Installation
Installation
up the bending forces. Illustration 7.12 shows the diameter of threaded bracket leg required for fixed point brackets with a particular bracket leg length and pipe diameter. Since the effect of force is felt most at the nodes and in particular at changes of direction, in practice, where bracket leg lengths are short, very strong, rigid fixed point constructions are often used at the nodes only. The intervening suspension points are then implemented as fixed point brackets, but with rather longer threads. The underlying idea here is that the bracket only needs to prevent buckling. - Clamp separations with rigid installation The fixed point brackets must be fixed at every node and further every 5 to 8 metres, depending whether it is hot, warm or cold pipework. Every 6 metres are regarded as standard. For the inter- vening brackets which are used as clamps, the separation distances provided for shell brackets in illustration 7.9 are used.
Illus. 7.10 Rigid installation. Forces occurring in the pipe at particular temperatures assuming a temperature during assembly of 10°C.
The change in length undergone by the pipe is directly proportional to the temperature difference. The force required to restrain this change in length is dependent on the cross sectional area of the pipe but also on the modulus of elasticity of the material as altered by higher temperatures. Illustration 7.10 shows the forces occurring at particular temperatures, assuming assembly free of tension at 10°C. Account has not been taken of the effect of relaxation of the HDPE material, which reduces the forces involved. - Brackets for rigid installation Where the forces of expansion cannot be transferred through a rail system (up to 160 mm) or bracket system (up to 75 mm), they must be transferred to the building structure by the fixed point brackets. Apart from using walls and floors to which the pipework can be fixed, (see illustration 7.2) fixed points can be provided using special fixed point brackets. Brackets can also be fitted between two welded sockets attached to the wall or another bearer via a threaded rod or gas pipe (illustration 7.11). It will be clear that as the length of the leg increases it must be more rigid to take
7.2.3 Cast-in pipework Pipework cast into concrete can be
regarded as rigidly installed. Any welded joints in HDPE must be allowed to cool first. It is recommended that the pipework is pressure tested and checked for leaks before the concrete is poured. The pipework must be well fixed to prevent flotation during pouring. Special brackets are available for this purpose (illustration 7.13). With HPE the bracket separation is around 8 x D (min. 0.75 metre, max. 1.5 metre). If pipework is cast vertically in concrete (e.g. columns, walls) the liquid concrete will produce an external over- pressure. Illustration 7.14 shows the resistance of various classes of HDPE pipes to external overpressure in kPa at 30°C. (SDR = external diameter/wall thickness)
Illus. 7.11Fixed point construction possibilities for rigid installation using HDPE in
diameters > 160 mm. Also possible as a fixed point in flexible installation.
Illus. 7.9 Bracket and band separation for HDPE waste pipework, flexible installation.
Note: The calculated class does not always accord with the nominal class.
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