The difference between NPT vs. BSP threads has more to do with where you live than their applications. Both NPT and BSP are pipe thread standards for screw threads used on pipes and pipe fittings to seal pipes.
BSP – UK, Europe, Asia, Australia, New Zealand and South Africa (most countries worldwide)
NPT – United States, Canada
Both threads have the same pitch and the familiar shaped peaks and valleys. However, the designs of the threads differ in two fundamental ways. With NPT, the peaks and valleys of the threads are flat. In BSP, they are rounded. Secondly, the NPT angle of the thread is 60 degrees and the BSP angle is 55 degrees.
NPT is the abbreviation for National Pipe Thread Tapered, which is the U.S. standard for tapered threads used to join pipes and fittings. They were established as a standard by the American National Standard Pipe Thread, commonly referred to as the national pipe thread standards. NPT is one of the U.S. national technical standards for both tapered and straight thread series that are used for various purposes such as rigidity and pressure-tight sealing.
BSP is the abbreviation for British Standard Pipe. The thread, as defined by the ISO 228 standard, uses Whitworth standard threads, and is among a number of technical standards for screw threads that has been adopted internationally for interconnecting and sealing pipes and fittings. It has been adopted as standard in plumbing and pipe fitting nearly worldwide.
The necessity to standardize screw threads began in earnest with the onset of the Industrial Revolution in the early nineteenth century. Standardization was needed to facilitate compatibility between different manufacturers and users.
The U.S. standard for tapered threads was established by William Sellers in 1864. Then, as president of Franklin Institute, he proposed a set of new standards for nuts, bolts and screws to replace the poorly standardized screw thread practice in the U.S. His thread design gained wide acceptance largely because the flattened peaks and valleys, and the 60 degree angle was easier for ordinary machinists to manufacture and produce.
Over two decades earlier in Britain, in 1841, Joseph Whitworth had proposed his thread design which was quickly adopted by many British railroad companies and became a national standard for the United Kingdom called British Standard Whitworth.
Even though up through the 1860s this standard was often used in the United States and Canada, it was not universally accepted, and often competed with many other standards used by a multitude of companies. Sellers thread eventually won out in the U.S. when his standard was used for work done under government contracts, and then became a standard for influential railroad industry corporations. Many corporations soon followed and it was adopted as the national standard in the U.S.
NPT and BSP tapered threads are utilized because unlike straight threads, a taper thread can be pulled tightly to make a fluid or air-tight seal. Straight thread applications are designed to simply hold pieces together, whereas a tapered thread, when torque is applied, will compress and seal the fitting.
Applications for tapered threaded pipes provide an effective seal for pipes transporting liquids, gases, steam, and hydraulic fluid. They are used in a wide range of industries—power plants, gas and oil, chemical, manufacturing, and shipping. For example, many pressure systems onboard ships use BSP while many applications in the oil and gas industry use NPT fittings. Both NPT and BSP threads have been adopted for use with a wide range of materials. In addition to steel and brass, the threads are used with bronze, cast iron, and plastics such as PTFE, PVC, and nylon.
Today, these two sealing systems are not inherently better than each other. Each thread works effectively in spite of the two different designs. In fact, for well over a century and a half later, the use and application of either thread has been dependent on where it’s manufactured.