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

The end connection method by which a valve joins the pipeline determines the strength, leak integrity, ease of installation and removal, and applicability to different pressure and temperature ratings of the valve-pipe joint. Four main end connection types are used in industrial piping: flanged, butt-weld, socket-weld, and threaded. Each has specific advantages and limitations, and the selection is governed by the pipe size, pressure class, temperature, application (permanent vs removable), and applicable piping code.

Flanged End Connections

Flanged connections are the most common end connection for process valves in sizes NPS 2 and above. A flange face is machined on each valve end, and the valve is bolted between mating pipe flanges through a full-face or ring-type gasket. Flanged connections are fully removable and reinstallable, making them the preferred connection for valves that require periodic maintenance or replacement. ASME B16.5 governs flange dimensions, facing types (raised face, ring-type joint, flat face), and bolt circle dimensions for flanges in Class 150 through Class 2500. Raised face (RF) flanges with spiral-wound gaskets are the standard connection for most process piping, while ring-type joint (RTJ) flanges are specified for high-pressure Class 600 and above service.

• Raised face (RF): standard facing for Class 150 through 2500, used with spiral-wound or ring gaskets

• Ring-type joint (RTJ): high-pressure facing for Class 600 and above, uses metallic oval or octagonal ring

• Flat face (FF): used with full-face gaskets for cast iron flanges to prevent flange cracking

• Male/female and tongue/groove facings: used for vacuum service and special leak-sensitive applications

• Stud bolting: flanged valves use studs and nuts per ASME B16.5 bolt circle and diameter specifications

Butt-Weld End Connections

Butt-weld (BW) end connections provide a permanent, fully continuous weld between the valve and the pipe, creating a joint with strength and leak integrity equal to or greater than the pipe itself. Butt-weld ends are beveled and prepared to ASME B16.25 dimensions to match the pipe wall thickness and allow full-penetration welding. Butt-weld connections are used for valves in critical, high-pressure, or high-temperature service where flange gasket leakage is not acceptable, or in corrosive service where flange bolt corrosion is a maintenance concern. The disadvantage is that removing a butt-welded valve requires cutting the pipe on both sides, which is more labor-intensive than unbolting flanged connections.

Socket-Weld and Threaded Connections

Socket-weld (SW) connections are used for small-bore valves in NPS 2 and below in Class 3000, 6000, and 9000 ratings. The pipe end is inserted into a socket machined in the valve end, and a fillet weld is applied around the outside of the socket. Socket-weld joints provide better leak integrity than threaded connections and are used in high-pressure, non-cyclic service. Threaded (screwed) end connections are the simplest and lowest-cost option for small-bore valves in non-critical, lower-pressure service. NPT (National Pipe Taper) thread is the standard in North America. Threaded connections are not recommended for hazardous service, cyclic service, or temperatures above 260 degrees Celsius where thread galling or joint leakage risk increases.