taylorjm said:
Many people think that teflon tape or pipe dope helps seal the threads. Neither does anything to seal the threads. All it does is lubricates the threads so you can tighten them down more. It's the tapered part of the threads that does the sealing, not the tape or the dope.
Click to expand...
This statement is partially true but mostly false.
PTFE tape (AKA "Teflon tape") is not a true sealant because it does not bond to the surface of the threads. However, it does far more than simply lubricate the threads so you can get the threads tighter. What it also does is get compressed into the root of the thread and creates a physical barrier to stop the spiral leak path that is inherent in NPT threads. In that sense, it is considered a thread sealant.
Pipe dope is designed to bond with the surface of the threads, so it is a true sealant. It does also provide lubricity to allow you to get a tighter make-up on the joint, but it's the ability of dope to fill the gap between the crest and root of the mating threads that allows dope to prevent leaks.
Due to the physical form of NPT threads, it is impossible to get a mechanical seal just from the mating metal parts (or plastic, whatever); you must
always use some type of sealant that fills the spiral leak path between the threads. That is why antiseize, oil, grease or other lubricants are insufficient for assembling NPT threads. If PTFE tape only provided lubricity, then other lubricants would also allow you to obtain leak-free connections.
There is a type of thread form that
does mechanically seal: NPTF, also known as "Dryseal" threads. They are cut differently than NPT. With NPTF threads, the crest of the threads are designed to crush into the root of the mating threads, thus preventing a spiral leak path.
Ironically, added lubricity when tightening pipe threads is not always a good thing as it can allow an inexperienced person to over-tighten the joint. This can lead to cracking or splitting of the female fitting.
This statement is partially true but mostly false.PTFE tape (AKA "Teflon tape") is not a true sealant because it does not bond to the surface of the threads. However, it does far more than simply lubricate the threads so you can get the threads tighter. What it also does is get compressed into the root of the thread and creates a physical barrier to stop the spiral leak path that is inherent in NPT threads. In that sense, it is considered a thread sealant.Pipe dope is designed to bond with the surface of the threads, so it is a true sealant. It does also provide lubricity to allow you to get a tighter make-up on the joint, but it's the ability of dope to fill the gap between the crest and root of the mating threads that allows dope to prevent leaks.Due to the physical form of NPT threads, it is impossible to get a mechanical seal just from the mating metal parts (or plastic, whatever); you mustuse some type of sealant that fills the spiral leak path between the threads. That is why antiseize, oil, grease or other lubricants are insufficient for assembling NPT threads. If PTFE tape only provided lubricity, then other lubricants would also allow you to obtain leak-free connections.There is a type of thread form thatmechanically seal: NPTF, also known as "Dryseal" threads. They are cut differently than NPT. With NPTF threads, the crest of the threads are designed to crush into the root of the mating threads, thus preventing a spiral leak path.Ironically, added lubricity when tightening pipe threads is not always a good thing as it can allow an inexperienced person to over-tighten the joint. This can lead to cracking or splitting of the female fitting.