A clean sweep for welding

Welds in midstream pipeline construction are considered critical and must pass inspection, which makes high quality and full penetration critical with each weld pass – especially the root pass. The root pass or ‘bead’ (the first pass of a multi-pass weld) lays the foundation for the quality that’s necessary in the completed weld. In addition, pipeline welding on the right-of-way presents several specific challenges that set it apart from other types of multi-pass welding.

Learning more about choosing the right products for the preparation and cleaning of multi-pass pipeline welds – and following proper technique – can help ensure the best results in these critical applications.

Multi-pass welding basics

Multi-pass welds laid on the right-of-way are used to join bevelled pipe, which could be a few inches to over 40 in. (more than 102 cm) in diameter, all with different thicknesses. Adding layers (or multiple passes) of weld to thick metal joints increases the strength of critical welds where testing is required and weld failure could be catastrophic.

Each pass must be thoroughly cleaned before the next pass is laid down to ensure there are no inclusions that would cause a weld to fail inspection – or worse, fail in operation. Therefore, the abrasives and power wire brushes used for grinding and cleaning between passes play a critical role in the quality of the finished weld.

In pipeline welding, the number of passes required depends on the diameter and thickness of the pipe and the specifications of the job. A smaller diameter pipe may require four passes: root, hot, fill and cap. Larger diameter pipe (such as 30 - 40 in./76 - 102 cm) may require root and hot passes followed by multiple filler and cap passes.

Pipeline welding challenges

There are several factors to keep in mind in the pipeline welding process that make these welds different from other types of multi-pass welds.

Environment: One big difference is that many pipeline welds are completed outdoors on the pipeline right-of-way, rather than inside of a shop. Because of this, pipeline welding typically uses shielded metal arc welding (stick welding) and may also include a mechanised element utilising gas metal arc welding (MIG welding). If MIG welding is used, a tent or enclosure is erected to help protect the shielding gas from the elements. The environment makes proper technique especially important, since the operator is typically not standing on a flat, level surface.

Very hot welds: The grinding and cleaning between weld passes happens on very hot welds during pipeline welding. In other multi-pass applications, there may be time allowed in the process for the weld to cool between each pass, but that is not the case on the pipeline. There is no time for welds to cool, so operators must use grinding wheels and a wire brush on welds that are 400+°F or 200+°C. This requires special attention to product selection and technique.

Multiple operators: Pipelines often have two welders on either side of the pipe. They each weld one-half of the pipe and meet in the middle. This process helps prevent the weld from flexing. Two helpers then grind and clean the weld immediately after.

Additional prep and cleaning: Part of the preparation for joining pipe in the field requires facing the land, which means using the flat side of the grinding wheel to create a nickel-sized area on the pointed portion of the bevel where the two pipe pieces can be connected by a tack weld. Operators may also need to transition the pipe, which involves grinding the pipe’s inside diameter, which may not be symmetrical between pipe pieces. It’s also important to remove any rust or pits that can form on the bevel when pipe pieces are stored outside. A flap disc is normally used for this application. In America, a metal backed flap disc is often preferred because it provides less flex and more control to the operator.

Tips for better results in pipeline welding

Properly preparing material for pipeline welding and thoroughly cleaning between each weld pass requires attention to several best practices.

Tip 1: Avoid inclusions

The most important step in successfully producing high-quality multi-pass welds is to make sure that full penetration is achieved and each pass is clean and free of inclusions. Otherwise, weld defects and impurities can be introduced and prevent welds from passing inspection. Repairs are costly, so time spent cleaning saves time in the long run. After the initial root pass, grind the weld down to create a U shape and grind through any impurities that have bubbled up. The root pass should be cleaned with a grinding wheel that fits into the gap or bevel, typically a 1/8 in. (3.4 mm) wheel. Cleaning the hot pass and subsequent fill passes is usually done with a bead wire brush (stringer bead brush), especially for stick welding because it produces a slag.

Tip 2…

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