Natural gas transmission pipeline refers to the pipeline transporting natural gas (including oil field associated gas) from the exploitation site or treatment plant to the city gas distribution center or users. This paper introduces the welding process of a gas pipeline transporting project, of which design pressure is 6.3Mpa, API 5L X60 LSAW steel pipe with an outer diameter of 508 ×11.1mm, and the total length is 10km. The welding process is mainly semi-automatic welding, and the manual arc welding downward welding is used in some parts. The paper including the selection of materials, welding sequence and groove shapes, inspecting to the welding check for the welding of the gas transporting pipeline.

Welding Material

According to the design requirements of the project: AWS E6010 cellulose electrode is used for manual arc welding, AWS E8010 electrode is used for filling and AWS E8018G low hydrogen electrode is used for capping. AWS E6010 welding rod is used for semi-automatic welding, and E71T8-NI1 or E71T8-K6 flux-cored wire is used for filling and capping. The electrode selected shall conform to AWS A5.1 and A5.5, and the wire to AWS A5.29.

Welding Equipment

Dc arc welder or semi-automatic welder shall be used in accordance with welding material requirements and construction conditions.

The Welding Procedures

This paper introduces the downward welding process of the back seal welding of the natural gas pipeline. All the layers of the base welding are completed by manual downward welding, and gas shielded welding is used for heat welding, filler welding and cover welding. The welding process of downward welding includes pipe assembly → pigging grinding → group pairing → welding.

  1. Preparation before welding. Before welding the pipe, remove impurities in the pipe and oil stains, rust and other impurities on the surfaces at both ends of the groove, and polish them within the range of 10mm on both sides of the pipe mouth to expose metallic luster and protect the groove. For the anti-sulfur natural gas pipeline, the welding joint is generally set as V-shaped, the Angle of the groove is 600±2.50, the blunt edge is between 0 and 0.5mm, and the groove gap is between 3.0 and 4.5mm.
  1. Assembly and positioning welding. Pipe assembly and positioning welding are the key to ensure the quality of down welding and good formation of pipe joint back. Position welding is a part of the formal weld, the best is single-side welding double side forming and to ensure the welding quality. The internal clamp can be used to complete the root welding pass at one time, and then the clamp can be removed after the root welding. 6~7 positioning welds are evenly distributed on the entire circumference of the welding port with the external clamp. The accumulated welding length shall not be less than 50% of the circumference of the pipe before removing the external clamp.
  2. When the end of the pipe is connected to the head, the external clamp is used for assembly; The latter shall remove the cutting marks on the surface of the groove. The vertical deviation between the cut surface and the centerline of the pipe shall be less than 1.5mm. The size of the groove is shown in the figure below.
  3. When two pipes are welded with the same wall thickness, the amount of staggering edges shall meet the requirements. When connecting with pipes with different wall thicknesses and the same outer diameter, the amount of interface wrong side shall not exceed 10% of thin wall thickness, and shall not be greater than 1mm; The wall thickness difference is less than 1mm can be directly welding; The wall thickness difference should be less than or equal to 30% of the thin wall thickness, and the processing of the pipe end of the thick wall pipe should ensure that the interface edge mismatch does not exceed 10% of the thin wall thickness. The joint weld of two pipes shall be staggered by more than 100mm, and the curvature radius of elastic laying of pipes shall be greater than 1000D.
  4. The principle is thin-layer multi-pass welding, the impurities such as slag and spatter should be removed between layers, and strict appearance inspection should be carried out. After confirming that there is no defect, the next layer of welding can be carried out.The arc starting or arc closing points of each layer of weld pass are staggered by 20~ 30mm, and the arc starting and arc closing points of root welding shall ensure penetration, uneven deposited metal in the weld pass shall be polished after root welding to avoid slag inclusion. Hot pass welding should be performed as soon as possible after root pass welding. The interval between root pass and hot pass welding should not exceed 5 minutes.
  5. Welding slag and spatter on the surface of the weld shall be removed in time after welding and the surface quality shall be carefully checked. The surface shall not have cracks, non-fusion, porosity, slag inclusion and other defects.
  6. Nondestructive testing.100% X-ray and ultrasonic inspection must be carried out for the entire welding process after welding. After the test is qualified, the post-welding heat treatment can be carried out. After the heat treatment, a 10% sampling ultrasonic test should be carried out on the welding joint. If cracks and other phenomena are found in the weld during the inspection, only one repair is allowed for this part.

Welding Parameters

Weld bead Process Welding material Diameter Polarity welding direction welding current, A Arc voltage, V Feeding speed, in/mm Welding speed, cm/min
Root welding GMAW E80C-Ni 1.2mm Reversed Downward 140-180 14-18 150-180 18-35
Hot-tie welding FCAW-S E818T-Ni2J

E818T-G

E551T8-K2

 2.0mm Received Downward 160-260 18-24 80-100 18-30
Filler welding FCAW-S E818T-Ni2J

E818T-G

E551T8-K2

 2.0mm Received Downward 170-280 18-24 80-120 16-28
Cover weld FCAW-S E818T-Ni2J

E818T-G

E551T8-K2

 2.0mm Received Downward 160-240 18-24 80-100 16-24

Welding procedure qualification, welding procedure, test and inspection, and welding procedure parameters shall be determined before downward welding is used.

  • Welded joints: Welded joints are prone to defects in pipe welding. The slag should be removed from the arc crater, the arc should be ignited on the top of the crater, the arc crater should be stopped slightly and gently swung until it is filled, and then welding at the normal welding speed.
  • Welding speed: the welding speed should be determined according to the diameter and wall thickness of the pipe. If the welding speed is small, the molten pool will flow downward, and porosity and slag inclusion will be produced. Too fast welding will make weld pass filling.
  • Fill bead welding: when welding fill bead, the wire feeding method of straightening is generally used. Tubes with large wall thickness may be row welded to fill a wider groove. The direct wire transport is easy to operate and can obtain good welding quality and mechanical properties. It is also possible to fuse the weld bead edges with a slight wiggle when feeding the wire.
  • Cover bead welding: in cover bead welding, the welding joint should be staggered with the filler welding joint. Before cover welding, the height of the fill weld bead to the pipe surface shall be less than 1.5mm.