Tapered Pipe Threads And Fittings: Making The Connection | ISM

Ensuring a gas or liquid-tight connection means understanding tapered pipe threads

NPT, National Pipe Taper (American) and BSPT (British standard Pipe Taper) are tapered pipe thread standards. Male and female tapered pipe threads wedge themselves together but need a sealant for a completely leak-free connection. Sealants fill any voids between the threads that could travel along the thread spiral.

Tube and pipe are not the same thing Tube and pipe are hollow structures designed to provide a path for the flow of fluids or gases. The main difference between tube and pipe is that pipe walls are thicker and stiffer. Tubing is never threaded because its walls are too thin. Pipe walls are strong enough to support cut or molded threads. Threaded pipe can provide gas or liquid-tight connections that are mechanically strong.

Pipe threads There are many national and international standards for pipe threads. They vary by intended use such as garden hose threads and fire hose threads among others. There are also historical developments behind “standard” pipe sizes and the shapes of threads. Examples of this are the differences between American and British threads. Units of measure play a part too.

The two basic categories of pipe thread

• Parallel or straight pipe thread

• Tapered pipe thread

Take a look at a selection of ISM's metal threaded fittings and plastic threaded fittings. These provide examples of the most common pipe thread types in use.

Tapered pipe threads Piping and fittings are primarily used to carry liquids and gases. Because of this, they need to have threaded connections that are gas or liquid tight. Tapered threads help make better seals. The male and female threads compress and wedge themselves together. As a result, these connections are stronger and leak resistant.

The two most common standards for tapered pipe threads

• NPT American National Standard Taper Pipe Thread
• BSPT British Standard Pipe Taper Thread

NPT pipe thread is the most common tapered pipe thread used in the United States and Canada and is incompatible with BSPT pipe threads.

Why NPT and BSPT pipe threads are incompatible Thread angle or included angle

• NPT threads have a 60-degree angle
• BSPT threads have a 55-degree angle

Thread shape

• NPT threads have flattened peaks and valleys
• BSPT threads have rounded peaks and valleys

Thread pitch (TPI, threads per inch)

• Each NPT and BSPT pipe thread size has a specific number of threads per inch

Some common acronyms for American tapered pipe thread types

• NPT National Pipe Thread Taper
• FPT, FNPT, NPT(F)* Female or internal tapered pipe threads
• MPT, MNPT, NPT(M) Male or external tapered pipe threads

*This is not NPTF. NPTF stands for National Pipe Taper Fuel. It is also called Dryseal American National Standard Taper Pipe Thread. The design of NPTF provides leak-free connections without using sealants.

There are parallel and tapered pipe threads for all pipe standards. NPS, National Pipe Straight, is the American standard for parallel or straight pipe threads. NPS threaded connections need gaskets or O-rings to seal them.

Sealant and tapered pipe threads Tapered pipe threads need a sealant for leak-free connections. It fills any voids between the two threads that could cause a spiral leak. They also function as lubricants between the male and female threads. Assembling parts and pipe made of dissimilar materials need special care. This is because sealants make it easier to over tighten fittings. Over tightening fittings can lead to damage and leaks.

Download the ISM Plastic Tapered Thread Fittings Installation Guide >>

Conclusion Understanding tapered pipe threads makes it easier to choose the best component, especially tapered pipe fittings. Parts with matching threads always perform better because matching threads are mechanically stronger and create better gas-tight and liquid-tight seals.

What kinds of problems have you had moving between different tapered pipe thread standards when sourcing components for your application? Help us by telling others what you learned.

Some additional reading

• NPT National Standard Taper Pipe Threads Size Chart from Engineers Edge
• NPS Nominal Pipe Size (North America) from Wikipedia
• BSP British Standard Pipe from Wikipedia

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