Valve End Connections: Flanged vs Threaded vs Welded — Which to Specify

1. Flanged End Connections

Flanged connections are the most common end type for industrial valves above 2" diameter. The valve body has integral flanges that bolt to matching pipe flanges with a gasket in between.

Raised Face (RF) — ASME B16.5

The standard flange facing for most industrial applications. The raised face (1.6mm or 6.4mm) concentrates gasket compression for improved sealing. Used with spiral wound, ring sheet, or compressed fiber gaskets.

• Pressure range: Class 150 to Class 2500
• Best for: General industrial, oil & gas, chemical, water — the default choice
• Gasket: Spiral wound 316/graphite (most common), PTFE, rubber

Ring Type Joint (RTJ) — ASME B16.5

A grooved flange face that accepts a solid metal ring gasket (R, RX, or BX type). The metal ring is compressed into the groove, creating a metal-to-metal seal. Superior for high-pressure and high-temperature service.

• Pressure range: Class 600 to Class 2500 (mandatory above Class 900 in many specs)
• Best for: High-pressure oil & gas, subsea, sour gas, critical process
• Gasket: Soft iron, SS316, SS304, Inconel 625 (for sour service)

Flat Face (FF)

The entire flange surface is the sealing face. Used with full-face gaskets. Required when mating with cast iron flanges (ASME B16.1) to prevent cracking from bending moments.

• Pressure range: Class 150 (up to Class 300 in some standards)
• Best for: Cast iron piping, HVAC, low-pressure water, PN10/PN16 systems
• Rule: Never bolt a raised-face flange to a flat-face cast iron flange without machining the RF down to FF

2. Threaded End Connections

Threaded (screwed) end valves have internal (female) pipe threads that screw directly onto external (male) pipe threads. No welding or flanges required — just thread sealant or PTFE tape.

NPT (National Pipe Taper) — ASME B1.20.1

The North American standard with 60° tapered threads. The taper provides the seal when tightened. Dominant in oil & gas, chemical, and industrial applications in the Americas and Middle East.

BSP (British Standard Pipe) — ISO 7/1, BS 21

The international standard with 55° Whitworth thread form. BSPT (tapered) is used for sealing connections; BSPP (parallel) requires a sealing washer. Common in Europe, Asia, Australia, and Africa.

3. Socket Weld End Connections

Socket weld (SW) valves have a recessed socket that the pipe inserts into. A fillet weld around the outside seals the joint. Provides stronger connections than threaded for high-pressure small-bore piping.

• Standard: ASME B16.11 (socket weld fittings), ASME B31.3 (process piping)
• Size range: 1/2" to 2" (sometimes to 4")
• Pressure: Class 150 to 600 (API 602 forged valves up to Class 1500)
• Gap requirement: 1.6mm (1/16") gap between pipe end and socket bottom per ASME B31.3 to allow for weld shrinkage

4. Butt Weld End Connections

Butt weld (BW) valves have beveled ends that are welded directly to the matching beveled pipe ends with a full-penetration weld. The strongest and most leak-tight connection for permanent installations.

• Standard: ASME B16.25 (butt weld ends), ASME B16.34 (valve dimensions)
• Size range: 1/2" to any size (most common above 2")
• Pressure: All classes, including Class 2500 and pressure-seal designs
• Bevel: 37.5° standard bevel per ASME B16.25, with 1.6mm root face

End Connection Selection Guide

Key Takeaways

• Flanged RF is the default for process piping above 2" — it provides removability and is universally understood by fabricators
• RTJ should be specified for Class 900+ or wherever metal-to-metal sealing is required by the piping specification
• Threaded is limited to 2" and below, utility service, and locations where hot work is restricted
• Socket weld bridges the gap between threaded and butt weld for small-bore process piping
• Butt weld is the gold standard for permanent, leak-free installations in high-integrity service
• Always verify end connection type against the project piping specification and line class tables

Frequently Asked Questions

Can I mix NPT and BSP threaded valves?

No. NPT (60° thread angle) and BSP (55° Whitworth thread angle) are not interchangeable and will not seal properly if mated together. While they may appear similar, the different thread angles cause progressive leakage. Always match NPT to NPT and BSP to BSP. When ordering valves for Middle East or international projects, confirm whether NPT or BSP is specified in the piping class.

Why is RTJ preferred over RF for high-pressure valves?

RTJ (Ring Type Joint) provides a metal-to-metal seal that is immune to gasket blowout, fire degradation, and creep relaxation — all risks with non-metallic gaskets used in RF connections. The solid metal ring in the machined groove creates a self-energizing seal that improves under pressure. For Class 900+ and all subsea/critical service, RTJ is specified by most major operator standards including Saudi Aramco (SAES-L-008).

What is the 1.6mm gap rule for socket weld connections?

ASME B31.3 requires a 1.6mm (1/16 inch) gap between the pipe end and the bottom of the socket before welding. This gap allows the weld to shrink without pulling the pipe out of the socket, and prevents stress concentration at the socket bottom during thermal cycling. The gap is created by inserting the pipe fully, then withdrawing it 1.6mm before tack welding.