Backing rings are commonly employed for welding carbon steel and low-alloy steel piping by the MMA process in steam power plants and other applications. While split rings are some­times used for non-critical applications, solid flat or taper-machined backing rings are pre­ferred for critical applications. Some designs of backing rings and the manner in which they are fitted are shown in Fig. 11.4. The figure shows that the pipe-end must also be suitably

machined on the inside diameter. Chemical composition of the ring is important as also the seat contact between the pipe-end and the ring. Guidance for the correct use of baking rings is available in relevant codes. Backing rings are rarely used for piping in oil refineries and chemical plants.

37 1/2° ± 22

3/16" nominal 1/16" ± 1/32" 3/16"

(Bore)

37 1/2° ±22

3/16" nominal 1/16" ± 1/32"

A B Break corners

3/4"

C DT (Bore) (Ring OD)

For wall thickness (T) 9/16" to 1" inclusive and tapered internal machining.

DS (Ring OD)

1/8-R min Break corners

For wall thickness (T) 9/16" to 1" inclusive and straight internal machining.

±1° Rounded 37 1/2° ±2 1/2°

3/16" nominal

. 1/16" ±1/32" /-А-

For wall thickness (T) greater than 1" and straight internal machining

Min. Break corners

10° ± 1°

±22 nominal

AB Break corners

C (Bore)

DT (Ring OD)

For wall thickness (T) greater than 1" and tapered internal machining

Rounded 37 1/2 K*| 3/16

C DS (Ring OD) (Bore)

Fig. 11.4 Edge preparation using flat or taper machined solid backing rings

Where the weld joint quality and especially its corrosion resistance are important, consumable insert rings are placed at the root, as mentioned earlier and illustrated in Fig.

11.2 and fused with a TIG torch, so that a sound root weld pass results. This technique dispenses with the addition of filler metal, which could interfere with the welding operation and cause lack of penetration. The subsequent passes, if required, are then deposited by the TIG process using a filler wire or by the MMA process. If instead of using an insert, the pipe-end is suitably machined at the root and autogenously welded, cracking or porosity is likely to occur because of the unfavourable base metal composition. Use of a consumable insert ring of properly balanced composition and dimensions:

(a) provides the best welding conditions even in horizontal fixed or 5G position, (b) minimises human element and thereby ensures weld uniformity, (c) gives the most favourable weld contour which can resist cracking arising from weld metal shrinkage, and (d) gives weld - metal composition which can guarantee optimum mechanical properties and corrosion resistance.

At this point, it is pertinent to mention that the various pipe welding positions are defined by standard symbols (1G, 2G, etc.) as shown in Fig. 11.5. Among these, 5G position is the most difficult and it calls for high welding skill. For this position, it is advisable to insert the consumable ring eccentric to the centreline of the pipe as shown in Fig. 11.6, so that it compensates for the downward sag of the liquid weld-metal and helps to obtain uniformly smooth root contour on the inside of the joint.

Fig. 11.6 Eccentric insertion of consumable insert ring for 5G position pipe welding

Consumable insert rings of proper shapes, diameters and chemical compositions to suit various metals and applications are provided by manufacturers in advanced countries.

In the installation of piping systems, tees, laterals, wyes and vessel openings have to be welded, and they normally involve intersection joints. Since such joints are difficult to weld, standard welding fittings supplied by manufacturers are used. These fittings possess bursting strengths equivalent to those of pipes of the same weight and they are designed to be con­nected by simple putt welds. Some examples of such fittings are shown in Fig. 11.7. Manufac­turers also provide factory-made nozzles, necks, outlets, tees, etc., specially designed for weld­ing to simplify the fabrication of piping.