Page 117 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 2 Common processes and techniques

The grades of low carbon steel have identical external diameters but the pipe
wall thickness will vary according to the grade – heavy grade having the thickest
pipe wall and light grade the thinnest.
Medium-grade pipe is the most common grade used in plumbing installations
but heavy grade may be used where a long system life is expected. Heavy-grade
pipe can also be used below ground. Light-grade pipe is seldom used, except in
some dry sprinkler installations for fire prevention.
Low carbon steel pipe is available in 6 m lengths, may be supplied with threaded
ends or plain ends, and is referred to by imperial pipe sizes, which are specified
as nominal bore. The common pipe sizes for domestic purposes are shown in
Table 2.23.
 Table 2.23 Common low carbon steel pipe sizes
Thread size/fitting size 1 / 8 ” 1 / 4 ” 3 / 8 ” 1 / 2 ” 3 / 4 ” 1” 1 1 / 4 ” 1 1 / 2 ” 2”
Nominal diameter mm 6 8 10 15 20 25 32 40 50

Bending low carbon steel pipe

There are two methods of bending low carbon steel pipe:
1 By hydraulic bending machine: this method uses a hydraulic bending
machine (see page 87 of this chapter) to bend the pipe. It uses an oil to exert
hydraulic pressure. The oil, being incompressible, exerts great force on the
pipe through the bending former to bend the pipe when the handle of the
machine is pumped. Steel is very tough to bend and tends to ‘spring’ back
once the bend is formed. Because of this, bends should be over-bent about
5° to allow for the bend springing back slightly. This is the method used in
domestic installations.
2 By heat: mainly used on industrial installations. This involves the use of
oxyacetylene torches to heat the steel almost to white hot to soften the
pipe. This allows the steel pipe to be bent easily by hand.
Here, we will look at how to bend a 90° bend and an off-set bend using a
hydraulic bending machine.

Bending a 90° bend, step by step
INDUSTRY TIP
1 Mark a line on the pipe at the required distance from the fixed point to the
centre line of the required bend (Figure 2.27). You may find it easier to
2 From this measurement, measure back towards the fixed point 1 nominal judge the angle of the bend
bore (the internal diameter) of the pipe to point A. by making a template from a
3 Place point A at the centre of the correct size bending former on the bending welding rod bent to 90° or by
machine. the use of a steel set square.
4 Pump the handle of the bending machine until an angle of 90° + 5° The template can be placed
(allowance for springing back) has been achieved. Make sure you are standing on the bending machine so
to the side of the machine. NEVER stand in front of it while bending is taking that you can see where to
stop the bend.
place.

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