Figure 1: Valve threads
To ensure compatibility and ease of maintenance, threads are standardized. For existing and new applications, it is necessary to identify the thread type to guarantee a proper connection of the joint.
The BSP, or Whitworth thread, is a family of thread standards that has been adopted internationally, except in the United States. This thread form is based on a 55° V-thread with rounded roots and crests, as seen in Figure 2. For a thread that conforms to BSP, the major diameter of the pipe thread is slightly smaller than the actual OD of the pipe, and the minor diameter will be very close to (smaller than) the inside diameter of the female thread. There are two types of BSP threads:
Figure 2: A BSPP male parallel thread profile (left) and a BSPT taperd male thread profile (right)
BSP threads are identified with letters each of which represents the type of thread and their associated standards1:
1ISO 7: Pipe threads where pressure-tight joints are made on the threads. ISO 228: Pipe threads where pressure-tight joints are not made on the threads.
Labeling Example: EN 10226 Rp 2 ½
This refers to a British Standard Pipe thread tapered (EN 10226) with an internal parallel form (Rp) and a nominal size of 2 ½.
The actual sizes of the most commonly used BSP threads are listed in Table 1 and Table 2 for BSPP and BSPT threads respectively. Table 3 provides data on pipe sizes associated with these threads.
Note: Each thread size is identified with a number which has little to do with the actual size of the thread. This discrepancy originates from amendments in industrial practices and standardisation throughout the history of standardisation of pipe threads. Therefore, always compare measurements with actual sizes listed in the tables.
|Table 1: BSPP (G) – British Standard Pipe Parallel|
|Nominal Thread Size||Major Diameter (mm)||Minor Diameter (mm)||TPI (in-1)|
|Table 2: BSPT (R/Rp) – British Standard Pipe Tapered|
|Nominal Male Tapered Thread Size (inch)||Nominal Female Parallel Thread Size (inch)||Major Diameter (mm)||Minor Female Diameter (mm)||TPI (in-1)|
|R 1/16||RP 1/16||7.723||6.490||28|
|R 1/8||RP 1/8||9.728||8.495||28|
|R 1/4||Rp 1/4||13.157||11.341||19|
|R 3/8||Rp 3/8||16.662||14.846||19|
|R 1/2||Rp 1/2||20.955||18.489||14|
|R 3/4||Rp 3/4||26.441||23.975||14|
|R 1||Rp 1||33.249||30.111||11|
|R 2||Rp 2||59.614||56.476||11|
|Table 3: British Standard Pipe Dimensions for Standard Thread Sizes|
|Nominal G / R size (in)||Corresponding Pipe|
|DN (mm)||Actual OD (mm)||Wall (mm)|
Figure 3: NPT thread profile mating with the top being a fitting internal thread and the bottom being a pipe external thread.
The American national pipe thread was created based on a 60° V-thread with flattened peaks and valleys (Figure 3) and is widely used in the US and Canada. There are two types of NP threads:
|Table 4: NPT – National (American) Pipe Thread Tapered|
|Nominal Thread Size (inch)||Major Diameter (mm)||TPI (in-1)|
Labeling Example: 3/8 - 18 NPT
National pipe threads are designated with their nominal size (3/8) followed by number of threads per inch (18) and the symbol for the thread series (NPT).
Note: NPS (National Pipe Straight) is not to be confused with NPS (Nominal Pipe Size) which is an American set of standards for pipes. For a given outside diameter, NPS (Nominal Pipe Size) provides multiple pipe schedules (wall thicknesses) while the pipe thread profile remains the same among them.
Figure 4: Metric Thread profile
The Metric Thread is one of the first internationally agreed general-purpose thread type. The V-shaped thread form has a 60° flank angle and male and female threads are both parallel (Figure 4). Metric Threads come in different pitch sizes for a given diameter: coarse pitch and fine pitches. Coarse threads have the default pitch size whereas fine threads have smaller pitch sizes and are used less often. As a result, coarse threads are identified by diameter only while fine threads are recognized by diameter as well as pitch size.
Metric threads come in two different pitch sizes for a given diameter:
Labeling Example: M8
This refers to a metric coarse thread with a diameter of 8 mm (which with reference Table 5, corresponds to a pitch size of 1 mm)
Labeling Example: M4 x 0.5
This indicates a fine thread with a diameter of 4 mm and a pitch size of 0.5 mm.
M 10M 10
|Table 5: Metric Threads (Coarse)|
|Thread Size (mm)||Major Diameter (mm)||Minor Diameter (mm)||Pitch (mm)|
Figure 5: Pitch gauge
To identify an unknown thread, verify the following:
Check Tables 1-5 to find a matching actual diameter corresponding to a nominal thread size. Confirm thread pitch or TPI with the values corresponding to that diameter.
Although some BSP and NPT threads might be tightened and engaged together fairly well, pitch and thread angle differences between them will allow spiral leakage. Specifically speaking, at 1/16”, 1/8”, 1/4”and 3/8” sizes, pitch difference results in misalignment of the threads. This misalignment might result in the threads to be fastened only partially. However, at 1/2” and 3/4” sizes, NPT and BSP have the same pitch which will allow them to engage well. Nevertheless, since there are differences in thread forms and thread angles, there will still be issues with this connection. For low end applications, if an appropriate thread sealant is used, these threads might be used together effectively. If you are creating your own thread, please read our tap and die set article. Table 6 shows the compatibility of threads and the required seal type to be used as a reference for your application.
|Table 6: Thread Compatibility Chart|
|My Fitting||Designation||Matching Fitting||Designation||Seal Type||Notes|
|BSPP Parallel Female||G||BSPP Parallel male||G||O-ring/washer||Rs or NPT male threads are incorrect installations here. Rs threads are slightly smaller than G threads, and NPT threads have a different form.|
|BSPT taper male||R||Tape/pipe dope|
|BSPT Parallel Female||Rp||BSPT taper male||R||Tape/pipe dope||G threads are incorrect installation here as they have different tolerances.|
|BSPT Taper Female||Rc||BSPT parallel male||Rs||Tape/pipe dope||Inserting an NPT male here is an incorrect installation, as NPT threads have a different form.|
|BSPT taper male||R||Tape/pipe dope|
|NP Taper Female||NPT||NP taper male||NPT||Tape/pipe dope||BSP threads are not to be used with NPT threads even if the threads seem to engage fairly well.|
|NP Parallel Female||NPS||NP parallel male||NPS||O-ring/washer||NPT thread is an incorrect installation here as sealing is not possible.|
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