Orbital tube to tubesheet welding

February 11, 2013 10:35 am

Orbital tube to tubesheet welding
Orbital tube to tubesheet welding demands specialised equipment that can produce reliable outstanding joint quality
Compared to manual welding, the planning of the orbital tube to tube sheet welding requires some more specific attention. The tubes have to be seamless (or without flattened weld), concentricity faults between the inner and the outer diameter must be limited to a minimum to allow the repeatability of the electrode positioning. With standard applications, (flush, protruding or recessed tubes) the torch is aligned at the inside of the tube whereas the welding is carried out at the external diameter. Concentricity faults would cause unacceptable variations of the distance between work piece and electrode and thus directly alter the arc length.
As with V-joints it is virtually impossible to ensure reliable melting of the base of the tube edge, especially in the vertically down position (fusion defects are to be seen on macrographic sections), these joints have to be replaced by J-preparations.Range of materials and tube dimensionsNearly all weldable metal and alloys are used in the field of tube to tube sheet applications, but the range of the tube dimensions is relatively restricted. Their diameter range covers 12.7 to 101.6 mm, the wall thicknesses are between 0.5 to 5 mm. Most of the tube diameters measure between 19.04 mm and 31.1 mm with wall thicknesses between 1.65 and 3.4 mm.
Welding equipmentIn most cases, the welding equipment used for tube to tube sheet welding is strictly adapted to the kind of application and the desired level of automation:
Welding equipment featuring three controlled axes (gas, current, and rotation) is composed of a portable power source and a closed welding head. This equipment allows for the execution of fusion welding without addition of filler wire.The welding equipment, including 4 controlled axes (gas, current, rotation, and wire), is composed of a stationary installed power source – portable power sources are rarely used for these applications: there is no need for the machines to be carried – and an open welding head. The equipment is suitable for single pass welding; two passes must be welded in two separate steps.
The welding equipment fitted with 5 controlled axes (gas, current, rotation, wire, and AVC) is composed of a power source designed to control 6 axes and a welding head of the type TS 2000 or TS 8/75 with AVC configuration. The equipment allows the chaining of several passes with filler wire, the raising of the torch between the different passes can also be programmed and is carried out without interruption of the weld cycle.
Welding equipment furnished with 6 controlled axes, (gas, current, rotation, wire, AVC, oscillation), comprises a PC Power Source and a welding head of the type TIG 20/160. The equipment allows multi-pass welding (2 or more passes); the torch can be displaced in radial direction.
Tube preparationIn some cases, if a good thermal conduction is required, the play between the tube and the bore must be eliminated by a slight expansion of the tube. Play is necessary for the assembly of the apparatus before the welds are carried out, but if clearances become too large, problems of repeatability may occur. However, it is difficult to specify a maximum amount of play; it depends on the demanded weld quality and the thickness of the tube.
Expert information: To get optimised centring tools for the tube to tube sheet welding heads, each order must be accompanied by information about the depth of the expansion and the tube diameter at the expanded zone as well as the original diameter.
The contact zone between the tube and the tube sheet must be clean. Grease, oil or other residues from the tube manufacturing or machining can cause the formation of unacceptable blowholes, with outlets on the surface or enclosed in the welds.
A strong expansion of the tubes inside the tube sheet must never be carried out before automatic welding. A strong expansion (with or without longitudinal grooves in the bore) causes almost always explosive degassing effects which make automatic welding impossible.
Welding of flush tubesDepending on the application, orbital welding of flush tubes with or without filler metal is possible. Different joint designs are shown in Fig. 1.
Welding of flush tubes without filler wireOften, the type 1 preparation is carried out for the welding of flush tubes; rarely the type 4 is used. In case of tube diameters between 10 mm and 25 mm or 10 mm and 32 mm the use of especially developed welding heads, for these applications without filler wire, is recommended.
It is the operator’s task to position the welding head and to start the weld cycle. The complete sequence is carried out automatically; the operator is not needed any longer at this machine. Thus, one operator can work simultaneously with several welding heads.
Typical application: Condensers of thermal-electric power plants. Here, the tubes with a wall thickness of about 1 mm are made of titanium whereas the tube sheet is designed and manufactured as titanium-cladded steel plate
Welding of flush tubes with addition of filler wireWelding equipment fitted with 4 or 5 controlled axes can be used for this application; the open tube to tube sheet welding head should be configured with devices adapted to the requirements:• Integrated or external wire feeder• With or without AVC• With or without shielding gas chamber (for the welding of titanium or zirconium)• Torch angle of 0° or 15°.
Expert information: The AVC function is recommended especially for the welding of flush tubes.
Generally, the tube end preparations are of the type 1, 2 or 3. If a preparation of the tube sheet is carried out, the V-joint can be avoided. With this type of preparation, there is always the risk of incomplete penetration of the root. A J-preparation (with or without radius) should be preferred, if the depth of the bevelled edge exceeds 1.5 mm, the tube end should be positioned at the half of it. The maximum value of the tube end to be recessed is 50 per cent of the tube thickness; the tube becomes flush by the weld.

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