Threaded connections are crevice traps. The threads hold a narrow gap where chloride sits, and bare 316 corrodes inside the thread. The cladding must cover the threads so no gap is exposed. A filled-PTFE film wraps the threaded length, and the threads are sealed with a filled-PTFE tape.
The glass-fiber composite is non-absorbent. The pre-treated film is dense. Material-selection service specifies the coated threads on the drawing. At each outage, the threaded connection is inspected for corrosion.
Procurement buys the coated threaded connections. The smooth surface prevents the crevice. The result is a connection that does not corrode inside the threads.
Plants that had threaded-connection corrosion found the cladding stopped it. The result is a 316 heater whose threaded joints stay clean and serviceable, because the filled-PTFE film covers the crevices that would otherwise hold the chloride.
Threaded connections are among the most neglected corrosion points on a heater. The threads form a narrow spiral gap where chloride sits, and bare 316 corrodes inside the thread even when the rest of the tube is protected. By the time the connection leaks, the thread is eaten away and the fitting must be cut out. A filled-PTFE cladding solves this by wrapping the film over the entire threaded length, so no thread gap is exposed. The glass-fiber composite is non-absorbent, so it does not wick water into the spiral, and the pre-treated film is dense. The threads themselves are also sealed with a filled-PTFE tape that resists cold flow. Material-selection service specifies the coated threads on the drawing, because leaving them bare is a hidden failure. At each outage, the threaded connection is inspected for corrosion and for free movement; a seized thread is a sign that the film has failed. Procurement buys the coated threaded connections as matched parts. The smooth surface prevents the crevice, and the non-stick film means the fitting can be broken and remade without seizing. Plants that had repeated threaded-connection corrosion found the cladding stopped it, once they stopped leaving the threads bare. The result is a 316 heater whose threaded joints stay clean, serviceable and leak-free, because the filled-PTFE film covers the spiral crevices that would otherwise hold the chloride and eat the fitting.
At each outage, the threaded connection is checked for free movement, because a seized thread signals that the film has failed. Procurement buys the coated threads as matched parts. The non-stick film means the fitting can be broken and remade without seizing. Plants that left the threads bare and saw repeated corrosion found the cladding fixed it once the threads were wrapped. The result is a joint that stays serviceable, because the filled-PTFE tape and film cover the spiral gap.
The filled-PTFE tape is itself glass-filled, so it does not cold-flow. At installation, the fitting is torqued to the manufacturer's value. The result is a thread that stays sealed, because the non-stick film and the reinforced tape cover the spiral gap and resist extrusion.
Installation torque is the other half of the crevice problem. Technicians use a torque wrench in diagonal passes, and they are trained never to over-tighten to stop a leak. The liner must wrap past the gasket face, leaving no exposed titanium ring. The glass-fiber composite seals the titanium at the joint, and the pre-treated filler resists cold flow. Material-selection service treats the gasketed joint as the primary crevice defense. Procurement writes the extension on the drawing. At each outage, the liner-to-flange interface is inspected for edge lift. The result is a flange that never develops an oxygen-starved pocket.
