Teflon tape is one of those great new products from the early 70’s. A customer’s recent stubborn coating leak confirmed why we discourage the use of Teflon tape on fluid fittings. In the wet coating industry there are two types of threads NPT (National Pipe Tapered) and NPS (National Pipe Straight). The industry standard in the wet coating business has the tapered NPT tread used on air connections and the straight NPS thread on fluid connections. The NPT thread design makes the seal between the male and female tapered threads on the air fittings. Where as the NPS design has the seal made internally inside the female swivel fitting and a tapered seat of the same angle on the inside of the male fitting.
An excellent pressure seal design is a smooth metal-to-metal contact as in the NPS internal design. We see field connections where people have teflon tape heaved on their NPS fluid connections. In our recent case the tape was so thick that internally the male and female tapered sealing surfaces were not mated and causing the leak.
If you have a leaking NPS fluid connection we will either change one or both of the fittings or seal the threads with an anaerobic sealant such as a thread lock sealant which hardens overnight from the lack of air.