- The choice of end connection type on a valve — flanged, butt-weld, socket-weld, or screwed — is not a detail to be left to default; it determines joint integrity, installation cost, maintenance access, pressure class capability, and the risk of leakage at the most vulnerable point in any piping system.
- Each end connection type has specific dimensional standards, design requirements, installation procedures, and appropriate pressure class and application boundaries that every piping and valve engineer must understand to make informed specification decisions.
- This guide covers all major valve end connection types, their governing dimensional standards, installation requirements, advantages and limitations, and the engineering logic that determines which connection type belongs on each valve in a process plant or pipeline.
Flanged End Connections
Flanged end connections are the most widely used valve connection type in process plant and pipeline service. They allow the valve to be disconnected from the piping system without cutting or welding — a significant maintenance advantage in plants where valve removal for service or replacement is a regular activity. The valve flanges bolt directly to matching pipe flanges, with a gasket between the mating faces providing the seal.
The dimensional standard governing flanged end connections in the oil and gas, petrochemical, and power industries is ASME B16.5 (Pipe Flanges and Flanged Fittings) for NPS ½ to NPS 24, and ASME B16.47 for larger sizes (NPS 26 to NPS 60). ASME B16.5 defines flange dimensions, bore dimensions, bolt circle and bolt hole dimensions, and face types (raised face, full face, ring type joint, tongue and groove) for each combination of NPS size and pressure class (Class 150, 300, 600, 900, 1500, 2500).
Raised face (RF) flanges — where a narrow raised ring inside the bolt circle carries the gasket — are the standard face type for the majority of process plant service. The raised face concentrates the bolt load on the gasket area, improving sealing efficiency compared to a full face. Ring type joint (RTJ) flanges use a metal ring gasket in a machined groove in the flange face, providing superior sealing performance for high-pressure and high-temperature service (Class 600 and above), and particularly for hydrocarbon gas service where the consequences of gasket failure are severe. Full face flanges (FF) — where the gasket covers the full face area to the outside of the bolt circle — are used with flat-faced cast iron flanges to prevent bending stress in the cast iron flange body from causing fracture.
The major limitation of flanged connections is their susceptibility to leakage at the gasket joint — particularly for fluctuating temperature service (thermal cycling can relax bolt loads, allowing gasket creep and subsequent leakage) and for volatile or toxic fluid service where any gasket leakage is unacceptable. The ongoing maintenance requirement for flanged joint re-tightening in high-temperature service is a well-known operational burden. For the highest-consequence services (highly toxic, highly flammable), process plant designers increasingly specify welded connections to eliminate all flanged joint risks from critical sections of piping.
Butt-Weld End Connections
Butt-weld (BW) end valves have their bodies terminated with a bevelled pipe end, identical to the pipe butt-weld bevels defined by ASME B16.25. The valve is incorporated into the piping system by butt-welding directly to the adjacent pipe, with a full-penetration groove weld around the full circumference of the joint. This creates a metallurgically continuous joint between valve and pipe — the highest integrity, lowest maintenance, lowest leak-risk connection available.
Butt-weld connections are mandatory in many critical service applications: high-temperature steam piping, high-pressure gas pipelines, highly toxic or flammable services, nuclear power piping, and any service where the consequence of gasket failure would be unacceptable. They are the standard end connection for all pipeline valves (API 6D), most high-pressure process valves (Class 600 and above in critical service), and subsea valve assemblies. The trade-off is that butt-weld valves cannot be removed from the piping system without cutting out the connecting welds — maintenance or replacement requires either cutting the valve out and welding in a replacement, or provision of flanged spade and spacer arrangements on either side to allow future valve removal.
The wall thickness of the butt-weld end must match the pipe wall thickness (same schedule) to ensure proper weld joint design. Transition pieces (weld pups with tapered bore) are used when the valve bore is larger than the pipe bore, or when valve and pipe wall thicknesses are significantly different. The weld procedure specification (WPS) and welder qualifications must be appropriate for the valve and pipe materials being joined.
Socket-Weld End Connections
Socket-weld (SW) connections are used for small-bore piping (typically NPS ½ to NPS 2, occasionally NPS 3). The pipe end inserts into a socket bore in the valve body, and a fillet weld is placed around the outside of the pipe where it meets the valve body. Socket-weld connections are defined in ASME B16.11 and ASME B16.5 for the fittings and flanges that accompany socket-weld valves.
Socket-weld valves are faster to install than butt-weld (the socket locates the pipe automatically and a single external fillet weld is simpler than a full groove weld preparation) while providing better integrity than threaded connections for high-pressure, high-temperature service. They are widely used for instrument isolation valves, small-bore process connections, drain valves, and similar small-bore duties in Class 800 and Class 1500 service.
Socket-weld connections have a known limitation: the socket creates a crevice between the pipe OD and the socket bore that cannot be fully weld-sealed and may be subject to crevice corrosion in corrosive services. For this reason, socket-weld connections are generally not recommended for severely corrosive service, caustic service, or services where chloride-induced crevice corrosion of stainless steel is a risk. ASME B31.3 limits the use of socket-weld connections in certain cyclic-loading applications due to the potential for fatigue cracking at the crevice root.
Screwed (Threaded) End Connections
Screwed or threaded valves use NPT (National Pipe Taper) threads per ASME B2.1, or BSP (British Standard Pipe) threads to BS EN ISO 7-1, to connect the valve to threaded pipe or fittings. They are the simplest and cheapest connection type to install, requiring no welding, no gaskets, and minimal tooling. Threaded connections are standard for NPS ½ to NPS 2 in Class 800 (3000 lb) or below, and for utility services (instrument air, water, low-pressure steam) at modest pressures and temperatures.
The significant limitations of threaded connections for pressure service are: the inherent leak risk from the tapered thread seal (which relies on thread-to-thread contact and often PTFE tape or pipe dope sealant to achieve sealing), the stress concentration at thread roots under cyclic loading, and the susceptibility to galling and seizing on repeated assembly-disassembly cycles particularly in stainless steel. For these reasons, threaded connections are excluded from many high-pressure, high-temperature, or toxic fluid process piping applications by the applicable code and company specifications. ASME B31.3 imposes limitations on the use of threaded connections in hazardous fluid services and for pipe larger than NPS 2.
The engineering decision between these end connection types is ultimately a balance of integrity (BW > SW > flange > threaded), maintenance accessibility (flange > BW/SW > threaded), installation cost (threaded < flanged < SW < BW), and appropriateness to the pressure, temperature, and fluid service. These trade-offs, assessed against the specific requirements of each piping system, are what produce the correct specification — and understanding them thoroughly is part of what it means to be a competent piping and valve engineer.