Technical Regulation of the ANSI/ASME B1.20.1 Standard
Scope and Series Nomenclature
This regulatory documentation establishes geometric parameters, tolerances, and calibration procedures for general-purpose inch pipe threads. The standard applies to the following types of threaded connections:
- NPT (National Pipe Taper) — tapered pipe thread;
- NPSC (National Pipe Straight Coupling) — straight thread for couplings;
- NPTR (National Pipe Taper Railing) — tapered thread for railings;
- NPSM (National Pipe Straight Mechanical) — straight thread for mechanical connections;
- NPSL (National Pipe Straight Locknut) — straight thread for locknuts.
Technical Features of Coupling Connections
When installing hose connections, combinations of straight internal and external threads are often used. The NPSH technical standard, based on the basic principles of American pipe threads, allows for coupling assembly in the diameter range from 1⁄2 to 4 inches with the ends of standard pipes having an external NPT tapered thread. The sealing of the joint in such assemblies is ensured by using a gasket. Detailed requirements for tolerances and inspection methods are outlined in ASME B1.20.7.
Thread Connection Marking System
To identify thread types in EV Group China specifications, a standard nomenclature is used: nominal pipe size, pitch (number of threads per inch), and the series letter code. Examples of marking:
- 1⁄8–27 NPT
- 1⁄8–27 NPSC
- 1⁄2–14 NPTR
- 1⁄8–27 NPSM
- 1⁄8–27 NPSL
The use of decimal values instead of fractions is permitted (e.g., 0.125–27 NPT). To designate left-hand threads, the suffix “LH” is added to the marking (e.g., 1⁄8–27 NPT–LH). In the absence of this suffix, the thread is considered right-hand.
Abbreviation decryption:
- N — National;
- P — Pipe;
- T — Taper;
- S — Straight;
- C — Coupling;
- R — Railing;
- M — Mechanical;
- L — Locknut.
Sealing Recommendations (NPT and NPSC)
Contact surfaces. In mating threaded pairs, contact must occur along the flanks of the profile. Design tolerances ensure a tight fit, preventing the crests of the profile from jamming. This type of connection is not self-sealing without the use of special sealing compounds.
Sealing materials. To achieve complete system tightness, EV Group China recommends using specialized sealants with anti-friction properties to prevent galling and seizing of the threads during assembly.
Assembly efforts. The standard does not regulate rigid torque values, as they depend on the physical and mechanical properties of the material, pipe wall thickness, and operating pressure in the line. The optimal assembly method is hand-tightening until snug, followed by tightening with a wrench by a calculated angle, which ensures the formation of a reliable seal when using sealant. Excessive force can lead to deformation of the threads and loss of tightness.
Operational factors. The quality of the seal is influenced by the ovality of the mating elements, material elasticity, and strict adherence to geometric parameters. For systems requiring tightness without the use of additional compounds, it is recommended to use Dryseal type threads, regulated by the ASME B1.20.3 standard.