NPT threads are designed to be used with some type of sealant. When a metal to metal seal is required NPTF threads are used. What's the difference?
NPT (National Pipe Taper) is a general-purpose tapered pipe thread, and NPTF (National Pipe Taper Fuel) is a dryseal tapered pipe thread. Visually, NPT and NPTF pipe threads look identical. Both thread types rely on thread interference for tight, robust connections. This thread interference is made possible primarily by their tapered design.
The two thread standards differ in how they create leakproof seals. Both require proper installation but leakproof NPT connections require the appropriate use of thread sealants. Correctly assembling NPTF connections create leakproof seals without the use of a sealant.
NPTF threads are used instead of NPT when:
The use of sealants could contaminate the fluids or gasses flowing through the system
Applications are critical or the thread sealant might fail because of high heat or high pressure
There are important issues regarding the installation, mixed-use, and reuse of NPT and NPT fittings:
NPT and NPTF fittings can be mixed but this requires using thread sealants
NPT and sometimes NPTF fittings can be reused but only under particular conditions and cautions
NPT and NPTF fittings both have special installation and assembly requirements for leakproof seals
