Fundamental of Orbital Pipe Welding

Typically, orbital pipe welding machines are divided by closed head (full function in place) and open head (full function orbital) [45]. For welding of tubes, mostly closed head machines are used and for welding of pipes with large diameters open head machines are used. Both types of these machines are shown in Figure 4 and Figure 6.

Figure 6 Open head welding machine for welding of pipes with large diameter [52]

3.2.1. Closed Head Mechanism (Full Function in Place)

For welds critical thin-wall tubing and pipes with small and medium-sized, fabricators select a closed head device [22, 46]. Standard closed orbital weld heads can be used for tube sizes from 1.6 to 162 mm (this number can be varies e.g. up to 200 mm [22] or 6.3 to 203 mm [46]) with wall thicknesses of up to 3.9 mm (0.4 to 12.7 mm [46]) [4]. The head housing in close head systems can be fixed or adapter types with limited range of accommodation [46]. Generally, during designing of welding heads, pipe standards and limitations are taken into account by manufacturers, in order to make the range of a single welding tool proportionately broad [22]. For welding of particular applications requiring extreme purity, thin-walled aluminum tube, small diameter tubes, titanium and stainless steels pipes, and so on, closed head welding system can be used [2, 22, 45].

This system performs autogenous welding and the tubes are maintained in a fixed position. The head cover the entire weld area while the tungsten electrode holder moves along the joint driven by a small dc gear motor inside of head. The enclosed chamber filled with inert gas (usually argon) which is responsible for shielding the end part of the electrode and the fusion zone and the heated regions of the component

to prevent the hot weld zone from oxidation [2, 22, 46, 51]. Arc length in this system remains fixed [46] and wire feed is and arc voltage control is not needed [45]. Figure 7 shows the principle of orbital GTAW closed head process explained [2].

Figure 7 Principle of orbital GTAW process [2]

In welding of stainless steels with this mechanism, it should be considered that inside surface of the pipe as well as outside should be protected from oxygen during whole process. The reason is that stainless steel pipes content chromium and carbon and during welding without proper protection will react with the oxygen to form chromium carbides, commonly known as coking. To solve this problem internal bore of the tube should be filled by an inert gas. This method is well known by back purging [51].

3.2.2. Open Head Mechanism (Full Function Orbital)

Generally, open head systems are used for pipe welding with large diameters and wall thicknesses (above 170 mm [22]) and are considered full function heads [4, 22, 45].

There are two approaches for designing of this welding system. In the first one, within of an external frame the welding heads mounted, introduced since 1960s, and its main application is on lay barges, planned for the wide, open space of the USA and USSR. In the second approach, welding system consists of welding head or

“bugs”, bands or chains, power supply, wire feeder, a programmable controller, shielding gas, and coolant. Bands or chains are a device attached to the pipe or tube to move the head around the joint. The controller may be mixed into the power supply and cable for delivery of power. The welding heads mounted on bands or chains and travels the circumference of the pipe along a track. In welding of thin wall tube, welding head mostly consists of water cooling system to reduce heat input. The main application of this welding system is onshore pipelines [7, 22, 46].

By starting welding procedure by operator, the weld head and its inside electrode house rotates in a precise orbit around the joint. The process is highly controlled, and there are at least four servo controlled axes (travel, wire, arc voltage control, and center steering) to ensure high quality welds that can be produced on a consistent and repeatable basis. Precise controlling of all parameters in welding head requires a state-of-the-art computer due to the complexity of this welding process [45, 46, 48]. It takes slightly more time to install these systems than to install an in-place head. This type also may require a longer straight length of pipe for mounting. In orbital multipass pipe welding, welding heads typically feature [1]:

1. Torch rotation

2. Filler wire feed capability

3. Electronic control of arc length (arc voltage control)

4. Torch oscillation (weave) capability with programmable width, speed, and independent end point (sidewall) dwell times.

The direction of welding head will change by changing of applications. If the position of pipe weld is horizontal (5G) both, a double up and double down technique (6 to 12 O’clock CW, followed by 6 to 12 O’clock CCW) can be used [47]. Also, weave technique is usually used in the 5G position welding. To ensure complete sidewall

fusion, most of available systems are possible to be programmed dwell or delay period at either end of the oscillation stroke. Orbital pipe welding with oscillation motion is a useful feature in pipe welding industries [53].

One way to improve productivity in open head mechanism is to mount two welding torches on one head. The improvement continues by reducing the distance between two wires and using shared gas and weld pool. In a further development, it has been proposed to replace the two torches of a system with tandem torches, creating the so-called “Dual-Tandem” process [54].