Example of size comparison between American standard NPT and British standard BSP
NPT tapered thread (American standard)
|
type |
Threads Per Inch (TPI) |
Effective engagement length |
Taper |
Application Scenario |
|
NPS 1/2 NPT Thread |
14 |
0.8 inches Approximately 20.3 mm |
1:16 |
Sealing connection of oil pipeline |
Dry seal application: NPTF (dry seal thread) is widely used in automotive fuel systems. It reduces the dependence on sealants by improving the tooth profile. For example, the oil circuit interface of a certain model of engine uses 1/4 inch NPTF thread.
BSP thread (British standard)
Straight thread (BSPP) and tapered thread (BSPT): 1/2-inch BSPP straight thread corresponds to the nominal diameter DN15 (15 mm), while BSPT tapered thread has stronger sealing ability. For example, the DN20 high-pressure pipeline of a chemical equipment uses 1/4-inch BSPT thread.
Unit difference: The pitch of BSP thread is expressed in threads per inch (TPI). For example, 1/8-inch BSP thread is 28 TPI, while the pitch of metric M10×1 thread is 1 mm. The two need to be converted by 25.4 mm/inch.
Metric thread compatibility example with ANSI/British standard
Metric Straight
Size comparison chart:
|
standard |
Nominal diameter |
Pitch/TPI |
unit |
|
Metric |
M20 |
1.5 mm |
mm |
|
NPT |
3/4″ |
14 TPI |
inch |
|
BSP |
3/8″ |
19 TPI |
inch |
Pipe thread dimensions unit Conversion Example
Pay special attention to the difference between nominal diameter (DN) and physical size. A 1-inch BSP thread corresponds to DN25 (25 mm), but its actual outer diameter is 33.25 mm.
Engineering cases of leakage caused by wrong selection
Risk of mixed standards
Due to a mistake, a project connected a 1/2-inch NPT tapered thread with a 1/2-inch BSPP straight thread. The thread angle (60° vs. 55°) and the taper could not match, resulting in leakage of high-pressure steam. Therefore, a unified standard BSPT connector was required in the project.
Improper sealing method
A refrigeration system used NPSM straight threads without adding gaskets, relying only on thread engagement, which caused serious leakage of low-pressure refrigerant. Later, it was changed to NPT tapered threads and matched with sealants to solve the problem.
Summary
Determine the unit and standard: First determine the unit, whether to apply the American standard, the standard or the metric system. Use the size table to choose whether to use straight threads or tapered threads.
Scenario selection: Straight threads, such as NPSM and BSPP, can be selected in low-pressure mechanical connection scenarios. Tapered threads, such as NPT and BSPT, should be selected in high-pressure scenarios.
Global design: When facing projects that are completed in multiple countries, it is more important to pay attention to the parameters between units, such as DN15 (1/2 inch) BSPP thread.
