How to Measure NPT Fittings?

When it comes to plumbing systems, understanding the components involved is critical to ensuring proper installation and operation. One of the most common types of fittings used in these systems is the NPT (National Pipe Taper) fitting. NPT is the standard for tapered threads in the United States and has been widely used on pipes and fittings for over 100 years. This article will take a closer look at what NPT pipe fittings are, the standards of NPT threads, and most importantly, how to measure NPT fittings accurately.

What is an NPT pipe fitting?

An NPT fitting is a threaded joint used in plumbing systems to create a seal between two pipes or between a pipe and a fitting. The “NPT” designation refers to the National Pipe Thread, which is the standard for tapered threads used on threaded pipe and fittings in the United States. The taper allows for a tighter fit when the threads engage, which helps create a seal that can withstand pressure and prevent leaks.

NPT fittings are commonly used in a variety of applications, including water supply, gas pipelines, and industrial piping systems. They are available in a variety of materials, including brass, stainless steel, and plastic, so they are flexible for different environments and applications.

NPT fittings

What are NPT threads?

NPT threads feature a tapered design, which means the thread diameter decreases along the length of the fitting. This tapered design allows the threads to fit more tightly together, creating a seal that is critical to preventing leaks.

What is NPT thread angle?

The NPT thread standard specifies a 60-degree thread angle and a specific pitch, which is the distance between the threads. Pitch is the distance between the threads. The threads are designed to be compatible with other NPT fittings, ensuring they can be easily connected and disconnected as needed. Please note that NPT fittings cannot be connected to all fittings. NPT pipe fittings are designed for tapered pipe threads.

How do NPT pipe fittings achieve a sealed connection?

NPT threads form a seal through the mechanical engagement of the threads, rather than relying solely on washers or sealants. NPT threads can use sealants (such as pipe dope or Teflon tape) to ensure a leak-proof connection. This helps it be used in high-pressure applications, especially NPT pipe fittings made of carbon steel and stainless steel. If it is an iron or brass NPT fitting, it is often used in pipes for fluid and gas transportation and is suitable for low-pressure applications.

Applications of NPT Pipe Fittings

Plumbing Systems: NPT fittings are used to connect pipes and fixtures. They create a leak-proof seal, which is essential for maintaining pipe installations.

Industrial Processes: NPT fittings are used in industries to transfer pressurized liquids and gases. They are used in manufacturing, chemical production, and the oil and gas industry, where secure connections are important.

HVAC Systems: NPT fittings are frequently used in heating, ventilation, and air conditioning (HVAC) systems. They help connect various components, ensuring efficient airflow and temperature control.

Fire Protection Systems: The reliability of NPT fittings makes them suitable for fire protection systems, where water or chemical delivery must be consistent and reliable.

Automotive Applications: In the automotive industry, NPT fittings are used for fuel and oil lines, where their secure connection prevents leaks and ensures optimal performance.

Disadvantages of using NPT fittings:

Susceptible to port expansion or cracking due to over-tightening

Required thread lubricant may cause system contamination

NPTF and NPSM thread styles

The two most common variations of this pipe thread standard are the NPTF and NPSM thread styles.

Where traditional NPT threads have a small gap between the crest and root of the mating thread that requires the use of a sealant, the NPTF thread root does not allow for this gap. It contacts and deforms the mating thread crest. NPTF or National Pipe Taper Fuel Style connections are widely used in fluid power systems. This type of connector is designed to provide a dry thread seal. However, lubricants are often still used to simplify assembly.

NPSM or National Pipe Straight Mechanical Threads are also often seen in fluid power systems. The female assembly contains a straight, non-tapered thread, and the male connector is inverted with a 30° chamfer on the female seat. This is a mechanical seal. If the NPTF male is chamfered correctly, it will also seat with the NPSM female. They are particularly well suited for manufacturers and end users who rely on versatility.

How to Measure NPT Fittings?

Measuring NPT fittings accurately is essential for ensuring that they will fit properly in your piping system. Here are the steps to measure NPT pipe fittings:

How to Measure NPT Fittings

1. How to measure npt threads

Measuring the NPT thread is to determine whether it is an NPT fitting. First of all, in appearance, NPT is a tapered thread that gradually becomes thicker, rather than a straight cylindrical thread. Secondly, confirm whether it is a 60° thread angle (NPT is 60°, BSP is 55°).

2. Identify the Size of the Fitting

NPT fittings are typically identified by their nominal size, which corresponds to the inside diameter of the pipe they are designed to fit. Common sizes include 1/8″, 1/4″, 1/2″, 3/4″, 1″, and so on. However, the actual outside diameter of the fitting will be larger than the nominal size due to the thickness of the material.

3. Measure the Outside Diameter

To measure the outside diameter of an NPT fitting, use a caliper or a tape measure. Place the measuring tool around the widest part of the fitting, ensuring that you are measuring the outside edge of the threads. Record this measurement in inches or millimeters.

4. Determine the Thread Pitch

The thread pitch is the distance between threads, which is crucial for determining the compatibility of fittings. To measure the thread pitch, you can use a thread gauge or a ruler. Count the number of threads in a specific length (usually one inch) and divide that by the length to find the pitch. For example, if there are 11 threads in one inch, the pitch is 11 threads per inch (TPI).

5. Measure the Taper Angle

While not always necessary, measuring the taper angle can be helpful in ensuring a proper fit. The standard taper angle for NPT threads is 1 degree 47 minutes (or approximately 1.75 degrees). You can use a protractor to measure the angle of the threads if needed.

Common NPT thread size reference table

6. Check for Compatibility

Once you have measured the outside diameter, thread pitch, and taper angle, you can compare these measurements to the specifications of the fittings you plan to use. This will help ensure that the fittings are compatible and will create a proper seal when connected.

NPT-style threads are very similar to BSP, some DIN styles, and other non-American thread styles. While visually very similar, differences in thread size, pitch, and angle make them incompatible, so care needs to be taken to correctly identify your connections to create a secure and leak-proof seal. For more information on how to measure NPT fittings, feel free to contact us.