When to use PTFE tape
PTFE can be used in many plumbing installations but should be used correctly and sparingly. It is used to seal those threads where the seal is made against the thread, as opposed to a plumbing washer.
Parallel BSP threads
BSP Male Threaded Adapter (note flat face at end of the thread where the seal is made by adding washer)
It is not necessary, and probably not advisable, to use PTFE tape on fittings that are installed where the seal is made against two faces which are usually BSP threads (British Standard Pipe). Usually, the seal is achieved using a plumbing washer made from rubber or fiber, with the male and female threads screwed together until the two mating surfaces meet and seal against the washer. If PTFE is used where the seal is made using a washer then there is always the danger that the PTFE will cause the thread to “clog”, locking the thread prior to the two mating surfaces sealing against the sealing washer properly. In this situation, and if extra security is required, it is advised to use a jointing compound in place of PTFE as this will be less likely to lock the thread.
Tapered BSPT Threads
BSPT Male Threaded Adapter (note lack of flat face)
Where there are no mating surfaces sealed with a washer the threads are often tapered threads (known as BSPT – British Standard Pipe Tapered) where the diameter of the threads gets slightly wider as they are screwed together. These threads are only compatible with each other and cannot be mixed with standard BSP non-tapered threads. In this situation, either PTFE or a combination of PTFE and jointing compound is advised. Because the threads simply get tighter as they are screwed together and there is no mating surface to seal the seal is actually made on the thread length, requiring the thread sealant and PTFE.
How to apply PTFE
These are two made rules of thumb, use sparingly and apply in the same direction of the thread. There is no optimum amount but it is generally accepted that two full turns of PTFE thread should be sufficient to seal a BSPT thread. The PTFE tape should be applied in a clockwise direction, looking down on the male thread. This will ensure that as the threads are screwed together, this action does not simply unwind and remove the PTFE thread.