NPT Fittings (National Pipe Tapered Fittings)



National Pipe Tapered thread is also called national pipe thread or National Pipe Tapered Fuel (NPTF), which is a US technical standard to measure tapered threads on pipes, tubes or fittings whether tapered or straight. The tapered threads provide a leak-proof seal for hydraulic systems. They are also called MPT (Male Pipe Threads). The NPT threaded fittings are available in different materials such as iron, brass, carbon steel, and stainless steel.

The national pipe taper connection is dependent on thread deformation. It is recommended for single or permanent assembly applications because frequent assembly or disassembly may result in thread deformation and wear and tear.

The NPT Fittings that seal forming a metal-to-metal connection these fitting usually deforms during the process of installation for providing a seal. This creates a leakage after the connection is formed and requires disassembly and then reassembly and replacement of fittings. In a few cases, tightening may solve the problem but consequently, it may split the fitting or the port. Usually, split ports cannot be replaced or returned. For NPT threaded connections, it is recommended to use a liquid thread seal rather than a Teflon tape. Both the sealants are considered potential contaminants for the hydraulic piping system.

The NPTF is a dry seal thread. The male thread mates with the female thread and a seal are formed when the two are crushed together i.e. threads deformation. Teflon and pipe dope are commonly used when additional sealing is needed. Although this connection is widely used in fluid piping systems, the National Fluid Power Association (NFPA) does not recommend it for hydraulic applications. Although the NPTF connector and BSPT connector are similar in appearance, the two are not interchangeable.

The NPT threads provide a seal when twisted as the edges of the threads squeezed together as compared to straight thread fittings in which threads only hold the components together but they do not provide the seal. The NPT fittings need to be properly lubricated or greased to decrease corrosion and make prospective disassembly easier.

They are commonly available in sizes including 1⁄8, 1⁄4,3⁄8, 1⁄2,3⁄4, 1, 1 1⁄4,1 1⁄2, and 2 inches. According to the ANSI and ASME the tapered size of NPT threads is the 1-inch diameter in 16 inches of length which is measured by the change in diameter of a threaded pipe over the distance of the thread. Larger sizes are not used commonly.

How to Measure for a leak-free seal?

If imperial system is followed, the number of threads per inch (TPI) is measured and the distance between each thread.

If the trial and error method is used, different gauges are used to determine the best fit. The tightest fitting is considered to be the best fitting. It is better if more number of threads are taken into account while determining the best fit. Hold the gauge against the light to identify gaps. This technique is easier to be followed in case of male threads than in the case of female threads.

After that measure the diameter of the thread with a caliper. In the case of male thread outer diameter is measure while in the case of female thread inner diameter is measured. It is recommended that while taking a male reading; hold the caliper at a slight angle for more accurate measure and for female reading; hold it perpendicular to the thread for more accurate measurements. These recommendations are made by the Hose Safety Institute.

Demerits of NPT Fittings:

Following are the demerits of using NPT fittings:

  • They are not considered suitable for high-pressure applications
  • Thread lubrication can contaminate the hydraulic system
  • Disassembly and reassembly may affect the performance and the system becomes prone to leakage
  • Over-tightening can result in cracking and leakage
  • It requires a thread sealant that can act as a contaminant for the system

Merits of NPT Fittings:

Following are the merits of using NPT fittings:

  • They are easily available in different sizes, shapes, and materials
  • It is used commonly and easily available in the market
  • It is easy to assemble and disassemble

How to Assemble National Pipe Tapered Threads?

There are four steps to follow to make sure that the national pipe taper threads are properly connected and sealed to provide a leak proof connection:

First STEP: The first step is to properly inspect the fitting and the port to make sure that both are free of toxins and impurities as well as any stresses or marks.

Second STEP:The second step involves applying the sealant whether liquid or Teflon on the male threads only. Do not wrap the Teflon tape more than one and a half turns in a clockwise direction. It is recommended to use the liquid sealant to be on a safer side.

Third STEP:The third is to twist the fitting into the port.

Fourth STEP: The fourth and final step involves using a wrench to further tighten the fitting properly.

It is important that once the connection is made do not try to make adjustments for better alignment or loosening the fittings because it will result in corrupting the seal and consequently lead to leakage.

The national pipe tapered threads were specifically designed for plumbing systems having a pressure of 60 psi, however, they are now used for hydraulic systems too. They are not considered suitable for high-pressure applications because they require a sealant to provide a secure seal. The NPT threaded connection is expected to leak as compared to the dry seal connection because it is not tightened properly or over-tightening resulted in cracking the female port. So, the tightening requirements differ depending on the type of sealant, assembly, disassembly, and reassembly.

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