Weld Nipples & Weld Fittings

Weld nipples and weld fittings add a removable connection point to a welded tube line. One end of the part is joined by welding to the tube or to a bulkhead, while the other end remains removable through a 24° cone compression or O-ring sealed connection. This allows equipment that needs servicing, such as a pump, valve block or cylinder, to be replaced without cutting the line. Weld bevel preparation, axial alignment and post-weld surface cleaning matter as much as part selection for the sealing and corrosion resistance of the connection.

Types of weld nipples and weld fittings

• Weld nipple coupling set: 24° cone body, nut and an O-ring weld nipple that is butt-welded to the tube.
• 24° weld nipple with O-ring: straight, reducing and 90° types; for thick-walled and high-pressure tubes.
• Weld elbow 90°: a right-angle transition from the welded line to a removable 24° cone connection.
• Weld-in bulkhead coupling: welded into a ship bulkhead, tank wall or shell plate, with a removable connection on each side.

Selection criteria

• Material: 316L is preferred for parts that will be welded. A carbon content of no more than 0.03% reduces the risk of intergranular corrosion in the heat-affected zone of the weld.
• Tube size: The OD and wall thickness of the weld end must match the tube; a difference in wall thickness creates a step and a stress concentration at the weld root.
• Removable end: The series (L or S) and size of the connection must be the same as those of the fitting body it mates with.
• Bulkhead penetration: The bulkhead thickness, the tube diameters on both sides and any watertightness requirement must be specified together.
• Location of the removable end: Choose a position where a wrench can reach and the equipment can be removed during maintenance.

Welding and assembly sequence

1. Slide the nut onto the tube before welding; it cannot be fitted after welding.
2. Do not leave the O-ring in place during welding: heat destroys the O-ring. The O-ring is fitted after welding and cleaning are complete and the part has cooled.
3. Cut the tube end square, prepare a weld bevel suited to the wall thickness, and tack the parts in axial alignment.
4. The welding method and filler material are determined by the project requirements and the stainless steel grade.
5. Remove heat tint around the weld by pickling and passivation or by a suitable mechanical method; do not use a brush that has been in contact with carbon steel.

Before the line is commissioned, pressure and leak-tightness verification is carried out to suit the application; on weld nipples, this is also the stage at which the O-ring is checked to be fitted and undamaged.

Which industries use them?

Bulkhead and tank penetrations in shipbuilding and ship repair, thick-walled pressure lines on offshore power units, high-pressure tubing in rolling mills, and removable equipment points connected to welded process lines in chemical plants. On sites where welding is not possible, or on lines that are modified frequently, DIN 2353 compression fittings are a more practical option.