The "Do's and Don'ts" explain how to apply Teflon tape correctly, and what thread sealant to use.
This is a common question that is asked by the person who is installing the piping system.Teflon sealant or tape is not the best for all threaded joints.The products are acceptable, but the potential for failures is overstated.There are threaded pipe joints.Designers strive to reduce the number of threaded joints in a system, yet they are essential for change of materials and convenience of repair.
The threaded two parts mate when tightened.There are voids where the crest and root don't meet.There is a small spiral leak path between the root and crest of the threads.During the joint assembly, the leak path needs to be sealed.A thread sealant is used to plug the leak passage.
One solution has been to wrap the male threads in Teflon tape as a means of packing the joint.Two additional problems are created by this.It has been used for thread seal because of its anti-friction lubricity which allows more turns to be taken on a tapered fitting.The wedging force of the male threads is increased by the thickness of tape.The packing ability of the tape has been lost when the threaded joint is backed off.Result: a leak or a drip.
Teflon tape is often banned in irrigation systems because of its shredding and contamination.Teflon tape joints work well in most metallic connections, but they increase the risk of failures in PVC joints.
During the process of tightening a joint the tape is sheared along the flanks, while lubricating, and gets packed into the voids between the root and crest of each thread.The packing process in metallic joints can cause stress in the male and female parts, but they are not affected by the strength of the materials.
Compression loads can be much greater than tensile for most materials.The female threaded part is stretched while the male component is compressed.The bulk of Teflon being packed into the root/crest voids does not have the strength to endure the combined loads of tightening.Since the stress and strain loads are greater, it is easier to split smaller diameter threaded joints.Teflon makes the threads more lubricated, inviting over-tightening.The joint does not feel tight to the installation.When the male and female threads are engaged, additional turns cause the female part to stretch.It is recommended to use a thread sealant and to assemble the joint with one turn and two turns at the most.
Over-tightening is the most likely cause of the splits in the female threaded parts.The female part with a crack that is parallel to the axis of the fitting is the cause of its failure.The origin of the crack can be found within the female threads.There is always a crack or split.The male threads cause stress to the female threads.
In a threaded joint, the greatest stress is generated within the meshed threads, so any cracks or failures will originate from the interior and progress to the exterior.Failure from cracking can be prevented by the wall thickness of the female threaded portion.
A compound that is non-hardening is the best for assembling threaded pipes.If you want to assemble the joint to finger tight, use a thread sealant and one and half turns, two turns at the most.When a joint is backed off, mechanically flexed or expanded with changing temperatures, Teflon tapes and hardening pastes can allow a leak path to develop.A non-hardening compound is forced by the internal fluid pressure into potential points of leak.All materials and media must be compatible with the seal compound.There are many brands of pipe sealants that contain oils.To be harmless to the joint materials and not affect the fluid in the piping system, a proper sealant must be approved by the manufacturer.Over-tightening is encouraged when a seal compound is not lubricating the joint.