Welding Method of Steel Pipe Fittings

(1) Butt-welding connections are mostly used for DN50 and above pipes of the same material. The two ends of the steel pipe fittings are provided with welding grooves. Compared with several other connection methods, due to its good sealing performance, strength, quality, corrosion resistance, and other advantages, and low price, it is widely used in high pressure and high-temperature piping systems.

(2) Socket welding connection is often used for the connection of small diameter pipes (below DN50). When small-diameter steel pipes are connected by butt welding, the internal cross-sectional area of the pipe may be reduced due to the dripping of the solder. The socket welding uses the fillet weld connection, so there is no problem of reducing the internal section of the pipe due to solder dripping, and the cost is low. However, due to the existence of gaps in the socket welding structure, it should be avoided when the conveyed fluid medium may produce crevice corrosion or severe erosion or when there is a smooth transition requirement for the connection part. The steel pipe fittings connected by socket welding are divided into socket steel pipe fittings and socket steel pipe fittings, which are generally determined by the type of steel pipe fittings and steel pipe fitting standards, and attention should be paid when selecting them.

(3) Pipe thread connection is mostly used for DN50 or less, and the connection of the detachable pipe part is required. Compared with the same detachable flange connection, the connection structure size is small. But the flange connection is more reliable, because the thread processing accuracy is difficult to ensure a complete seal, and the strength is lower than that of the welded connection. Therefore, pipe threaded connections are usually used to transport water, air, and other fluid media that are less harmful even if they leak, or pipe welding may cause the coating to peel off in the pipeline. For working conditions that may produce crevice corrosion, severe erosion, or cyclic loading, pipe threaded joints should be avoided.

(4) Flange connections are mostly used for pipe connections on special occasions, such as cast iron pipes, lined pipes, and connections with equipment. Our country's flange connection steel pipe fitting standard has GB/T 17185, which adopts ANSIB16.5 non-equivalently.

The main differences are:

1) The pressure-temperature grade of flange pipe fittings is not specified in the standard;

2) According to the actual situation of our country, the common basic flange pipe fittings (equal diameter elbow 45°, 9oo, equal diameter tee, cross, 45 tees, reducer) are the main ones, excluding reducing tee, Reducing cross, etc.