The tubes (or pipes) and their welding They have been widely used for decades in almost every engineering application, from nuclear, thermal and petrochemical plants to currently covering a wide spectrum of industries of all kinds.
Therefore, the effectiveness of the preparation and the pipe welding It has a direct impact on the safety and quality of the products they transport. The use of different methods and machinery, as well as technological developments to achieve pipes that meet the highest quality standards have given rise to what we know today as “piping process”, that is, a set of operations that, starting from individual tubes, lead to the installed and functioning pipeline.
The piping process It can be done both in the workshop and in the field, and it can be both manual and automated. However, current demands and the growing demand for equipment capable of carrying out field installations characterized by speed, precision and low cost have driven automated techniques with great impetus.
What are the stages of the piping process?
Basically, the treatment of the individual tubes to form a complete installation consists of two parts that we will detail below.
1) Preparation of tubes for welding
In this stage, the cutting and beveling of the tubes is carried out, which can be done both cold and by thermal processes. If the union of the tubes is by means of flanges, at this stage the flange facing by processes using lathes or milling machines.
Cutting is simply the operation of cutting tubes of any size, either perpendicular to its length or at an angle (bevel). There are no limitations in terms of diameter or wall thickness. Depending on the size and type of pipe to be cut, various methods can be used, and these may involve equipment that is mounted both on the outside and on the inside of the pipe.
Traditional flame cutting methods cause heating of the tube and leave a heat affected zone (DO) which is unacceptable. For this reason there are also cold methods, capable of making fast cuts and with a finish ready for welding, which responds to basic requirements, such as straight or beveled cuts.
The pipe beveling is the process where an angle is formed between the edge of the tube end and a plane perpendicular to the surface. The standard bevel angle in a tube preparation for welding is 37.5 degrees. It can also be beveled to other angles and special shapes to obtain simple, compound, “J” or “V” bevels, which are made by automatic beveling machines. Now, why is it preferred to bevel the tubes? Because not only is chamfering the usual method of preparing joints for welding, but it is also indicated for removing burrs from cut ends, either for safety or aesthetic reasons.
In the event that the pipes contain flanges, the preparation process includes the fronted or counterfaced of these, that is, the rectification of the surface of the flanges so that it is perfectly flat. Various cutters and speeds can be used to achieve a wide variety of surface finishes that properly match flange sealing materials.
Figure 1 – Schematic representation of flange facing
2) Tube welding
It is the stage in which the individual tubes already prepared are joined to form the pipe. It can be done by traditional TIG equipment, with or without filler material, MIG/FCAW, or by automatic orbital welding equipment that uses any of these welding processes.
Let us now see what are the most used techniques both for the preparation and for the welding of the tubes.
1) Preparation
cold methods
The cold cutting and beveling of the tubes is carried out with rings, generally mounted on the external diameter of the tube, whose size varies from those used for small pipes to large gas pipelines.
The following video will give us an idea of some of the types of rings used and how we should proceed to place them on the tubes and make the cuts.
As we can see, the cutting and beveling of tubes with rings can be used both in the workshop and in the field and has a series of advantages, such as:
- Ease of use, as the rings can be quickly mounted and removed from the tube.
- High resistance to simultaneous cutting and beveling operations.
- The rings are suitable for all types of materials, from cast iron to high strength alloys (eg stainless steel).
- Wide availability of sizes, covering tubes between 15 and more than 1000 mm in diameter.
- Diversity of drive mechanisms: hydraulic, pneumatic or electric (with servomotors).
heat methods
Heat cutting can be done using a variety of techniques offered by different manufacturers. The most used are:
portable system with flame
The equipment that performs this process also consists of a ring, although in this case a orbital cut, that is, the cutting torch slowly travels through the ring around the stationary tube.
It is a very versatile and easy-to-handle system, which can be applied with diameters that vary from 350 mm to 1500 mm and produces clean and precise cuts and bevels.
The head that incorporates the cutting torch includes a manual regulation guide for its correct location for both cutting and bevelling.
Plasma/oxyfuel cutting and beveling tables
Already within large machinery and, particularly, to meet the demands of the oil and gas industry, plasma/oxyfuel cutting and beveling tables with CNC control offer a multiplicity of features.
The CNC control can make fully programmable and automated cuts, both vertical and superficial, straight, bevel, “V”, fishmouth and even holes in tubes 12 meters long or even more, whose diameter varies between 50 and 500mm.
All these features are exemplified in the following video.
Plasma beveling devices
Another option offered by plasma technology for pipe beveling (for example, for mining and wind industry applications) is to install suitable fixtures on CNC plasma cutting equipment.
One such device for achieving straight bevels may be a turtle that rides on a rail.
If you want to make curved bevels, beveled holes and complex beveled parts, you can use a robotic beveling head installed on the gantry of a table, although this requires a six-axis CNC plasma cutting table.
The advantage of performing the cutting and beveling operations on the same equipment significantly reduces operating costs and ensures excellent quality and precision of the cuts obtained, which eliminates subsequent work.
fronted
The front equipment They range from portable lathes to large machines, suitable for shop and field. They are also of the ring type, which can be mounted on the external or internal diameter of the tube, and the cutting tool describes a circular movement around the flange.
We see an example in the following video.
2) Welding of prepared tubes
There are various ways of welding pipes, from the traditional technique used by plumbers and other workers using small diameter carbon steel pipes, to the more complex techniques applicable to welding pipes on site or in large workshops.
This is not to say that the general methods of, for example, oxyacetylene welding, stick SMAW, TIG, and flux-cored FCAW are not applicable in the field. On the contrary, they are still in use and the video below shows an example.
However, most modern techniques for pipe welding on an industrial scale are based on two basic types used in the shop or field.
stationary welding
In these systems, the welding torch is in a fixed position and the tube, deposited on a bed with rollers (see figure below), is the one that rotates.
Figure 2 – Tube holder roller bed
It is one of the most widely used systems in the oil and gas industry and various welding processes can be used, such as plasma arc welding, MIG and TIG.
orbital welding
In this fully automated method, the welding torch rotates around a fixed tube, either in a horizontal or vertical position. Moving the electrode circumferentially around the tube has more advantages than stationary welding.
For starters, orbital welding does not require skilled welders like stationary welding, as a single operator can control multiple machines and follow the entire procedure. Automatic orbital welding systems perform a series of operations in a controlled manner, where variables such as travel speed, arc separation, welding current control, etc., are monitored to ensure a higher degree of precision, provide uniform results , consistent, repeatable and high quality, making these systems portable, affordable, fast, accurate and cost effective in a wide spectrum of applications.
Figure 3 – Automatic orbital welding equipment
Other advantages of automatic orbital welding for tubes are higher productivity, faster welding speed, lower distortion, and limited and controlled heat input, leading to a much smaller HAZ.
We can appreciate the operation and capabilities of orbital welding for tubes in the video below.
Currently, the expansion of the use of pipes extends to the electronic, food, pharmaceutical, paper, aerospace, automotive, construction and air conditioning industries, among others, so the search for increasingly portable, precise, automated and low-cost applications to the piping process is ongoing and will undoubtedly lead to new technological advances in the years to come.
