When it comes to connecting plumbing systems, you must know both the type of pipe thread and size. This way, you will have perfect connections with zero leakages or malfunctions.
Therefore, at any given time, you must choose a suitable pipe thread. With that in mind, let’s explore some of the different types of pipes threads:
Parts of Pipe Thread
Knowing what a pipe thread looks like is critical. It will help you understand the various classification criteria. Ideally, normal threads for pipe have the following key features:
- Thread dimension – it refers to the inner and outer diameters
- Thread angle – angle determines thread shapes and use. You can get the angle by determining the difference between pipe threads. Alternatively, you can determine the angle by measuring the valley forming two consecutive peaks.
- Thread pitch – the distance between two adjacent peaks of course concerning the crest.
- Crest – uppermost section of a thread
- Root – lowermost section of the thread
Male Pipe Threads vs Female Pipe Threads
This is the first and the most basic type of pipe thread classification criteria. Ideally, it describes the exact location of the threads on pipe systems. Simply put:
- Male pipe threads – threads on the outer pipe surface are referred to as male.
- Female pipe threads – these threads are on the inner surface of the pipe system.
Therefore, looking at the threads on pipes, you can easily tell whether they are male or female. However, other classifications may require a little expertise and maybe refer to the types of pipe thread charts for reference.
National Pipe Tapered (NPT) or National Pipe Straight (NPS)
A common thread in North America. Although these thread types were common in brass and steel piping systems, they have been adopted for use in other materials.
Usually, when it comes to reliable sealing mechanisms, the tapered threads offer a better option than the straight options. For the national pipe tapered thread, they have these unique features:
- Pipe thread crest and troughs make an angle of 60 degrees
- Usually, the taper angle is 1°47’
To determine the pitch, you will measure the threads per inch.
British Standard Pipe Tapered (BSPT) or British Standard Pipe Straight (BSPS)
These types of pipe thread for fittings are common in the Commonwealth countries. Comparing the NPT thread to BSP threads, the latter has rounded crests and troughs. Furthermore, crests and troughs make an angle of 55 degrees in the BSP threads.
Furthermore, the BSPT is characterized by a tapered seal while the BSPP is straight or parallel.
Again, it is worth noting that the BSPP is also referred to as the “G” thread. On the other hand, the BSPT is also called the “R” thread. Therefore, you should not confuse the two types of pipe threads.
National Unified Special (UNS)
UNS threads are used alongside the compression pipe fitting systems. They are common in Canada and the United States.
Although the UNS shares many properties with the NPT, you cannot interchange these pipe threads.
API Line Pipe Threads
These are unique pipe threads designed to withstand high temperatures and pressure in the oil and gas industries. Although the threads are similar to the NPT, their distinct features include:
- Tapered structure
- Peaks have a “round” shape
- Thread angle is 30°
Ideally, these threads are not suitable for low-pressure applications.
Buttress Pipe Threads
If you are considering pipes and fittings for heavy loads, then you should consider buttress threads. These pipe threads have inclined planes making them suitable for high-pressure applications such as hydraulic systems.
You can easily achieve tight sealing with the buttress pipe threads.
Square Pipe Threads
Square threads feature distinct rectangular shapes which contribute to the extremely high-strength design. They are a perfect choice for high-power applications.
Compression Thread Type
Compression threads do not mate with other types of pipe threads.
How to Choose Best Thread for Piping System
Certain variables determine the type of threads for piping systems. Usually, it is never an issue to do with guesswork.
More importantly, you must consider compatibility, compliance, and functionality. So, let’s look at a few variables you must consider:
Geographical Location
In most cases, if you live in the USA, most likely your piping will adopt the NPT or NPS. On the other hand, if you live within the EU market, your threads will be BSP.
Even if you don’t live in these regions, the pipe fittings manufacturers will specify the thread standards. So, it will be easier to choose an appropriate pipe thread type.
Choosing either Male or Female Pipe Threads
Well, this will depend on whether the threads are on the inner or outer section of the pipe.
Straight Pipe Threads vs Tapered Pipe Threads
The straight pipe thread has nearly the same cross-section throughout the thread. On the other hand, the tapered pipe threads tend to have a narrow cross-section as they extend outwards.
At times, you may require a seal on the pipe threads to form a tight sealing mechanism.
Consider Types of Pipe Thread Sizes
Determining the pipe thread sizes can be a challenging task. It is because the different types of pipe threads at times do not follow a standard size.
However, you can use special measuring instruments such as a caliper to determine the outer and inner diameters. That is, for female pipe threads, you will measure the inner diameter. While for the male threads, you will measure the outer diameter.
Let’s look at these types of pipe threads chart:
Sample NPT Thread Chart
| Nominal Size | Actual OD | Hand Tight Turns | Wrench Turns |
| 1/8 | 0.405 inches | 4.5 | 2.5 |
| 1/4 | 0.54 inches | 4 | 3 |
| 1 | 1.32 inches | 4.5 | 3.25 |
Sample BSP Thread Chart
| Nominal Size | Major Diameter (OD) | Minor Diameter (ID) |
| 1/8 | 0.38 inches | 0.34 |
| 1/4 | 0.52 inches | 0.45 |
| 1 | 1.30 inches | 1.19 |
Even with all these, compatibility is still a critical factor you must consider. Only use pipe threads that are compatible with your applications.
At the same time, the discussion on different types of pipe threads will not be complete without mentioning the following:
- JIS pipe threads standard – this is a Japanese industrial standard.
- SAE thread type – these are threads as outlined by the Society of Automotive Engineers
- Metric thread types – these are common thread types in Europe.
Conclusion
The variations in the types of pipe thread identification systems vary depending on the location like in the case of NPT and BSP. Therefore, even as you choose or fabricate pipe threads, it is important to follow the best practices.
At D&R, we are here to offer free consultations on pipe threads and valve systems – talk to our engineers now.