National Pipe Thread Taper (NPT thread gauge) is another important thread gauge obeying the U.S. standard that is meant for the tapered threads used on threaded pipes and fittings. In comparison to straight threads which are found on a bolt, a taper thread will be ideal to pull tight hence, it makes a fluid-tight seal.
In America, William Sellers have set the standard for nuts, bolts, as well as screws which ultimately became the National Pipe Tapered Thread (NPT thread gauge) in the year 1864. Threaded pipes are popular for providing an effective seal for pipes that have the core purpose of transporting liquids, steam, gases, and hydraulic fluid.
These threads controlled by thread gauges are now used in materials apart from steel and brass. The material may include PTFE, PVC, nylon, cast iron, and bronze. The taper end on NPT thread gauge allows them to form a seal which is when torqued, the flanks of the threads gets compressed against each other. They may also get opposed to parallel or straight thread fittings or compression fittings where the threads negligibly hold all the pieces together and do not provide any kind of seal.
As the thread body gets tapered (0.75 in/ft or 62.5 mm/m) by a thread gauge, a larger diameter is busy compressing into a smaller diameter that finally forms a seal. In this situation, no clearance remains among the crests and roots of all the threads due to the taper).
This means that NPT thread gauge fittings must be burr free and should be lubricated with the help of a lubricating material. This material can be lubricating paste or tape. The use of lubricating tape helps to limit and control corrosion on the threads. If it is not done, the corrosion can make future disassembly nearest to impossible.
NPT thread gauge is also defined by ANSI/ASME standard; B1.20.1. Pipe threads are very different from the usual machine screw along with the bolt threads. These are designated as NC (national coarse) and NF (national fine.) The biggest difference among the two is the taper on pipe threads.
The taper rate for all NPT thread gauges is stated to be 1 in 16 (3⁄4 inch per foot or 62.5 millimeters per meter) which is measured by the change of diameter (of the pipe thread) over the entire distance. The angle present between the taper and the center axis of the pipe is explained as the tan−1(1⁄32) = 1.7899° = 1° 47′ 24″.
On the other hand, apart from NPT thread gauges and other thread gauges, the NPTE and NPS threads have a 60° included angle. They also have a seller’s thread form that has flattened peaks and valleys.
The NPTF, as compared to NPT thread gauges, is semi-compatible variant that is called as National Pipe Taper Fuel (NPTF). It is also called Dry-seal American National Standard Taper Pipe Thread as defined by the ANSI B1.20.3.