British Standard Pipe Threads:
For industrial fittings, BSP threads, an abbreviation for British Standard Pipe, reign supreme in the lands of Australia and the Commonwealth countries, adding a touch of sophistication to the tapestry of plumbing and engineering. As well-known BS standard malleable iron fittings factory from China, we will explain BSP Thread fittings thread connection below:
Within the realm of BSP threads, two distinguished types emerge:
- BSPP: Known as the General Purpose thread, both the female and male threads don this moniker. A harmonious blend of precision and grace, BSPP threads exude a sense of unity in their parallel design.
- BSPT: The Resolute and Reliable thread, where the female thread stands parallel while the male thread tapers with determination. Often, the female thread may adopt a tapered form, known as Rc, a rare sight in the vast landscape of BSP threads. It is not uncommon to encounter a fusion of terminology, where the female is referred to as BSPP (parallel) and the male as BSPT (tapered), despite both embodying the essence of BSP threads.
In a delightful convergence of uniformity, both BSPP and BSPT threads share the same pitch, angle (set at 55 degrees), and shape characterized by rounded peaks and valleys. As we delve deeper into the intricate world of BSP threads, let us explore the majesty of each thread size, where the minor diameter, though meticulously detailed in the table below, may exhibit subtle variations based on the thread gauge’s place of origin.
The below table gives the major and minor diameter for each BSP Trade Thread Size. let us embrace the legacy and craftsmanship that define the British Standard Pipe:
| Trade Size | Threads per inch | Pitch | Major Diameter | Minor Diameter | Gage Length | ||||
|---|---|---|---|---|---|---|---|---|---|
| Inch | mm | Inch | mm | Inch | Mm | Inch | mm | ||
| 1/8 | 28 | 0.0357 | 0.907 | 0.383 | 9.728 | 0.3372 | 8.565 | 0.1563 | 3.97 |
| 1/4 | 19 | 0.0526 | 1.337 | 0.518 | 13.157 | 0.4506 | 11.445 | 0.2367 | 6.012 |
| 3/8 | 19 | 0.0526 | 1.337 | 0.656 | 16.662 | 0.5886 | 14.95 | 0.25 | 6.35 |
| 1/2 | 14 | 0.0714 | 1.814 | 0.825 | 20.955 | 0.7336 | 18.633 | 0.3214 | 8.164 |
| 3/4 | 14 | 0.0714 | 1.814 | 1.041 | 26.441 | 0.9496 | 24.12 | 0.375 | 9.525 |
| 1 | 11 | 0.0909 | 2.309 | 1.309 | 33.249 | 1.1926 | 30.292 | 0.4091 | 10.391 |
| 1 ¼ | 11 | 0.0909 | 2.309 | 1.65 | 41.91 | 1.5336 | 38.953 | 0.5 | 12.7 |
| 1 ½ | 11 | 0.0909 | 2.309 | 1.882 | 47.803 | 1.7656 | 44.846 | 0.5 | 12.7 |
| 2 | 11 | 0.0909 | 2.309 | 2.347 | 59.614 | 2.2306 | 56.657 | 0.625 | 15.875 |
| 2 ½ | 11 | 0.0909 | 2.309 | 2.96 | 75.184 | 2.8436 | 72.227 | 0.6875 | 17.463 |
| 3 | 11 | 0.0909 | 2.309 | 3.46 | 87.884 | 3.3436 | 84.927 | 0.8125 | 20.638 |
| 4 | 11 | 0.0909 | 2.309 | 4.45 | 113.03 | 4.3336 | 110.073 | ||
National Pipe Thread:
NPT threads, a revered symbol of precision and reliability, proudly bear the title of National Pipe Thread, representing the pinnacle of American craftsmanship in the realm of threading excellence. Let us embark on a journey to discover the nuances of NPT threads, known by various monikers such as MPT, MNPT, NPT (M) for male external threads, and FPT, FNPT, or NPT (F) for their female internal counterparts.
A meticulous dance of thread and sealant ensues, for a leak-free seal is paramount in the world of NPT threads, with the exception of the resilient NPTF variant. Much like its BSP counterpart, NPT threads adhere to the principle of Trade Size over actual diameter, adding a touch of intrigue to the art of measurement.
In a harmonious alignment of precision, both male and female NPT threads share the same pitch, angle (set at 60 degrees), and shape characterized by flat peaks and valleys, a testament to the uniformity and elegance of American threading standards. The below table gives the Threads Per Inch, Pithc and Major Diameter for NPT Threads.
| Trade Size | Threads per inch | Pitch | Major Diameter (O.D) | ||
|---|---|---|---|---|---|
| Inch | mm | Inch | mm | ||
| 1/8 | 27 | 0.03704 | 0.94082 | 0.405 | 10.29 |
| 1/4 | 18 | 0.05556 | 1.41122 | 0.54 | 13.72 |
| 3/8 | 18 | 0.05556 | 1.41122 | 0.675 | 17.15 |
| 1/2 | 14 | 0.07143 | 1.81432 | 0.84 | 21.34 |
| 3/4 | 14 | 0.07143 | 1.81432 | 1.05 | 26.67 |
| 1 | 11 ½ | 0.08696 | 2.20878 | 1.315 | 33.4 |
| 1 ¼ | 11 ½ | 0.08696 | 2.20878 | 1.66 | 42.16 |
| 1 ½ | 11 ½ | 0.08696 | 2.20878 | 1.9 | 48.26 |
| 2 | 11 ½ | 0.08696 | 2.20878 | 2.375 | 60.33 |
| 2 ½ | 8 | 0.125 | 3.175 | 2.875 | 73.03 |
| 3 | 8 | 0.125 | 3.175 | 3.5 | 88.9 |
| 4 | 8 | 0.125 | 3.175 | 4.5 | 114.3 |
BSP vs NPT Threading Standards:
The BSP (British Standard Pipe) and NPT (National Pipe Taper/Straight) standards are the two most common pipe threading systems used worldwide. However, they are not interchangeable due to differences in their design specifications.
BSP utilizes the internationally recognized ISO 7 tapered and parallel thread forms. Common BSP designations include:
- BSPT (British Standard Pipe Tapered) – External tapered thread
- BSPP (British Standard Pipe Parallel) – External parallel thread
- Rp (ISO 7/1) – Internal parallel thread
- Rc (ISO 7) – Internal tapered thread
NPT conventions follow standards set by the American National Pipe Thread (NPS/NPT). Popular types are:
- NPT – National Pipe Tapered thread
- NPS – National Pipe Straight thread
While BSP and NPT pipe sizes may have similar outside diameters, their threads are not compatible. BSP uses a 55-degree thread angle with rounded roots and crests, whereas NPT specifies a 60-degree angle with flat surfaces.
This makes physically joining BSP and NPT fittings dangerous, as they are designed for different thread forms and engagement principles. Intermixing the standards risks leaks or mechanical failures, especially in high-pressure applications.
Pipefitters must carefully select only BSP or only NPT components to ensure proper sealing and structural integrity throughout a piping system. The threading standards are not a simple matter of different pitches – their thread profiles are fundamentally distinct. NPT and BSP thread pitches (threads per inch) are listed below. To determine pitch, use a thread gauge or count the number of threads that fall into a 1″ span.
| Trade Size | Pitch (Threads per Inch) | |
|---|---|---|
| NPT/NPS | BSP | |
| 1/8 | 27 | 28 |
| 1/4 | 18 | 19 |
| 3/8 | 18 | 19 |
| 1/2 | 14 | 14 |
| ¾ | 14 | 14 |
| 1 | 11 ½ | 11 |
| 1 ¼ | 11 ½ | 11 |
| 1 ½ | 11 ½ | 11 |
| 2 | 11 ½ | 11 |
| 2 ½ | 8 | 11 |
| 3 | 8 | 11 |
| 3 ½ | 8 | 11 |
| 4 | 8 | 11 |
| 5 | 8 | 11 |
| 6 | 8 | 11 |
Measuring Threaded Pipe & Fittings:
The first step in identifying a thread is to determine if it is tapered or parallel. To do this, measure the diameter of the first, fourth/fifth, and last full thread:
- Tapered Thread: The diameter will increase from first to last thread on a male fitting, or decrease from first to last on a female fitting.
- Parallel Thread: The diameter will be consistent across all measured threads.
Once the thread type is known, measure the diameter to find the trade size:
- Parallel Thread: Measure any full thread and reference the measurement.
- Tapered Thread: Measure the fourth or fifth full thread and reference the measurement.
Note that the actual measurement does not equate to the nominal/advertised size. For example, a 1/2″ fitting might measure 9/16″ across the threads. The measured diameter then needs to be cross-referenced to thread identification tables to correlate it with the correct trade size designation. These tables will list diameters that correspond to standard BSPT, BSPP, NPT, and NPS sizes.