Industrial Valve End Connections: Flanged, Threaded, Socket Weld, and Butt Weld

Overview of End Connection Types

An end connection is the interface between the valve body and the adjoining piping. It must transfer fluid pressure without leakage, accommodate thermal expansion and pipeline loads, and be maintainable or replaceable in service. The main types used in industrial piping are: Flanged (RF, FF, RTJ); Butt Weld (BW); Socket Weld (SW); Threaded (NPT, BSP/BSPP/BSPT); Wafer; Lug; and Grooved/Victaulic. The valve end connection must always match the piping class specified in the project Piping and Instrument Diagram (P&ID) and the site Piping Material Specification (PMS).

Flanged End Connections (ASME B16.5 and B16.47)

Flanged valves are the most common in process plants for NPS 2" and above. The flanges are integral to or welded onto the valve body and mate with matching pipe flanges, bolted together with a gasket. ASME B16.5 covers NPS 1/2" through NPS 24" in Classes 150, 300, 600, 900, 1500, and 2500. ASME B16.47 covers NPS 26" through NPS 60" in Series A (formerly MSS SP-44, used by the oil industry) and Series B (formerly API 605, used by the chemical industry). The raised face (RF) is standard for ASME Class 150 and 300; Ring-Type Joint (RTJ) is used for Class 600 and above and for critical high-integrity joints.

Butt Weld (BW) End Connections

Butt weld valves have bevelled pipe ends that are directly welded to the mating pipe using a full-penetration groove weld, typically qualified per ASME IX and inspected by RT or UT. ASME B16.25 governs butt weld end preparation. Butt weld is preferred for: high-pressure and high-temperature service where flange leakage is unacceptable; cryogenic and toxic service; NPS 2" and above in critical piping classes; offshore and subsea piping where flanged connections are minimised to reduce leak points. Disadvantages: valve replacement requires cutting the weld, making in-line maintenance slower. A valve with a short pup piece welded to the valve ends allows future removal with a grinder without disturbing the main pipe joint.

Socket Weld (SW) End Connections

Socket weld valves have sockets (bores) into which the pipe is inserted, then fillet-welded around the outside. ASME B16.11 governs socket weld fittings in Class 3000, 6000, and 9000. Socket weld is used almost exclusively for NPS 2" and below (small bore piping). The pipe must be withdrawn 1.6 mm before welding to allow for thermal expansion — failing to leave this gap can crack the weld during heat-up. Socket weld is not recommended for applications where crevice corrosion is a concern, as the gap between socket and pipe traps stagnant fluid.

Threaded End Connections (NPT and BSP)

Threaded valves use tapered or parallel threads machined on the valve end ports that engage with matching pipe threads. ANSI/ASME B1.20.1 defines NPT (National Pipe Taper) — the US standard taper thread; BSP (British Standard Pipe) per ISO 7/1 (BSPT — taper) and ISO 228 (BSPP — parallel) is used in Europe, the Middle East, and India. Threaded connections are used for NPS 2" and below in utility services — instrument impulse lines, sample connections, drain/vent valves, and small auxiliary connections. They should not be used in high-vibration, high-cycle, or cryogenic service where thread loosening or seal failure risk is elevated.

Wafer and Lug Body Types for Butterfly and Check Valves

• Wafer Body: Valve is sandwiched between two pipe flanges and clamped by long bolts passing through the flange bolt holes — the body itself has no integral flanges. Most compact and lowest-cost option for large-bore isolation duty. Cannot be used at end-of-line. Standard per API 609 Category A.
• Lug Body (Lugged): Body has integral threaded lugs around the circumference that align with pipe flange bolt holes — bolts thread into the lugs. A lug butterfly valve can be unbolted from one side while maintaining isolation on the other side, making it safe for end-of-line installation. Heavier and more expensive than wafer. Standard per API 609 Category B.
• Double-Flanged Body: Body has integral pipe flanges on both ends — highest cost and heaviest, but fully independent of the adjacent pipe flanges; required for very large-diameter valves (NPS 48"-96") and high-performance triple-offset butterfly valves in Class 600 and above.
• Grooved/Victaulic: Valve ends have circumferential grooves that engage with split couplings — used in fire protection (NFPA 13), HVAC, and large-bore water systems for fast installation and disassembly.

End Connection Selection Guide