SS pipe fittings and flanges for butt-weld, socket-weld, threaded and tube lines

Butt-weld fittings are the standard for process piping from about 2 in NB upward, and in smaller sizes wherever the line must be fully welded and free of crevices. Dimensions follow ASME B16.9, which runs from NPS 1/2 to 48. Material follows ASTM A403, where the grade reads WP304, WP316L and so on. Elbows come in long radius (centre-to-face 1.5 times the nominal size) and short radius (1 times).

A403 also gives each fitting a class. WP-S fittings are made from seamless pipe or bar. WP-W fittings are made from welded pipe, with radiography required only on welds the fitting maker adds. WP-WX means all welds are fully radiographed, and WP-WU means all welds are fully examined by ultrasonics. Light-wall CR fittings are made to MSS SP-43 rather than B16.9.

The fitting wall must match the pipe schedule. A Sch 10S elbow welded to Sch 40S pipe leaves a step inside the joint and a thinner component in the line.

Small-bore lines, typically NPS 2 and below, often use forged fittings to ASME B16.11 made from ASTM A182 F304, F316 or F316L forgings. Socket-weld fittings come in Classes 3000, 6000 and 9000 and threaded fittings in Classes 2000, 3000 and 6000. The pipe is pushed into the socket, pulled back about 1.5 mm to leave an expansion gap, and fillet welded. Threaded fittings suit utility, air and instrument lines where a welder is not practical.

Unions, which let a section be removed without cutting, are made to MSS SP-83. Threads deserve a line of their own on the order. B16.11 fittings default to NPT, but much equipment in India uses BSP threads, and the two do not seal together.

Compression fittings join tube rather than pipe, and they are sized by tube OD. The nut drives one or two ferrules onto the tube, which grip and seal without welding. They work only if the tube is right: annealed, within OD tolerance, round, and softer than the ferrule. Pair them with seamless or welded instrument tube from our stainless steel pipes and tubes range, and keep ferrules and bodies from the same maker. Common tube sizes are 1/4, 3/8 and 1/2 in OD, along with metric 6, 8, 10 and 12 mm. Inch and metric fittings look alike but do not interchange, since a 1/4 in tube is 6.35 mm, so check the nut marking before ordering spares.

Stainless flanges to ASME B16.5 cover NPS 1/2 to 24 in pressure Classes 150, 300, 400, 600, 900, 1500 and 2500, forged to ASTM A182. The common types are weld neck, slip-on, socket-weld, threaded, lap joint and blind, with raised face, flat face or ring-type joint facings. A weld neck flange is bored to match the pipe schedule, so give the schedule with the size. Flanges for the same line in carbon or alloy steel are on our carbon and alloy steel fittings page.

• Item and type: 90° long radius elbow, equal tee, socket-weld union, weld neck flange.
• Size, and both sizes for reducing tees.
• Wall or class: schedule for butt-weld fittings and weld neck bores, class for forged fittings and flanges.
• Material standard and grade: ASTM A403 WP316L-S, or ASTM A182 F304L.
• End connection: butt-weld, socket-weld, NPT or BSP thread, tube OD for compression.
• Flange facing and finish, if not standard raised face.
• Test certificate and any third-party inspection.

• Fittings in one schedule and pipe in another.
• NPT fittings arriving for BSP-threaded valves, or the reverse.
• 304 fittings installed in a 316L line, which leaves the joints as the weakest point in chloride service.
• A Class 150 flange ordered for a line designed to Class 300.
• Compression fittings fitted on hard or oversized tube, which then weeps at the ferrule.