Valve End Connections: Flanged, Threaded, and Welded Joints

Valve end connections determine how the valve integrates with the process piping system. The selection of end connection type affects installation cost, leak potential, replaceability, and suitability for the process conditions. Common end connection types include flanged, threaded, socket weld, butt weld, and wafer/lug (for butterfly and check valves). Proper specification of end connections ensures valve installation meets code requirements and achieves reliable, leak-free service.

Flanged End Connections

Flanged end connections (ASME B16.5 for pipe flanges, ASME B16.47 for large diameter flanges) are the most common type for process plant valves. Flange ratings (Class 150, 300, 600, 900, 1500, 2500) define the pressure-temperature capability based on material. Flanged valves are easily installed, inspected, and replaced without cutting pipe. The flange face type (raised face, ring joint, flat face) must match the mating pipe flange. Gasket selection (spiral wound, ring joint, sheet gasket) depends on fluid service and flange pressure class.

• ASME B16.5: pipe flanges up to 24 inches, Class 150 to 2500

• ASME B16.47: large diameter flanges (26 to 60 inches)

• Raised face (RF): standard for Class 150-2500, accepts spiral wound gaskets

• Ring type joint (RTJ): high-pressure service, metallic ring gasket

• Flat face (FF): used with cast iron valves and full-face gaskets

Threaded End Connections

Threaded end connections (NPT per ASME B1.20.1) are used for small-bore valves (typically 2 inches and below) in lower-pressure, lower-temperature applications. Threaded connections are economical and simple to install but have limitations: they are subject to vibration-induced loosening, limited to moderate pressure-temperature ratings, and cannot be used for lethal or cyclic service per ASME B31.3 requirements. BSPT (British Standard Pipe Taper) threading is specified for international markets.

Butt Weld End Connections

Butt weld end connections provide the strongest, most leak-proof valve installation, making them preferred for high-pressure, high-temperature, toxic, and flammable fluid service. Butt weld valves are integrated into the piping system by welding, eliminating the leak paths present at flanged and threaded connections. Valve removal requires cutting the weld, making maintenance more time-consuming and costly. Wall thickness and material at the weld end must match the adjoining pipe to ensure a compatible weld joint. Inspection of butt welds per ASME B31.3 is required for high-pressure and lethal service.