(pic. 62.1 – Arc welding)
Arc welding. Tacks equally spaced along the border of the joint are designed to fix and centralize steel pipes. Electrodes «transfer» electric current to the root weld making a welding.
Rolled and fixed (more complicated to realize) joints in manual weldingare welded minimum in two layers (the thickness of the walls is no more than 6 mm) or in three layers (the thickness of the walls is more than 6 mm). Welded the fist seam the surface should be cleaned from metal splashes and slag, then the next seam is realized by the electrode of bigger diameter.
The requirements to the welding seam are the following: full penetrated weld, steadiness and lack of non-melted areas. The diameter of the welding rod is chosen considering the thickness of the pipe wall.
Welding steel gas pipes with the diameter of ≥ 900 mm (without using base ring which simplify the welding and centralizing) there is realized inside back weld of the root.
Automatic submerged arc welding is realized on base layer made by manual arc welding or by semi-automatic carbon dioxide welding.
Breakdown of the welded arc in the end of the welded seam is unacceptable to avoid crater creation. It’s necessary to hold the electrode for one or two seconds and then to interrupt the arc by abrupt movement up and back.
Important: welded seam all around the pipe should be ended with 10-15 mm overlap to its beginning. Here we create «welded lock» (or «tail») – «covering» welded area by the end of the seam, turning and moving it 10 mm sideward.
(pic. 62.2 – Gas welding)
Газовая сварка.Gas welding. Te edges are heated up and melted at one time with welding rod which fulfills joint area. Gas welding, as a rule, is realized by one layer. One pass provides not so big intension in the end of the seam («lock» and «tail»), excluding the lack of penetration.
The following conditions allow to use gas welding with acetylene: diameter of the gas pipe less than 150mm, wall thickness - ≤ 5 mm (with scarfed edges) and ≤ 3mm (edges without scarfs).
The following conditions are for gas welding by propane-butane mixture: diameter of the gas pipeline – less than 150mm, wall thickness - ≤ 5mm, pressure - ≤ 0,005 MPa.
Realizing gas welding of joints by reducing flame it is used a filler rod (diameter 1.5-3 mm), which should correspond to steel grade in certain degree. The edges should be cleaned first. Mounting a «spool» which is welded in line segment of underground gas pipeline, should have the lengths ≥ 200 mm.
Welding of X-type and T-type joints requires not only exact perpendicular positioning of the pipes axis, but also matching of stub pipe axis joined by welding to the center of the pipe hole.
It is not allowed to weld stub pipes for gas pipeline branching in circumferential (transverse) seam area; minimum distance between welding seam and circumferential seam on the gas pipeline is 100 mm.
Cutting in branches with the diameter ≤ 50 mm to indoor gas line the following distance is allowed between circumferential seams and welded in stub pipe: ≥ 50 mm.
(pic. 62.3 – Protection of welded area
from bad weather)
Realizing butt-welded joining of the pipes we should get straight, little bit convex seam without any sinks, cracks, pores, craters and undercutting. Slants and molten metal inside the pipe are also not allowed.
Organizing welding works in the open air it is required to protect working area from the wind and moisture (snowfall, rain, fog, strong wind).
Similar articles:
- SNiP 3.05.02-88 Gas supply
- GOST 16037-80 Welded joining of steel pipes.
- SNiP III-42-80 The main pipelines.
- Welding of steel gas pipelines: methods and general conditions
- Welding of steel gas pipelines: preparatory works and assembling
- Welding of steel gas pipelines: quality control of welded joints
