NPT (National Pipe Tapered) style pipe threads have been widely used for over 100 years. NPT is a U.S. standard for tapered threads used on pipes and fittings.
They are used to effectively seal pipes for fluid and gas transfer. The nominal pipe size can be identified by physically measuring the thread diameter, then subtracting 1/4″.
They are available in iron or brass for low-pressure applications and carbon steel and stainless steel for high-pressure.
NPTF (National Pipe Tapered Fuel) style connections are widely used in fluid power systems.
They have a tapered thread by which a seal is made by deformation of the threads. NPTF Threads are measured at the thread diameter and subtracting 1/4-inch to establish the nominal pipe size.
NPSM (National Pipe Straight Mechanical) connections are also often found in fluid power systems. The female component incorporates a straight thread with an inverted 30° seat. The male component has a straight thread and a 30° internal chamfer.
A seal is made by compression of the 30° seat on the chamfer. This is considered a mechanical connection. If an NPTF male is properly chamfered it will also seal with an NPSM female connection.
NPT, NPTF, and NPSM are standards for screw threads commonly used in North America and are defined by the American National Standards Institute (ANSI). They are mainly used for sealing pipes and fittings in plumbing and are designed to be interchangeable.
National Pipe Tapered
NPT: National Pipe Tapered. NPT is a standard for tapered threads used on pipes and fittings. The tapering of threads allows for a tighter seal as the male thread is screwed into the female thread. They’re used for general purpose applications and can provide a fluid-tight seal with the aid of thread sealants such as PTFE tape or pipe dope.
National Pipe Taper Fuel
NPTF: National Pipe Taper Fuel. NPTF threads are similar to NPT threads but are designed to provide a fluid-tight seal without the need for additional thread sealants. This is achieved by a slightly different thread design that allows for metal-to-metal contact between the male and female threads, thus making a seal.
National Pipe Straight Mechanical
NPSM: National Pipe Straight Mechanical. NPSM is a standard for straight (i.e., not tapered) threads. Because they are straight, they do not provide a fluid-tight seal by themselves and often require a gasket, O-ring, or some form of thread sealant. These threads are commonly used for mechanical, as opposed to fluid connection purposes.
While these threads are somewhat interchangeable, there are differences in their sealing properties and applications. Always make sure to choose the correct thread standard based on the specific requirements of your application. It’s also important to note that while NPT, NPTF, and NPSM are widely used in North America, other parts of the world commonly use different standards like the British Standard Pipe (BSP).
Comparison of NPT, NPTF, and NPSM thread types
NPT
NPTF
NPSM
Full Form
National Pipe Tapered
National Pipe Taper Fuel
National Pipe Straight Mechanical
Shape
Tapered
Tapered
Straight
Sealing Mechanism
Requires additional thread sealant for a fluid-tight seal (like PTFE tape or pipe dope).
Designed to make a fluid-tight seal without additional sealant, due to metal-to-metal contact between threads.
Requires a gasket, O-ring, or sealant as it does not provide a fluid-tight seal by itself.
Application
General purpose applications where pipes need to be sealed.
Applications where a leak-proof seal is needed without the use of additional sealants, often used in fuel and oil lines.
Commonly used for mechanical connections, rather than fluid sealing purposes.
Interchangeability
Somewhat interchangeable with NPTF and NPSM, but differences in sealing properties must be considered.
Somewhat interchangeable with NPT and NPSM, but differences in sealing properties must be considered.
Somewhat interchangeable with NPT and NPTF, but differences in sealing properties must be considered.
Keep in mind that while these thread types are designed to be interchangeable, there can be complications when mixing different types due to their varying sealing mechanisms. Always ensure that the thread type chosen is suitable for the specific application.
