Stove pipe welding is one of the chief methods used in the field welding of pipelines for oil, gas, water etc., where the speed of joining pipes is critical in the speed of pipeline construction (ditching, hauling, stringing, etc.). It is a variant of the manual/shielded metal arc welding (MMA/SMAW) technique used for positional welding, enabling steel pipelines to be laid at high production rates.
In pipe joining, the rate of progress is limited by the root pass and the hot (second) pass. In order to speed up the deposition of these two passes without compromising the quality of the weld, the welding is carried out downward from 12 o’clock to 6 o’clock, since the process is faster than the upward direction, especially for pipes of wall thickness below 25 mm. Furthermore, this allows the use of two pairs of welders working simultaneously on both sides of the pipes, as opposed to only one pair in the upward position.
The root pass is the most critical, and requires skilled welders. Cellulosic or cellulosic-iron powder coated electrodes are used [ISO 2560-A (B) - E XX X XXX C or AWS A5.1-2004 EXX10 or EXX11]. These do not require drying and are coated with cellulose, an organic compound with high levels of hydrogen which gives a high burn-off rate, forceful arc and a light, fast-freezing slag - all very suitable for the vertical-downward technique. The coating also provides a gas shield which is less affected by wind than other electrodes (though weather protection may still be required).
The weld preparation typically consists of a 60-70° bevel (inclusive angle), with a 1-2mm root face and 2-3mm root gap. Stringer beads are deposited into the root at high speeds (250-300 mm/min). This is immediately followed by a hot pass which refines the root pass and reduces the risk of hydrogen cracking associated with these consumables. A minimum level of preheat may be required for the same reason. Filler runs, stripper runs and capping run(s) complete the welding.
The welding of pipelines is usually performed by a team of welders; the larger the diameter of pipe, the greater the number of welders. In most cases each welder performs the same weld run(s) on each successive joint.
- BS EN ISO 2560:2009 Welding consumables. Covered electrodes for manual metal arc welding of non-alloy and fine grain steels. Classification.
- ANSI/AWS A5.1-2004 Specification for carbon steel electrodes for shielded metal arc welding.