Female TEE 1/4 NPT

NPT (National Pipe Thread) Fitting Sizing

NPT is a widely used standard for pipe threads in the United States, particularly in fluid and gas systems. NPT fittings use tapered threads, which provide a tight, leak-proof seal when connected. The size designation of NPT fittings is different from other systems like AN, as it refers to the internal diameter of the pipe and the thread count.

Key Features of NPT Fittings:

• Tapered Threads: NPT threads are designed to create a seal by wedging together the male and female threads as they are tightened. This creates a pressure-tight seal that works well for liquids and gases.
• Pressure Rating: NPT fittings are commonly used in systems that operate at moderate to high pressures, but their pressure tolerance depends on factors like material, thread engagement, and the quality of the sealing surface.
• Sealant Use: Because of the tapering, NPT fittings often require the use of a thread sealant (like Teflon tape or pipe dope) to ensure a leak-proof seal.

1. NPT Sizing System

NPT sizes are given as a nominal pipe size (NPS) and the number of threads per inch (TPI). The size refers to the inside diameter (ID) of the pipe it’s designed for, while the TPI refers to how tightly the threads are spaced.

Common NPT Sizes:

• 1/8" NPT: This is the smallest NPT fitting commonly used. It refers to a pipe with a nominal diameter of 1/8 inch.
  • Outer diameter: 0.405 inches
  • Threads per inch: 27 TPI (threads per inch)
• 1/4" NPT:
  • Outer diameter: 0.540 inches
  • Threads per inch: 18 TPI
• 3/8" NPT:
  • Outer diameter: 0.675 inches
  • Threads per inch: 18 TPI
• 1/2" NPT:
  • Outer diameter: 0.840 inches
  • Threads per inch: 14 TPI
• 3/4" NPT:
  • Outer diameter: 1.050 inches
  • Threads per inch: 14 TPI
• 1" NPT:
  • Outer diameter: 1.315 inches
  • Threads per inch: 11.5 TPI
• 1 1/4" NPT:
  • Outer diameter: 1.660 inches
  • Threads per inch: 11.5 TPI
• 1 1/2" NPT:
  • Outer diameter: 1.900 inches
  • Threads per inch: 11.5 TPI
• 2" NPT:
  • Outer diameter: 2.375 inches
  • Threads per inch: 11 TPI

2. NPT vs. NPS

• NPT (National Pipe Thread): These threads are tapered, meaning the diameter of the male threads decreases as the fitting is screwed in, which helps form a seal. This taper allows NPT threads to make a tight, leak-resistant connection.
• NPS (National Pipe Straight): These are straight threads (not tapered) and are used for mechanical joining, not sealing. NPS fittings often require an additional sealing method, such as an O-ring or a gasket.

3. Thread Geometry

• Tapered threads: The taper on the male threads creates a wedging effect, where the threads get tighter as you turn the male fitting into the female fitting.
• Thread angles: NPT threads are typically cut at a 60° angle to provide better grip and tighter seals.

4. How to Determine NPT Size

Unlike AN fittings, which are based on the outside diameter of the hose or fitting, NPT sizes are determined by the nominal pipe size, which refers to the inner diameter of the pipe. However, the size is not always the same as the actual dimensions of the fitting. For example:

• A 1/2" NPT fitting might have an outside diameter of 0.840 inches, but it fits a pipe with an inner diameter (ID) of 1/2 inch. The nominal size refers to the ID, not the outer diameter.

5. NPT Fitting Types

NPT fittings come in various configurations, including:

• Straight: Connects two pipes or tubes in a straight line.
• Elbow: Allows you to change the direction of the pipe (45° or 90°).
• Tee: Splits the flow of fluid into three directions.
• Union: Allows easy disconnection of two pipes or tubes.
• Coupling: Joins two pipes or tubes of the same size.

6. Common Uses

NPT fittings are used in a variety of applications, including:

• Plumbing: Connecting pipes for water, gas, and drainage.
• Hydraulics and Pneumatics: Joining pipes and components in fluid systems.
• Automotive: Oil and fuel line connections.
• Industrial systems: Connecting piping in factories, pressure vessels, and systems requiring pressure-tight seals.

7. Identifying NPT Fittings

To identify NPT fittings, you can look at several characteristics:

• Tapered threads: NPT threads are easy to distinguish from other thread types by their tapering, which gives the male threads a conical shape.
• Threads per inch (TPI): NPT fittings have a specific number of threads per inch, depending on the size.
• Tools: NPT threads can be identified by special thread gauges designed to measure the pitch and angle of the threads.

8. Important Considerations

• Torque: When tightening NPT fittings, you must use the correct torque to avoid over-tightening or under-tightening. Too little torque can result in leaks, while too much can damage the threads.
• Sealing Compounds: Most NPT connections require sealing compounds like Teflon tape or pipe dope to ensure a leak-proof connection. Always apply these to the male threads before assembly.

Summary of NPT Fitting Sizing:

• 1/8" NPT: 0.405 inches O.D., 27 TPI
• 1/4" NPT: 0.540 inches O.D., 18 TPI
• 3/8" NPT: 0.675 inches O.D., 18 TPI
• 1/2" NPT: 0.840 inches O.D., 14 TPI
• 3/4" NPT: 1.050 inches O.D., 14 TPI
• 1" NPT: 1.315 inches O.D., 11.5 TPI
• 1 1/4" NPT: 1.660 inches O.D., 11.5 TPI
• 1 1/2" NPT: 1.900 inches O.D., 11.5 TPI
• 2" NPT: 2.375 inches O.D., 11 TPI