GB2288562A - Purging apparatus for welding - Google Patents

Abstract

When producing high quality metal welds it is essential to remove all oxygen containing gas from the area around the weld. A purging apparatus for purging a hollow section (20), comprises two compressible foam stoppers (1, 3) linked by a flexible coupling (2), one of the stoppers (1) having a tube (4) passing through it so that, in use, the two stoppers substantially block each end of a part of the hollow section (20), and an inert gas is fed to that part through the tube (4) to expel oxygen from that part of the section. Preferably, the two foam stoppers (1, 3) are made from open cell foam, so that they are gas permeable, thereby allowing gas purged from the part of the section (20) to leave through the stoppers. Preferably the foam stoppers (1, 3) are made from heat resisting foam. The invention also has utility where only a single compressible foam stopper (1) is provided having a tube (4) passing through it so that it can be used to supply an inert gas to a hollow article or whole pipe system (20). <IMAGE>

Description

WELDING APPARATUS When producing high quality metal welds it is essential to remove all oxygen containing gas from the area around the weld. This is particularly true when welding stainless steel, Cunifer (Trade Mark) and Incalloy (Trade Mark) as well as when welding non-ferrous metals. If any oxygen containing gas remains during welding, the root penetration of the weld bead may be oxidized, leading to an unsatisfactory weld. Normally, in welding such as MIG or TIG welding, an inert gas is supplied to shroud and shield the weld site as the weld is being formed. However, a special problem arises where a hollow section is being welded only from the outside. In this case it is necessary to purge any oxygen containing gas from the inside of the hollow section, as well as supplying an inert gas shroud or shield to the weld site from the outside. The most common hollow sections to be welded are tubular sections such as pipes.

Especially with pipes of large diamter or length, it is time consuming to purge the pipe using an inert gas.

Typically, an inert gas feed tube is taped in place in the pipe and then inert gas supplied to the pipe at a flow rate of at least 0.5 cu.m per hour. To purge an entire pipework system this must be supplied for many hours so that the pipe contains less than 1% oxygen.

An alternative system has been proposed which significantly reduces the purge times, and the volume of inert gas required. In this system, two inflatable bladders are provided which are connected together by a tube. The device is positioned in the pipe bridging the weld site. The bladders are inflated typically with an air line and then inert gas is introduced into the space between them and around the inside of the weld site. A vent is provided on one of the inflatable bladders to define an opening between the bladder and the side wall of the pipe through which oxygen and inert gas purged from the pipe is vented.

Using this system, the time required for purging is significantly reduced. For a pipe of 40 mm diameter, the purge time may be as low as thirty minutes to reduce the oxygen content in the pipe to less than 1%.

This arrangement has not however achieved widespread acceptance. The bladders are not very robust and accordingly are easily damaged and punctured when they contact rough metal edges of the pipes or weld spatter left by the welding operation. Equally they are easily damaged and punctured if they are removed too soon after the welding has been completed so that they come into contact with hot just welded regions. This can also happen if they are not positioned correctly with respect to the weld site. Any puncture, however small, is serious because the air used to inflate the bladders then is jetted from the puncture into the space being purged. This completely nullifies the effect of the purge gas. In an effort to increase the robustness of the bladders they are made from tough material which is not very expansible. Accordingly a large number of purging devices are required for pipes of different diameter. Further because the bladders are generally cylindrical they are only suitable for pipes.

According to the present invention, a purging apparatus for purging a hollow section, comprises two compressible foam stoppers linked by a flexible coupling, one of the stoppers having a tube passing through it so that, in use, the two stoppers substantially block each end of a part of the hollow section, and an inert gas is fed to that part through the tube to expel oxygen from that part of the section.

Preferably, the two foam stoppers are made from open cell foam, so that they are gas permeable, thereby allowing gas purged from the part of the section to leave through the stoppers. However the stoppers prevent oxygen from penetrating into that section by diffusion. Both of the stoppers may include a tube, one of which allows for inert gas to be introduced into the part of the section, and the other of which allows for purged gas to leave the part of the section. In this case it is preferred that the outlet tube is of smaller flow capacity than the inlet tube. However the open cell nature of the foam reduces turbulence within that part of the section and so it is preferred to allow the purged gas to leave through the foam stoppers. Again to reduce turbulence a diffuser may be fitted to the outlet of the tube leading into the part of the section. This discourages mixing between the purge gas and that already present and allows a gradual dissipation of gas from within that section through the foam material.

The use of stoppers made of foamed material is particularly beneficial because even if their edges get nicked and damaged by swarf or weld spatter this does not materially affect their efficiency. Equally, the foam stoppers are considerably more compressible and therefore a single foam stopper can be used with a far wider variety of shapes of sections and diameters of pipe than is the case with the prior inflatable bladders.

Preferably the foam stoppers are made from heat resisting foam so that they are not materially damaged when they come into contact with a hot, just welded, part of the section.

Preferably the two foam stoppers are linked together by a thin, flexible, metal rod which can both accommodate any corners or junctions in the pipe and yet resist both tensional and compressive forces to enable the purging apparatus to be pushed into or pulled out of the section.

The metal rod may have the form of a helical spring the turns of which touch one another but preferably it simply comprises one or more thin rods. The metal of the rods is preferably the same as the metal to be welded so as not to introduce a source of contamination. The metal may be, for example, stainless steel.

The invention also has utility where only a single compressible foam stopper is provided having a tube passing through it so that can be used to supply an inert gas to a hollow article or whole pipe system. In this case, the invention is not used to isolate only a small section but it does, nevertheless, provide a convenient way of introducing an inert purging gas into the article or whole pipework system.

It is possible to replace the stoppers, for example with stoppers of different dimensions or shape to allow the apparatus to be used in articles of different shape and pipes of different diameter but, in general, because the foam material is so compressible this is not usually necessary. The apparatus may also be arranged so that the flexible connection between the two stoppers can be replaced easily, for example with a connection of a different length where the section requiring welding is longer. Again this is not usually necessary.

The inert gas introduced into the section defined between the two stoppers is usually argon.

Examples of the present invention will now be described in accordance with the accompanying drawings, in which:- Figure 1 is a section through a butt joint in a pipe to be welded using a first example; and, Figure 2 is a section through a butt joint between a pipe and an elbow using a second example.

The first example of apparatus consists of a first stopper 1, a flexible connector 2 and a second stopper 3.

The first stopper includes a short length of tubing 4 passing through it by which inert gas can enter the space between the stoppers 1 and 3. The stopper is mounted and retained on the tubing 4 by a pair of washers 5 and 6 held at one side by crimped clips 7 and 8 and at the other by a surrounding plastics tube 9. The flexible connector 2 is formed by two lengths 10 of stainless steel wire of 1.5 mm diameter fixed onto the crimped clips 7 and 8 at one end and held by further crimped clips 11 and 12 at their opposite end. The second stopper 3 is also mounted on a length of tubing 4 and retained between washers 13 and 14 held on one side by crimped clips 11 and 12 and on the other by a plastics tube 15.

The stoppers 1 and 3 are made of an open-cell heat resisting foam having a cell size of the order of 1 to 2 mm. The stoppers have a length of the order of 50 mm.

The apparatus is pushed into a pipe 20 by first compressing the foam stoppers 1 and 3 so that they have a size less than the diameter of the pipe 20. When inserted, the foam stoppers expand to fill the entire cross sectional area of the pipe 20. The apparatus is pushed along the pipe until one of the stoppers 3 is positioned downstream of the butt joint 21 forming the site to be welded and the other stopper is upstream of it.

The stainless steel wires 10 are flexible so that the apparatus can accommodate a pipe 20 which includes a bend or elbow as shown in Figure 2.

Inert gas, for example argon, is fed into the section between the foam stoppers 1 and 3 through the tubing 4. As the inert gas is introduced, oxygen is forced out of the section through the gas permeable foam stoppers 1 and 3 and through the joint 21 thereby purging oxygen from that section. The supply of inert gas is maintained throughout the welding process, so that no oxygen can re-enter the section of pipe through the gas permeable stoppers 1 and 3 or through the butt joint 21.

Gas diffusion through the foam stoppers 1 and 3 also prevents any welding gases building up in the space between them and so prevents such gas build-up blowing out molten metal from the weld or the formation of welds with a hollow root.

The second example of apparatus is generally similar to one end of the first example. This it includes a compressible foam stopper 1 mounted on a tube 4 and sandwiched between washers 5 and 6 held in place by clips 7 and 8. The second example may be used on its own to introduce gas into a short, closed hollow section or to a whole pipework assembly that is to be flushed. However, it is preferred that they are used in pairs with one on the inlet end and one on the outlet end. The second example is preferred for short sections where access is available at both ends, as shown in Figure 2, or for purging entire pipework assemblies.

A convenient way, not shown, of retaining the pairs of washers 5 and 6 or 13 and 14 on the tubing 4 is by swaging the tubing 4 to form raised ribs on the outside of the pairs of washers 5 and 6 and 13 and 14.

Claims (10)

1. A purging apparatus for purging a hollow section, comprising two compressible foam stoppers linked by a flexible coupling, one of the stoppers having a tube passing through it so that, in use, the two stoppers substantially block each end of a part of the hollow section, and an inert gas is fed to that part through the tube to expel oxygen from that part of the section.

2. A purging apparatus according to claim 1, in which the two foam stoppers are made from open cell foam, so that they are gas permeable, thereby allowing gas purged from the part of the section to leave through the stoppers.

3. A purging apparatus according to claim 1 or 2, in which both of the stoppers include a tube, one of which allows for inert gas to be introduced into the part of the section, and the other of which allows for purged gas to leave the part of the section.

4. A purging apparatus according to claim 3, in which the outlet tube is of smaller flow capacity than the inlet tube.

5. A purging appratus according to any one of the preceding claims, in which the foam stoppers are made from heat resisting foam.

6. A purging apparatus according to any one of the preceding claims, in which the two foam stoppers are linked together by a thin, flexible, metal rod which can both accommodate any corners or junctions in the pipe and yet resist both tensional and compressive forces to enable the purging apparatus to be pushed into or pulled out of the section.

7. A purging apparatus comprising a single compressible foam stopper having a tube passing through it so that it can be used to supply an inert gas to a hollow article or pipe system.

8. A purging apparatus according to claim 7, in which the foam stopper is made from open cell foam, so that it is gas permeable.

9. A purging apparatus according to claim 8, in which the foam stopper is made from heat resisting foam.

10. A purging apparatus substantially as described with reference to Figure 1 or Figure 2 of the accompanying drawings.