Ever wonder what the difference is between an NPT (National Pipe Taper) and NPTF (National Pipe Taper Fuel) threads are? It’s not just the spelling.
They are both the same in many ways,
– both are used to carry fluids and gasses
– both have the same diameter, thread per inch combination, sizes, 1/16-27, 1/8-27, ¼-18, etc.
– both have an included thread angle of 60 degrees.
– both have a thread that tapers 3/4 per inch per foot or 1/16 per inch.
– both have flatted crest and roots
– both have the same pitch diameter at the same location along the taper
– both have the same length of thread in the “hand tight” and “wrench tight” thread length.
(I will explain “hand tight” and “wrench tight” later)
So, what’s different? First of all, the application. NPT threads are used in different applications. One of them being internal and external fittings used for mechanical purposes other than plumbing, like shelving supports. But the most common applications are those that carry air, fluids or gasses where the assembly includes some type of sealants like Teflon tape or sealing compound that is not functionally objectionable. That is that it can withstand the temperature or pressure it may be subjected to or it will not dissolve or contaminate the fluids or gasses that flow through.
NPTF threads, also called “Dryseal”, are used for applications that carry fluids or gasses where the use of sealants such as Teflon tape or sealing compounds are functionally objectionable due to dissolving and/or contaminating the fluids or gasses that flow through the fittings and pipe. Obtaining a seal without sealants is where NPTF also gets the name “Dryseal”.
The second difference is the width of flats on the crest and roots of the thread. But before we discuss this, we need to first explain what “hand tight” and “wrench tight” means. When you screw together an internal and external pipe or pipe fitting by hand and it comes to a stop, the length of overlapped threads of the internal and the external fittings are called “hand tight” threads. The threads beyond the “hand tight” threads on both the internal and external fitting are called “wrench tight” threads. These threads are provided for the required wrenching necessary to complete the assembly.
NPT threads are dimensioned so that the crests and roots, when assembled “hand tight “, result in a space at the tops and bottoms of the threads and contact with the thread flanks. After wrenching, the threads will reduce this space but will not eliminate it entirely. This space at the crest and root spiral along the thread from the beginning to the end of the assembled fitting. If a sealant is not used, whatever is flowing through the pipe will leak from the pipe and fitting, therefore a sealant is needed to prevent this.
NPTF threads, on the other hand, are dimensioned so the crests and roots, when assembled “hand tight “, come in contact with each other and results in a space between the internal and external thread flanks. When wrenching the threads, the crests and roots are crushed together till the thread flanks come together forming a seal around the entire thread form, eliminating the need for sealant, thus the name Dryseal.
Because of the differences between NPT and NPTF threads, the tooling to produce parts and the gages used to inspect them are different. If the job at hand calls for NPT, use tooling and gages for NPT and if it calls for NPTF, use NPTF tooling and gages.
The US standards that cover NPT and NPTF threads are:
– NPT product threads and gages, ASME B1.20.1 Pipe Threads, General Purpose.
– NPTF product threads, ASME B1.20.3 Dryseal Pipe Threads.
– NPTF gages, ASME B1.20.5 Gaging for Inch Dryseal Pipe Threads.
– NPT and NPTF taps, ASME B94.9 Taps: Ground Thread with Cut Thread Appendix and USCTI Taps, Ground Thread
Now that you know NPT and NPTF threads are different, can you answer these questions?
Can I use NPT tooling to produce NPTF parts or vice versa?
Can I use NPT gages for evaluating NPTF threads or vice versa?
Can I mix NPT and NPTF pipe fittings?
Can I call North American Tool for help when encountering a pipe thread application?
I hope your answer to all the questions would be No, except for the last one.
We at North American Tool look forward to helping you with your tooling and gage needs for pipe thread applications.