Valve End Connections: Flanged, Threaded, Butt-Weld, and Socket-Weld Options

The end connection type of a valve determines how it is attached to the surrounding piping system, and this choice has significant implications for installation cost, maintenance access, system integrity, and suitability for the process service. The four most common valve end connection types used in industrial piping are flanged, threaded (screwed), butt-weld, and socket-weld, each suited to specific applications, pressure classes, pipe sizes, and operating environments.

Flanged End Connections

Flanged valves use bolted flange connections that allow the valve to be easily removed and replaced without cutting the pipe. The valve flanges are mated with corresponding pipe flanges, with a gasket between the mating faces to provide sealing. Flanged connections are the most common type for valves NPS 2 and larger in process industry piping. Standard flange dimensions are defined in ASME B16.5 for NPS 1/2 to 24, and ASME B16.47 for larger sizes. Flange faces include raised face (RF), flat face (FF), and ring type joint (RTJ), each requiring specific gasket types.

• Raised face (RF): standard for most process piping, used with spiral wound or ring gaskets

• Flat face (FF): required when mating to cast iron or ductile iron flanges to prevent flange cracking

• Ring type joint (RTJ): metal ring seal for high-pressure, high-temperature service above Class 600

• ASME B16.5 flanges: NPS 0.5 to 24, Classes 150 to 2500

• ASME B16.47 flanges: NPS 26 to 60, Series A (MSS SP-44) and Series B (API 605)

Threaded (Screwed) End Connections

Threaded valves use pipe thread connections (NPT in North America, BSP in the UK and many other regions) to attach to threaded pipe or fittings. Threaded connections are typically limited to NPS 2 and smaller due to the difficulty of making leak-tight large-diameter threaded connections and the stresses imposed on threads by thermal cycling and vibration. PTFE tape, pipe dope, or anaerobic thread sealant is applied to the threads to provide sealing. Threaded valves are widely used in utility systems, instrumentation, HVAC, and low-pressure industrial applications where the convenience of screwed installation outweighs the limitations.

Butt-Weld End Connections

Butt-weld end valves are welded directly into the pipe, with the valve end beveled to match the pipe wall bevel for a full-penetration groove weld. This connection type provides the highest integrity, lowest leakage potential, and greatest resistance to vibration and thermal cycling of any end connection type. Butt-weld valves are specified for high-pressure, high-temperature, corrosive, or cryogenic services where flanged connections might leak or where reducing the number of potential leak points is a priority. The main limitation is that maintenance requires cutting the valve out of the pipeline, making replacement more labor-intensive.

Socket-Weld End Connections

Socket-weld connections insert the pipe into a recessed socket machined into the valve end and weld around the pipe outside diameter. This fillet weld is simpler than a butt-weld and does not require pipe end beveling, reducing fabrication cost for smaller valve sizes. Socket-weld connections are typically used for NPS 2 and smaller in higher-pressure applications where threaded connections are less desirable. A small gap (approximately 1.6 mm) must be left between the pipe end and the socket bottom before welding to allow for weld shrinkage and prevent cracking. Socket-weld connections are not recommended for crevice-corrosion-sensitive services.