Understanding NPT and Other Pipe Thread Types

The Evolution of Screw Threads

In the nineteenth century, many different types of screw threads were required for hydraulic and pneumatic circuits as well as fastening components. Manufacturers consequently began to design their own fastening techniques. There were compatibility issues as a result. To solve the incompatibility issue, the English mechanical engineer and inventor Sir Joseph Whitworth created a standard threading method in 1841. The Whitworth thread shape has rounded roots and crests and is based on a thread angle of 55 degrees.

What is NPT thread?

In America, William Sellers set the standard for nuts, bolts, and screws which became the National Pipe Tapered Thread (NPT) in 1864. His 60 degree thread angle, in common use by early American clockmakers, enabled the American Industrial Revolution. These thread forms later became the American National Standard.

Whitworth threads were chosen to be self-sealing connecting threads for pipes, with at least one thread being cut with a taper. The British Standard Pipe thread (also known as the BSP Taper or BSP Parallel thread) was created from this. Today, low carbon steel pipes are joined using the Whitworth thread, which is typical worldwide.

What does NPT stand for?

The best known and most widely used connection where the pipe thread provides both the mechanical joint and the hydraulic seal is the American National Pipe Tapered Thread, also referred to as NPT. Npt pipe fitting has a tapered male and female thread that seals with Teflon tape or jointing compound.

Pipe threads used in hydraulic circuits can be divided into two types:

Jointing threads are pipe threads for joints made pressure tight by sealing on the threads and are taper external and parallel or taper internal threads. The sealing effect is improved by using a jointing compound.

Fastening threads are pipe threads where pressure tight joints are not made on the threads. Both threads are parallel and sealing is affected by compression of a soft material onto the external thread, or a flat gasket.