The alchemies of the weld zone. Guidelines for effective revamping of an ERW tube line

Upgrading and retrofitting an ERW longitudinal welding line for carbon steel tubes
New welding generators are increasingly performing in terms of energy saving. Replacing the existing one with a latest-generation generator can result in considerable savings, depending on the starting situation.
The high level of automation available nowadays makes recent welding boxes easier to use. Machine operations become more repetitive and manageable, especially for less experienced operators. A suitable welding box design makes rolls change faster and safer.
All these and others are valid reasons for carrying out a revamping of production lines for welded tubes. In these cases, revamping focuses on the welding area of the line.

The main technological components are:
⊛ Welding coil, connected to HF generator.
⊛ Impeder and possibly inside weld bead scarfing units.
⊛ Welding box, which mechanically closes the edges of the open tube against each other.
⊛ Welding rolls, mounted on the box.
Arranging and designing these components is a prerogative of the designer, who has to factor in the tube to be produced, as well as the inherent characteristics of each element. 

However, the optimizations of the individual components are often in conflict with each other! Finding a compromise solution is the responsibility of the line designer or the customer, who purchases elements separately from four different suppliers.

Welding coil

The scope of the coil is to transfer high-frequency power to the edges of the tube. An electric field flows by electric induction on the open tube. By skin effect, generated by the appropriate frequency at which the generator oscillates, the current concentrates on the edges of the strip, that is, at the two edges facing each other. The current heats the edges by the Joule effect up to the welding temperature. The diameter and position of the coil determine the success of the process. The efficiency of power transfer impacts energy efficiency and therefore operating costs. The efficiency of the process increases by tightening the diameter of the coil, thus limiting the dispersion of the electric field. A diameter that is too narrow, however, does not allow the passage of the junction between two strips. In the junction area, the geometry of the open tube is difficult to predict and not repetitive. This issue should be adequately investigated at the design stage. It is advisable to avoid making assumptions on energy performance that are not achievable in practice, and to keep the diameter as small as possible. The position of the coil should be as close as possible to the welding point, that is, the point where the two heated edges of the open tube finally meet. The welding rolls are located between the coil and the welding point. The size of the rolls must be compact to allow the coil to approach. In general, heating of metal parts near the coil could happen. This would result in energy inefficiency due to unused power to heat the edges of the open tube. 

Impeder position and length

The impeder is placed inside the tube, hanging from a holder normally attached in the last fin pass area. Typically made of ferrite, it helps direct the coil-induced field to the tube.
In smaller tubes, the diameter of the impeder is affected by the inner diameter of the tube itself. Its ideal position is close to the welding point, an area that is hardly free from drops of melted metal. But again, the optimization of a component must be waived to make the use of the machine viable.

Welding box and rolls

The increase in the diameter of the rolls of the welding box allows the inserting bearings of greater diameter, resulting in a longer lifetime. With the same line speed, the rollers are slower, so the life of the bearings increases even more. Similarly, the shaft on which the bearing is mounted also increases in rigidity.
Mechanically speaking, the diameter of the rolls should be increased as much as possible. However, this condition would move the coil away from the welding point, which is undesirable.
The size of the rolls with the greatest angle of ‘hugging’ on the tube affects the difference in peripheral speed with which the roller profile touches it.

In conclusion, it is fundamental to cross-check the constraints imposed by each component (and given by the corresponding supplier!) The aim is to identify the overall most effective and efficient solution. An overview of the process is essential to deliver the best possible solution to the production staff.

About the author:
Jacopo Grassino is a consultant with 20 years of experience as a plant manager in leading companies in the engineering and steel industry. His expertise ranges from management control, production start-up, strategy, and team management. For more info, you can contact him by calling +39 3396821463, emailing [email protected], or via LinkedIn.