitate the use of wire rope or chain slings, the sling should be padded with rubber or silicone padding, minimum 10mm thick and tension kept to a minimum. Even with padding, check for indentations prior to installation. It is not worth risking a 1,000 heater for a 20 wire rope sling. The manner of installation is just as important to heater life as the material specification. Include lifting methods in procurement papers and train installers A few minutes of care at the time of installation saves months of unnecessary downtime. Wire Rope & PFA don't go along. If it leaves a mark, it means the heater is doomed to fail early.