Understanding Pipe Threads

Pipe threads are a surprisingly complex topic — and an important one as well. 

In many cases, the thread on a pipe or piping system component is a critical part of safe operation and an effective seal. 

Worse still, choosing the wrong pipe thread type or size could cause irreparable damage to the components in use, costing additional time and money to rectify the situation.

This guide will look at the science behind pipe threads and the various standards and types of pipe thread common throughout the world. 

We’ll also provide tips and tricks for determining your proper pipe thread type and best practices for safely using threaded pipe in your operations.

How Pipe Thread Works

Threads can take on various shapes and sizes, offering a tight seal for nearly any piped material, including air, gas, liquids, hydraulic fluids, slurries, and more.

Once reserved for brass and steel, you can find threaded pipe in a variety of materials today, including bronze, cast iron, nylon, PTFE, and PVC.

When combined with a mirrored version of themselves, these threads harness mechanical force to create a secure seal or connection between both pipes themselves and piping components.

Depending on the thread type used, the connection may require additional treatment, such as tapes, coatings, or sealants, to lubricate or reinforce the seal.

For Example:

Pipe Thread Standards

Pipe thread standards define thread pitch, thread angle, and thread dimensions, allowing for increased consistency across manufacturers and product lines. 

Common standards include:

• National Pipe Thread or American National Standard Pipe Thread (NPT)
• British Standard Pipe (BSP)
• Japanese Industrial Standard (JIS) Tapered Pipe Thread (PT)
• SAE (SAE International) Straight Thread
• Metric Tapered/Parallel Thread (M)

As you can likely tell, your geographic location (or where the pipe or component in question was manufactured) will often impact the standard used.

To further complicate things, while various standards are often incompatible, each of these standards can also have different thread types that are not compatible even within the same thread standard. 

While they might appear to thread together in some cases, in most, you won’t achieve a proper seal. 

You might even irreparably damage the pipe, threads, or components joined. Precisely understanding what thread type and size you’re dealing with is essential.

We’ll be looking at the National Pipe Thread standard for this guide — the most widely used standard in the US and Canada in accordance to ASME B1.20.1.

Additional thread types within the NPT standard include:

• NPT: National Pipe Tapered
• NPS: National Pipe Straight
• NPSC: National Pipe Straight-Coupling
• NPSF: National Pipe Straight-Fuel
• NPSH: National Pipe Straight-Hose
• NPSI: National Pipe Straight-Intermediate
• NPSL: National Pipe Straight-Locknut
• NPSM: National Pipe Straight-Mechanical
• NPTF: National Pipe Taper-Fuel
• NPTR: National Pipe Taper-Railing
• PTF-SAE SHORT: Pipe taper, fuel, SAE, short

Determining Pipe Thread Type

If you want to ensure an effective fit and seal, it’s critical to know the exact type of threads with which you’re working. 

In many cases, the fitting, pipe ends, ports, or other terminating locations will include a stamp that describes standards and measurements.

However, if you don’t see any information, you can use a few standard tools to gather additional details and limit thread options to a handful of choices.

• A calliper: Callipers are typically affordable and offer greater accuracy and ease of use. However, a steel straight rule can work as well. 
• Pitch gauges: Laying these in the thread makes it easy to find a perfect measure of the threads-per-inch measurement of your pipe, fitting, or other components.

With these tools and some basic observations, you can gather information about the component in question and determine the thread type.

Things to consider include:

Is the thread male or female?

Male threads are on the outside of the piece, while female threads are on the inside. 

The designation will have little influence on performance but is an essential part of ensuring that connections are possible and secure.

Do you have the right schedule of pipe?

According to ANSI ASME B36.19 schedules 5S and 10S wall thicknesses do not permit threading in accordance with ASME B1.20.1.

Is the thread tapered or parallel?

Tapered threads subtly reduce in diameter as they extend from the end of the pipe or component. 

The tapered shape allows the pipe to establish an effective seal using metal-to-metal wedging and compression. 

NPT standards typically require a 3/4-inch decrease in diameter over 1 foot of distance. 

If you cannot detect a taper through visual inspection, using a calliper to measure the first, middle, and final threads should make any taper apparent. 

Parallel threads (also known as straight threads) are all equal and typically rely on an o-ring, gasket, washer, thread tape, or other treatment to establish an effective seal.

What is the thread’s pitch size?

The way you find this measurement varies slightly depending on the standard in use.

For most thread types, the pitch size is the number of threads in an inch of threaded pipe or component. 

However, by metric standards, the pitch size describes the distance between threads.

Both details are easily found using pitch gauges or through visual observation and calliper measurements.

Thread Diameter

Finally, you’ll want to use the calliper to get the outside diameter on male threads and the inside diameter on female threads. 

The pipe diameter gives you an excellent starting point for determining the nominal pipe size of your threading. 

You might find that your measurements don’t line up precisely with standards with most nominal sizes. 

These minor discrepancies are typical of nominal sizing. 

However, in most cases, you’ll find an obvious answer that correlates with your findings.

Putting It All Together

With the information gathered, you can then compare your findings to the sizing charts for each standard. 

In most cases, starting with the most common standard in the region in which the pipe or component is manufactured is a good starting point. 

However, if you don’t find a match, check the standards expected in the regions the pipe or component was designed to be used in. 

We’ve provided an example of a typical NPT basic dimensions chart below. 

However, most standards offer tables that you can easily find with a quick search in your preferred search engine.

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Pipe Thread Chart

ASME B1.20.1 (NPT/API)

BASIC DIMENSIONS OF AMERICAN NATIONAL STANDARD TAPER THREADS, NPT (ASME B1.20.1)

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So what can you do if threads don’t quite match up or you need to join multiple pipes or components? 

Adapting Pipe Threads through Fittings

Threaded pipe fittings are likely your answer.

Like a threaded pipe, threaded fittings are available in a range of materials depending on your needs and budget. 

Frequently used threaded pipe fittings include:

In many cases, using fittings and adapters is a cost-effective means of ensuring compatibility between various components in your piping process with minimal risk and comparable performance designs without fittings.

Pipe Thread Best Practices

Exact requirements will depend on your industry, regulations, budget, system designs, and a wide range of other variables we just can’t account for in a single guide. 

However, considering these basic points should improve the safety and service life of your threaded pipe products.

1. Be mindful of hand-tightening and wrench-tightening tolerances. Over-tightening threaded pipe — especially tapered pipe — can lead to compression and damage of the threads. This could lead to structural concerns, impact the quality of the seal provided, and make maintenance or removal more difficult later.
   
2. Avoid using sealant tapes to match differing thread types. As mentioned above, just because two different thread types might appear to thread together, doing so is always a gamble. While using sealant tapes might be an acceptable stopgap measure while a proper solution is sourced and applied, it should never be the primary solution.
   
3. Take care when aligning thread connections. Always ensure proper alignment when connecting threaded parts. If you experience substantial resistance, stop tightening immediately and inspect the connection. Cross-threading — or connecting pipe threads at an improper angle — can cause irreparable damage to threads and result in an ineffective seal. Lubricating connections with tapes and treatments can help to reduce cross-threading risks further and reduce thread integrity risks.
   
4. Always verify seals. While threaded pipes provide an outstanding seal in a variety of conditions, be sure to verify seals prior to unmonitored operation. In some cases, an external sealant, o-ring, washer, or gasket might be required to achieve the desired level of seal, particularly with straight or parallel thread patterns.
   
5. When using fittings, be sure to consider the materials and match them to your piping and other components when possible. This will minimize galvanic corrosion risks and ensure that your fitting provides comparable pressure ratings, environmental resistances, and overall performance to the rest of your piping system.

Key Takeaways

• Threads are defined by a variety of standards — typically influenced by region.
• Threads of different standards are rarely compatible and mixing standards without the use of fittings is not recommended.
• NPT is the most common thread standard in North America.
• Tapered threads provide an excellent seal and are most common. 
• Straight or parallel threads typically require a gasket, seal, or o-ring to achieve effective seals making them less common.
• Fittings offer a cost-effective means of adapting various thread types, styles, and sizes. 
• Always consider performance variance and corrosion risks when using threaded pipe with other components of differing materials.
• Never force threaded pipes and adhere to recommended hand-tight and wrench-tight specifications to avoid cross-threading or damaging the pipe threads.
  
  

As a leading provider of stainless steel piping and piping system components across North America and Western Canada, Unified Alloys has a thorough understanding of the myriad threaded pipe products on the market today. Our expert sales analysts can help you choose a product that meets your required specifications and will provide long-lasting, safe performance. Contact us today to discuss our selection of threaded pipe, threaded flanges, and other piping system components!