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.
Depending on the dimensions and the required weld thickness one or two passes are necessary. One tour of the torch is always applied in case of a pass for tightness; the layers for mechanical resistance often require a second tour.
Welding of protruding tubesProtruding tubes are always welded with addition of filler wire, but in some cases the weld is beginning with a fusion pass. As shown in Fig. 2, different joint designs are possible.
Protruding tubes are always welded with addition of filler wire, but in some cases the weld is beginning with a fusion pass. As shown in Fig. 3, different joint designs are possible.• Torches with an angle of 15° are preferentially used in case of thin-walled tubes (1.6 mm to 2.11 mm), thus melting the inside can be avoided• Torches with an angle of 30° are applied for thick-walled tubes (from 2.5 mm onwards) if there is sufficient space with regard to the tubes around (reduced pitch).
In any case, to avoid melting down the tube edge, the tube length measured from the ground of the groove must exceed at least 5 mm.
Expert information: If equipment fitted with five controlled axes is used, the AVC has to be operated in the relative height mode. Thus it is possible to adjust the distance between electrode and tube plate to get the best result; independently of the torch position.
Special attention must be paid to the training of the operators; differently to orbital tube-to-tube welding, where the mechanical adjustments of torch and wire guide are carried out in the same plane, tube to tube sheet welding requires three-dimensional operation.
Welding of recessed tubesDifferent joint designs are shown in Fig. 4. Welding equipment fitted with 4 or 5 controlled axes and an open tube to tube sheet welding head can be used for the application D, E and F.
The preparation of the type G is frequently used in the petrochemical industry; welding equipment with six controlled axes and a TIG 20/160 welding head with separate clamping device have to be used. This type of application generally requires a specific project to study the best adaptation of clamping tools and welding procedures.
Expert information:  Different to those applications with protruding tubes, in the case of recessed tubes a V preparation of the tube plate is possible. If joint preparations of the type E or F are applied, the tubes may protrude slightly from the base of the groove.Depending on the dimensions, and the required weld thickness, one or two passes are necessary. One tour of the torch is always applied on the first pass for tightness; layers needed for mechanical strength and wear resistance will often require a second tour.Particular application:  Welding behind the tube plate of a double-walled collector for air-cooling or fluid condensation. The AVC operation is indispensable to make the weld in this application.
Internal bore welding behind the tube sheetTo avoid gap corrosion between the tube and the tube sheet, gapless joints are welded from the inside of the tubes at the backside of the plate. This type of application requires extended accuracy of the workpiece preparation and welding. Some possible joint designs are shown in Fig. 5.
A joint preparation of the type X is not recommended: the greater mass difference between the tube and the plate excludes the possibility to achieve a sufficient penetration.The joint preparation of the type Y overcomes the penetration problem by creating a welding zone with a better balanced mass of the tube and the plate.
For three reasons, by the joint preparation of the type Z the weld conditions become quite similar to those of a standard orbital tube-to-tube weld operation:• By the recess the tube is aligned on the bore.• Melting down the collar offers some additional metal which increases the mechanical strength of the weld.• The concave form of the weld is reduced.
Expert information: Unlike classic tube to tube sheet applications, the internal bore welding operations behind the tube sheet require a gas protection of the root, (at the outside of the tube). Only with a preparation of the type X, where the tube end is positioned sufficiently deep in the bore (e.g. half of the tube wall thickness), a root protection is not necessary. The protection can be provided by flooding the entire apparatus with inert gas or, if the backside of the plate is accessible, by a local protection applied tube after tube.
With a tube I.D. of more than about 35 mm, the use of welding tools with filler metal is possible.
If relatively thick walled tubes of 3 mm to 3.6 mm, (depending on the base material), are to be welded, a horizontal weld position with the plate at the bottom with the Weld Head also horizontally positioned, is recommended.The distance from the face of the plate to the welding joint must be very precise, (close tolerance). The operator cannot see the torch position inside the tube, he has no possibility of adjustment and he cannot watch the welding process.
Welding equipment fitted with three or four controlled axes can be used for this application, in the case of a joint preparation of the type X, five controlled axes are necessary. The welding heads must be equipped with a particular lance for internal bore welding
Similar application: If nipples have to be welded on a collector, (this is typical application in the field of power plant equipment construction), identical base materials are used, and the joint preparation and the precautions to be taken are similar to those of internal bore welding behind the tube sheet.
ConclusionsTo conclude, the importance of orbital TIG (GTAW) welding remains critical if sophisticated applications require reliable outstanding joint quality.
Contact:Polysoude IndiaA. B. KulkarniHead Of RepresentationsShri Shanta Durga Niwas, Plot No. 7,Shivaji Co-op. Hos. Soc., Off Senapati Bapat Marg, Pune – 411 016, MaharashtraTel: +91-20-40035931, Fax: +91-20-40035930ab.kulkarni@polysoude.in