Laying GRP pipes above ground:
The distance between the supports, the type of supports, the anchorage and the fixing points are defined in the design stage. In general, pipes should be supported by concrete cradles surrounding the pipes over an arc of 150° with a layer of rubber of at least 4 mm in thickness in between.
During the laying operations, it is necessary to take all possible steps to avoid damaging the product. In particular, the following aspects should be kept in mind:
- tightening of the fixed points (clamps) should be carried out with the pipe at the temperature foreseen during the working conditions; the design values should be scrupulously respected;
- bolts of flanges should be tightened at specified loads;
- elbows should be anchored when it is foreseen that the pipe will undergo torsion stresses;
- fittings not made of FRP, such as valves, steel flanges, and the like, should be supported individually;
- battery limits, such as connections to equipment or pipelines, made of different materials should not transfer additional stresses to the FRP pipeline;
- all tee connections should be supported so as to avoid interactive stresses between the main line and branch lines.
General information of this kind obviously cannot cover all the problems associated with the laying of pipes above ground. Therefore, during the design stage, stress should be analysed for each individual case and the most suitable support system should be selected.
Furthermore, when pipes are installed in contact with flammable products, all metal parts should be earthed to prevent the build-up of electrostatic charges leading to sparks.
Laying GRP pipes below ground:
The underground pipes are laid on a bed of sand or screened material compacted to the level anticipated in the design. Then, the backfilling is carried out at the sides and above the pipe in layers of 20-30 cm, each with the same material employed for the bed.
The first layer of the backfill should be applied with great care since it provides a firm support under the pipe. Each layer must be compacted with methods and systems adapted to the nature of material employed, taking care to avoid any damage to the pipe during this operation.
An increase of 1% - 3% of the vertical diameter of the pipeline during the laying operations is beneficial in order to compensate the reduction of the vertical diameter, which occurs during the pipeline operation.
It is impossible to provide full instructions for the laying operations, since each installation is a case on its own.
However, the following recommendations can be useful:
- the trench should be dug with its walls as vertical as possible;
- any contact of the pipe with large stones or rocks should be avoided;
- in case of installations below the water table, continuous drainage should be provided in the trench until the backfilling has been carried out;
- crossing areas subject to heavy external loads (permanent or mobile) must be analysed very carefully to establish if special steps must be taken, e.g. different stiffness to fit the situation, increased burial depth, use of a sheet metal or a reinforced concrete cover;
- in soils with a low modulus of reaction, such as clayey soil, or grounds with a high content of organic substances, the width of the trench should be increased and pipes should be surrounded with compacted and selected backfilling material;
- the behaviour of the underground pipes is greatly affected by the surrounding soil. It is therefore essential, when carrying out the laying operations, to comply with the requirements of the design.
An engineer designing the underground pipelines should take great care in examining the soil parameters. In case of doubts, they should assume the most pessimistic conditions.
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