1. Definition and Core Purpose
2. Design and Structure
Features tapered threads (internal or external) along the pipe or fitting.
No bolts, gaskets, or separate components are required for the connection itself-only the threaded surfaces.
Sizes are defined by nominal pipe size (NPS) and thread pitch (e.g., NPT 1/2", NPT 2").
Consists of a circular "face" with a bore (matching the pipe's inner diameter) and a series of bolt holes around the perimeter.
Requires additional components: bolts, nuts, and a gasket (made of rubber, metal, or composite materials) to seal the joint.
Designs vary by type (e.g., weld neck, slip-on, threaded) and standards (e.g., ANSI/ASME B16.5, DIN), with features like raised faces, flat faces, or ring-type joints (RTJ) for different pressure/temperature needs.
3. Installation and Assembly
Assembled by screwing the male thread into the female thread until tight (often with a "hand-tight plus a quarter turn" rule).
No welding or bolting is needed, making installation quick in low-complexity setups.
Disassembly is possible but may damage threads if over-tightened repeatedly.
Requires precise alignment of bolt holes between mating flanges.
Involves placing a gasket between the flange faces, then tightening bolts evenly to compress the gasket and create a seal.
May require welding (for weld neck flanges) or threading (for threaded flanges) to attach the flange to the pipe, adding installation steps.
4. Pressure, Temperature, and Application Limits
Suitable for low to moderate pressure systems (typically up to 1,000 psi, depending on material and thread quality).
Less reliable in high-temperature environments, as thermal expansion can loosen the seal.
Common in plumbing, small-diameter fluid lines, and non-critical industrial applications (e.g., compressed air, water).
Designed for high-pressure, high-temperature, and critical systems (e.g., oil and gas pipelines, chemical processing, power plants), with pressure ratings up to 25,000 psi (ANSI Class 2500) or higher.
More robust against thermal cycling and vibration, as the bolted gasket seal maintains integrity under stress.
Preferred for large-diameter pipes, where threaded connections would be impractical or weak.
5. Sealing Mechanism
NPT: Relies on the interference of tapered threads to create a metal-to-metal seal, augmented by thread sealants to fill micro-gaps.
Flange: Uses a gasket (compressed between two flange faces) to block fluid flow, with the bolts ensuring uniform pressure on the gasket.
6. Flexibility and Maintenance
NPT: Offers limited flexibility-threaded joints are rigid, and misalignment during installation can cause leaks. Maintenance may require cutting and rethreading damaged pipes.
Flange: Allows for easier disassembly and maintenance (e.g., replacing gaskets or components) by simply unbolting the flange. Better tolerance for minor alignment errors between pipes.
NPT is a simple, threaded connection for low-pressure, small-scale applications, while flanges are robust, bolted connections designed for high-performance, large-scale, or critical systems.
