When working with fluid or gas systems, selecting the correct type of threaded fitting is crucial for ensuring leakproof connections. Two common types of tapered threads are National Pipe Taper (NPT) and National Pipe Taper Fuel (NPTF). Understanding their differences and applications can help you achieve reliable, leak-free seals.

NPT (National Pipe Taper) Threads

NPT threads are designed primarily for mechanical connection and sealing of pipes. The threads have a taper of 1/16 inch per inch, which helps create a tight seal as the threads are engaged. However, due to the design, NPT threads typically require a sealant like Teflon tape or pipe dope to fill the gaps between threads and ensure a leakproof connection. Without a sealant, small gaps can allow for leakage, particularly with gases or under high pressure.

NPTF (National Pipe Taper Fuel) Threads

NPTF threads, also known as Dryseal threads, offer an enhanced seal without the need for additional sealants. The threads are designed to crush into each other as they are tightened, effectively creating a metal-to-metal seal. This self-sealing feature makes NPTF threads ideal for fuel and high-pressure applications where a reliable seal is critical. The primary advantage of NPTF threads is their ability to prevent leaks through the thread roots and crests, even in high-pressure systems.

Key Differences and Applications

• Sealant Requirement: NPT threads generally require a sealant, whereas NPTF threads are designed to seal without one.
• Application Suitability: NPTF threads are preferred for fuel, hydraulic, and high-pressure applications due to their superior sealing capability.
• Interchangeability: While NPT and NPTF threads can sometimes be used together, achieving a reliable seal often necessitates careful consideration of the application and may still require sealants when using NPT threads.

Thread Shapes and Descriptions

Thread forms are the cross-sectional shapes of threads or screw threads, with each type varying based on the design and parameters of a particular thread standard. The specific thread designs or forms of NPT and NPTF pipe threads are particularly suitable for fluid and gas handling applications.

NPT (National Pipe Taper)

• Standard: Based on ANSI/ASME B1.20.1
• Description: NPT threads are general-purpose tapered pipe threads.
• Applications: Suitable for a wide range of plumbing and pipe-fitting tasks.
• Characteristics:
  • Tapered thread design, which provides a mechanical seal.
  • Requires thread sealant (such as Teflon tape) for a leak-tight seal.

NPTF (National Pipe Taper Fuel)

• Standard: Based on ANSI/ASME B1.20.3
• Description: NPTF threads are dryseal tapered pipe threads.
• Applications: Ideal for fuel and other critical fluid applications where leaks must be avoided.
• Characteristics
  • Also has a tapered design but with tighter tolerances than NPT.
  • Designed to create a mechanical seal without the need for additional sealant, though sealant can still be used for added security.

Basic Screw Thread Terms

Understanding the parts and dimensions of screw threads is crucial for designing and working with threaded connections. Below is an overview of the key terms associated with screw threads, particularly focusing on tapered pipe threads such as NPT (National Pipe Taper) and NPTF (National Pipe Taper Fuel).

Thread Parts

Thread Crest

• Description: The thread crest is the continuous helical ridge that runs along the top of a threaded male connection or inside a nut or female port.
• Shape: For NPT and NPTF threads, the crests are flat or truncated.
• Visual Reference: See points B and D in the referenced diagrams.

Thread Root

• Description: The thread root is the opposite of the crest, located at the bottom of a thread.
• Shape: NPT and NPTF thread roots are flat or truncated. NPTF roots have a shorter truncation than NPT roots, making them shallower.
• Visual Reference: See points A and C in the referenced diagrams.

Thread Flank

• Description: The thread flank is the surface or straight side of the thread that connects the thread root to the thread crest.
• Function: The flank provides the angular interface that engages with the corresponding thread in a connection.

Thread Dimensions

Major Diameter

• Description: The major diameter is the largest diameter of a thread, measured from crest to crest for male threads or root to root for female threads.
• Measurement: Calculated as A×2 based on the thread centerline axis.
• Application: For both NPT and NPTF threads, the major diameters are the same. However, NPTF threads have tighter tolerances, resulting in taller crests.
• Visual Reference: Indicated by A in the referenced diagrams.

Minor Diameter

• Description: The minor diameter is the smallest diameter of a thread, measured from root to root for male threads or crest to crest for female threads.
• Measurement: Calculated as B×2 based on the thread centerline axis.
• Application: NPTF threads have smaller minor diameters compared to NPT threads, resulting in shallower roots.
• Visual Reference: Indicated by B in the referenced diagrams.

Thread Centerline Axis

• Description: The centerline axis (C) is the central axis of the thread, which also aligns with the central flow path through a fitting or pipe.
• Significance: This axis is crucial for ensuring the symmetry and proper engagement of the threaded connection.

How NPT Threads Work

Connection Mechanism

NPT (National Pipe Taper) threads create a secure connection through thread tapering. When tightened with a wrench, the taper causes the male and female thread flanks to wedge together. This wedging action induces elastic compression, leading to thread deformation. This deformation acts like spring tension, holding the threads tightly together. The combination of thread deformation and friction (interference fit) between the flank surfaces locks the threads, ensuring a strong, vibration-resistant connection.

Reusing NPT Threaded Pipe and Fittings

Before reusing NPT fittings, inspect the threads for damage. Reuse is generally not recommended due to potential permanent thread deformation from overtightening. Repeated assembly and disassembly can also damage threads. A general guideline is to replace male NPT threaded fittings after two to three uses.

However, a fitting may still be reusable and leakproof if:

• Only the usual amount of thread sealant is needed to create a good seal.
• No more than six threads from the male end go into the female port for a hand-tight fit.

Proper installation is crucial for the potential reuse of NPT components without leaks or damage. Over-tightening can permanently distort tapered threads, limiting their reuse.

Sealants for NPT Fittings and Connections

NPT pipe thread design allows slight clearance between the thread crests and mating roots, creating a potential spiral leak path along the male thread crests. This is why NPT connections require a thread sealant to ensure they are leakproof.

How NPTF Threads Work

NPTF (National Pipe Taper Fuel) threads are a type of dry-seal thread commonly used in hydraulic and pneumatic systems. They are designed to create a leak-proof seal without the need for sealing compounds like Teflon tape. Here’s how they work:

Tapered Design

NPTF threads are tapered, meaning the diameter of the thread decreases as it moves away from the opening. This taper helps create a tighter fit as the male and female parts are threaded together.

Metal-to-Metal Sealing

The primary feature of NPTF threads is their ability to form a metal-to-metal seal. When the male and female threads are tightened, the peaks of the threads on both sides are compressed against each other, creating a strong seal. The taper ensures the threads become progressively tighter, and this compression is what forms the leak-proof connection.

Dry Seal vs. NPT Threads

NPT (National Pipe Taper) threads are similar to NPTF but require a sealing compound or tape to prevent leaks. NPTF threads, on the other hand, are designed to seal without the need for additional sealant, although it can be used for extra security.

Thread Interference

NPTF threads are manufactured with tighter tolerances than NPT threads. This ensures that when the male and female threads are tightened together, the thread flanks interfere, meaning the threads press against each other in such a way that a seal forms due to deformation.

Application

NPTF threads are typically used in fuel, hydraulic, and pneumatic systems where a leak-free, high-pressure connection is essential. They are common in industries like automotive, oil and gas, and manufacturing.

By forming a leak-proof connection through metal deformation, NPTF threads help maintain system integrity and are ideal for applications where fluids, air, or gases need to be securely contained.