^ Fig. 12. Example of welding protruding tubes to a tubesheet. With a Polysoude Ts 2000 welding head.
Article By Patricia Dauxerre, Communications Department, Polysoude, Nantes, France
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Article By Patricia Dauxerre, Communications Department, Polysoude, Nantes, France
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Fig. 1. Shemes of standard applications in the field of tube-totube-sheet welding
Fig. 7. Macrographic section of a fully penetrated tube-to-tube-sheet
joint welded behind the tube plate.
Boilers and heat exchangers are used in all kinds of industries, with the heaviest equipment being found in the plants of the chemical or petrochemical industries, and in electric power stations.
Compared to manual welding, orbital tube to tube-sheet welding requires more detailed attention to such things as the tube type, the material, the end preparation, and the dimensions of the required weld thickness, for example. To use orbital welding equipment, it is strongly recommended to choose seamless tubes (or those without a flattened weld), concentricity faults between the inner and the outer diameter must be limited to a minimum in order to allow the repeatability of the electrode positioning. With standard applications, (flush, protruding or recessed tubes –see Fig. 1) 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 in the distance between the workpiece and the electrode and would thus directly alter the arc length.
Nearly all weldable metals and alloys are used in the field of tube-to-tube-sheet applications, but the range of tube dimensions is relatively restricted. Their diameter range covers 12.7 to 101.6 mm, with wall thicknesses being between 0.5 to 5 mm. Most of the tube diameters measure between 19.04 mm (3/4”) and 31.1mm (1.5’’) with wall thicknesses between 1.65 and 3.4 mm.
Fig. 2 J-preparation & V-preparation.
In most cases, especially in the vertically down position, joints have to be machined by J-preparations (Fig. 2). In fact, with V-joints it is virtually impossible to ensure reliable melting of the base of the tube edge; fusion defects are therefore often observed on macrographic sections.
If good thermal conduction is requested, the gap between the tube and the bore must be eliminated by a slight expansion of the tube. A gap is necessary for the assembly of the apparatus before the welds are carried out, but if clearances become too great, problems of repeatability may occur. However, it is difficult to specify a maximum amount of gap; it depends on the demanded weld quality and the thickness of the tube. But 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) almost always causes explosive degassing effects, which make automatic welding impossible.
To get optimised centring tools for the tube-to-tubesheet 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.
Fig. 3. Composed of a portable power source
Fig. 4. Closed welding head
Fig.10. Example of welding flush tubes to a tube-sheet
with three Polysoude TS 8/75 welding heads
using external wire feeders
Fig. 5. composed of a power source
Welding equipment
In most cases, the welding equipment used for tube-totube-sheet welding is strictly adapted to the kind of application and the desired level of automation; four main types exist using a power source and a dedicated welding head.
In most cases, the welding equipment used for tube-totube-sheet welding is strictly adapted to the kind of application and the desired level of automation; four main types exist using a power source and a dedicated welding head.
- For the execution of fusion welding without the addition of filler wire , the welding equipment features three controlled axes (gas, current, rotation) is composed of a portable power source (Fig. 3) and a closed welding head (Fig. 4).
- The welding equipment, including four controlled axes (gas, current, rotation, 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 five controlled axes (gas, current, rotation, wire, AVC) is composed of a power source (Fig. 5) designed to control six axes and a welding head of the type TS 8/75 with AVC configuration. The equipment allows the chaining of several passes with filler wire. Raising the torch between the different passes can also be programmed and is carried out without interruption of the weld cycle. This welding equipment is also used for low protruding tubes from the tube-sheet. The AVC slide is not mounted in the same axis as the tube axis but with an angle to optimise the arc length control.
- The welding equipment furnished with six controlled axes, (gas, current, rotation, wire, AVC, motorised diametre adjustment), comprises a PC power source and a welding head of the type TIG 20/160. The equipment allows multi-pass welding (two or more passes); the torch can be displaced in a radial direction.
Depending on the application, the orbital welding of the tubes with or without filler metal is possible but is either less or more easy following the tube sheet preparation. Several different joint designs exist but the choice will influence the weld penetration of the root pass and consequently the strength and the quality of the product. For orbital welding of flush tubes with or without filler metal, the different joint designs are shown below.
Welding of flush tubes without filler wire
Type 1 preparation is most often carried out for welding flush tubes; Type 4 is rarely used. With regard to tube diameters between 10–25 mm or 10–32 mm, the use of especially developed welding heads,for these applications without filler wire, is recommended. The typical application is condensers of thermal-electric power plants. In case of stainless shell and tube, the tube wall thickness is in between 1 – 2mm. In case of titanium shell and tube, the titanium tube wall thickness is less than 1 mm. The shell is explosion cladded by titanium.
Fig. 8. Standard preparation
Fig. 6 Welding of flush tubes
Fig. 9. Welding equipment fitted with four or five controlled axes
Fig. 11. Protruding tubes
Fig. 13 TIG 20/160 welding
Fig. 14. Example of recessed tube welding.
Welding of flush tubes with addition of filler wire
Welding equipment fitted with four or five controlled axes can be used for this application; the tube-to-tube-sheet welding head should be equipped with an on-board or external wire feeder, with or without AVC, with or without a shielding gas chamber (for the welding of titanium or zirconium) and a torch angle of 0° or 15°. The AVC function is recommended especially for the welding of flush tubes.
Generally, the tube end preparations are of type 1, 2 or 3. If a preparation of the tube sheet is carried out, the V-joint can be avoided because there is always the risk of incomplete penetration of the root. A J-preparation should be preferred, if the depth of the bevelled edge exceeds 1.5 mm, the tube end should be positioned at halfway. The maximum value of the tube end to be recessed is 50% 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 the case of a pass for tightness; the layers for mechanical resistance often require a second tour.
Welding of protruding tubes
Welding of protruding tubes
Protruding tubes are always welded with addition of filler wire, but in some cases the weld begins with a fusion pass. Different joint designs are compatible (Fig. 11):
Protruding tubes are always welded with addition of filler wire, but in some cases the weld i begins with a fusion pass. Note that torches with an angle of 15° are preferentially used with thin-walled tubes (1.6–2.11 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 ground of the groove must exceed at least 5 mm.
Welding of recessed tubes
Different joint designs are shown below (Fig. 13):
Welding equipment fitted with four or five controlled axes and an open tube-to-tube-sheet welding head can be used for the application D, E and F.
The preparation of type G is frequently used in the petrochemical industry; welding equipment with six controlled axes and a TIG 20/160 welding head with a separate clamping device has to be used. This type of application generally requires a specific project to study the best adaptation of the clamping tools and the welding procedures.
The case of recessed tubes is different to the applications with protruding tubes in that 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.
For internal bore welding behind the tube sheet, extended accuracy of the workpiece preparation and welding are required. Each configuration must be study to find the right solution.
Conclusions
It is extremely important to stress the importance of orbital TIG (GTAW) welding if sophisticated applications require reliable outstanding joint quality. For several decades, the French company Polysoude has developed and manufactured appropriate gear and can offer a wide range of standard machines or adapt it for specific demands. The modular design of the devices, i.e. welding heads and power sources, allows the proposal of tailor-made solutions to exigent customers, always taking into consideration the special constraints of the particular project.