Overview of Industrial Instrument Fittings
In automation and chemical instrument systems, instrument fittings are key components for connecting pipelines, instruments, and cables. The most commonly used connection methods include ferrule, NPT/G thread, cable gland, and flange. The main material is 316/316L stainless steel, and sealing is achieved by metal gaskets, lens gaskets, or O-rings.
1. Classification by Connection Structure (Most Common)
1. Ferrule Fittings
Ferrule fittings achieve sealing through ferrule + nut biting into the pipe wall, with pressure resistance up to ≤32MPa and temperature resistance ≤450℃. They are standard for instrument impulse lines (φ6/8/12), and the material is usually 316L.
Taking E+H as an example, ferrule fittings are divided into two standards: standard threaded ferrule and welded seat ferrule.
- Standard type (double ferrule structure): sealing is achieved by tightening the nut, allowing repeated disassembly and assembly, suitable for industrial-grade pipelines and instrument installation, without welding.
- Welded seat type: the ferrule base is first welded onto the pipeline, then the instrument is tightened with a nut, suitable for high-temperature, high-pressure, or corrosive media, with faster sealing. Mostly used for pressure-related instruments such as pressure transmitters.
The figure below shows a tuning fork level switch installed with a welded seat.
2. Threaded Fittings (NPT, G, M)
| Thread Type | Standard and Angle | Common Specifications | Typical Applications |
|---|---|---|---|
| NPT | American 60° taper | 1/8, 1/4, 1/2 | Most commonly used for pressure gauges and transmitters |
| G | British parallel pipe | G1/4, G1/2 | Low-pressure gas lines, explosion-proof cable glands |
| M | Metric | M20×1.5 | Thermocouples, junction boxes, cable glands |
3. Cable Glands (Cable Sealing Fittings / Glands)
Cable glands have explosion-proof/waterproof functions, with common specifications such as M20×1.5, NPT1/2, G1/2. They are mainly used for cable entries of instrument junction boxes and explosion-proof cabinets.
4. Flange Fittings (RF/FF)
Flange fittings are suitable for medium-high pressure/large diameter applications, with pressure ratings from Class150 to 2500. Transmitters and control valves commonly use 1/2"~4" RF flanges.
2. Classification by Functional Form
- Straight: straight pipe connection, most commonly used.
- Elbow (90°): used for pipeline direction change.
- Tee: one path splits into two, used for impulse or sampling.
- Reducing: transition between different pipe diameters, such as φ12→φ8.
- Adapter: thread or specification conversion, such as NPT1/2→G1/2.
- Plug/blind plug: seals spare ports.
3. Common Materials and Sealing Methods
| Category | Options | Applicable Conditions |
|---|---|---|
| Material | 316L (preferred), 304, brass, Monel | 316L general purpose; Monel for strong corrosion |
| Metal gasket | Copper, graphite, lens gasket | High pressure/high temperature |
| O-ring | Fluororubber (FKM), EPDM | FKM oil/corrosion resistant; EPDM water/steam resistant |
4. Quick Selection Mnemonic
- Impulse line (φ6/8/12) → ferrule fittings (316L).
- Pressure gauge/transmitter → NPT1/2 thread.
- Explosion-proof wiring → M20×1.5 cable gland.
- High purity/high pressure → VCR metal seal.
To learn more, you can add the WeChat below for consultation, or join the instrument communication group to exchange ideas, or call 153 2371 3323. More content is also available on the official account and website http://ainstru.com/.