Socket Weld vs Threaded Valves to API 602: Which End Connection to Specify

Technical guide · Supreme Valves India · Updated 2026-10-24

Small-bore forged valves, NPS 2 and below in most piping classes, make up a large share of the line items in refinery, power and utility packages. Almost all of them are built to API 602, and the end connection is usually socket weld or threaded. The two joints behave differently under corrosion, vibration and fire, and project specifications restrict where each can be used.

API 602 in brief

API 602 covers compact forged steel gate, globe and check valves in sizes NPS 4 and smaller, with threaded, socket weld, butt weld or flanged ends. For threaded and socket weld ends it introduces Class 800, an intermediate rating between Class 600 and Class 900. Typical body materials are ASTM A105 (carbon steel), A350 LF2 (low temperature carbon steel), A182 F11 and F22 (chrome-moly) and A182 F304 or F316 (stainless).

Socket weld ends

The pipe slides into a counterbored socket in the valve end and is joined with a fillet weld around the outside. Socket dimensions follow ASME B16.11 practice, as referenced by ASME B16.34 and API 602.

• Expansion gap. The pipe is withdrawn about 1.6 mm (1/16 inch) from the socket bottom before welding. Without the gap, weld shrinkage and thermal expansion can crack the fillet weld.
• Crevice. The gap and the annular space between pipe and socket form a crevice that can't be cleaned or inspected. In services prone to crevice corrosion, such as some chloride or acidic services, socket welds may not be permitted.
• Cyclic loading. ASME B31.3 discourages socket welds in severe cyclic conditions because the fillet weld root is a stress concentration.
• Strength and leak tightness. For normal service, a socket weld gives a leak-tight, permanent joint with good resistance to vibration and fire, and the pipe ends don't need bevel preparation.

Threaded ends

Threaded valves have female taper pipe threads, normally NPT to ASME B1.20.1, or BSPT where specified. The joint is sealed by thread engagement and a thread sealant or tape.

• Easy installation and removal without welding, useful for utilities, instrument air, water and small temporary connections.
• Leak paths. Thread joints can loosen under vibration and thermal cycling, and sealants can degrade at high temperature.
• Reduced wall. Cutting threads reduces the effective wall thickness of the pipe, which matters for corrosion allowance.
• Fire. Thread sealants can fail in a fire, so many specifications prohibit threaded joints in flammable service unless they are seal welded.

Seal welding means running a fillet weld over the exposed threads after assembly. It stops leakage but removes the advantage of easy disassembly, and the threads no longer contribute reliable strength once overheated. Where specifications require seal welding, they often prefer socket weld ends instead.

What ASME B31.3 and typical piping classes say

Process piping codes and owner specifications set the rules rather than the valve standard. Common patterns are:

• Socket weld ends as the default for NPS 1-1/2 or NPS 2 and below in hydrocarbon service, with butt weld or flanged above.
• Threaded ends limited to utility services (water, air, nitrogen) or to instrument connections, sometimes seal welded.
• No threaded joints in toxic, flammable or hydrogen service, and no socket welds where crevice corrosion is expected.
• Butt weld ends for high-temperature, high-pressure or severe cyclic service, even in small bore.

Welding forged valves in line

For socket weld valves, follow the manufacturer's installation instructions and the qualified welding procedure. Points that often come up:

• Soft-seated valves, such as small ball valves with PTFE seats, may need seats protected or removed during welding, or should be specified with extended ends (nipples) so the heat stays away from the body.
• Post-weld heat treatment requirements for chrome-moly materials can affect seats, packing and gaskets. Discuss PWHT with the supplier before ordering.
• For stainless steel valves, control heat input to avoid sensitisation, and confirm whether low carbon grades (F304L, F316L) are required.

Butt weld ends on small valves

API 602 also allows butt weld ends, either machined directly on the body or on integral extensions. They avoid the crevice of a socket weld and allow full radiography, so they are specified for severe service such as high-pressure steam, hydrogen and lethal fluids. Give the pipe schedule so the bevel bore matches.

RFQ checklist for small-bore forged valves

• Valve type (gate, globe, check, piston or lift check), size and class (800, 1500, 2500).
• Body material and trim (for example A105 with trim 8, stellite-faced seat and stainless stem).
• End connection: SW to ASME B16.11, NPT to ASME B1.20.1, or BW with pipe schedule.
• Bonnet type: bolted, welded or pressure seal.
• Bore: reduced or full port.
• Any extended ends, NACE MR0175 requirements, and testing beyond API 598.

Send your small-bore line list and we will check end connections against your piping class and confirm availability.

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