General Considerations

 

1. PVC pipes are usually joined using the push-fit elastomeric jointing ring or solvent cement techniques. The push-fit technique has advantages as it is less dependent on weather conditions and provides allowances for pipe movement.

The solvent cement technique is usually restricted to smaller diameter pipes.

2. The sub surface material to be excavated should be assessed for its suitability as backfill material, i.e. free from large sharp stones, heavy clay, etc. If the material is unsuitable for bedding and surround to the pipe then imported material should be utilised and the surplus spoil removed from site

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3. The properties of PVC make it suitable for areas subject to minor ground movements due to seismic forces, mining subsidence, compaction of filled sites or the disturbance caused by the activities of other utilities in the vicinity. In larger sizes greater than DN 160 special attention to the flexibility of the joints is necessary

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4. Where PVC is to be used in environments with temperatures greater than 20°C for prolonged periods, the allowable operating pressure should be reduced in accordance with the manufacturers recommendations, to maintain the expected life of the pipe.

5. Corrosive ground (e.g. ground with low pH or high sulphate characteristics) has no known effect on PVC but all metal fittings, ancillary equipment, bolts etc should be carefully protected against corrosion in the normal way.

6. Contaminated ground, however, must be considered carefully. PVC is resistant to most chemicals, but is vulnerable to petroleum products and certain solvents. Where concentrations of such contaminants exist, PVC should NOT be used unless suitably protected. Where any doubt exists, soil sampling should be undertaken and

specialist advice sought.

7. Where the natural ground water table is high, or the construction trench is liable to flooding, special consideration should be given to the possibility of flotation of the pipe when empty. This particularly applies to the larger diameters where special

anchoring or weighting may be necessary prior to backfilling.

 

8.. Direct connection of PVC to sources of high frequency should be avoided and a flexible joint should be used to isolated such vibration.

Laying Considerations

 

1. Gradual changes of direction of PVC pipelines can be accommodated by pipe deflection but every effort should be made to keep the pipe as central as possible within the trench to enable correct side-fill compaction.

 

2. PVC should generally be installed in straight runs in order to reduce the stresses induced when the pipe is bent. It is possible in some circumstances however to bend the pipe in a radius no less than 200 times the pipe diameter. Elastomeric ring

joints will provide for some deflection of the pipe in the vicinity of 3° but it is unacceptable for the trailing joint to have an angular deflection greater

than 1°C.

 

3. During the pipe laying of continuous fusion joint systems, allowance should be made for the movement likely to occur due to the thermal expansion/contraction of the material. This effect is most pronounced at the end connections to fixed positions and at branch connections.

 

4. In cold conditions allowance should be made for expansion with push-fit joints to accommodate subsequent thermal expansion. Once a pipeline is installed the temperature variation is usually very small and is not likely to induce any significant stress or movement in the pipe system.

 

5. Whenever possible, a minimum distance of 300mm from obstructions should be maintained. This distance is often possible when laying parallel to other services but not always practicable when crossing other services. A separation distance of 75mm may be allowed for a square crossing but suitable protection should be provided

from possible joint loading, interference, damage or contamination.

 

6. PVC is not a conductor of electricity and no attempt should be made to use PVC pipework as a means of earthing electrical equipment. Similarly, because of its high electrical resistivity, caution is required in the use of the material where the avoidance of static electricity may be an important consideration.

 

 7. PVC is a poor conductor of heat but can burn when subjected to a naked flame. Upon removal of the source of ignition burning then ceases.

 

8. The installation of flanged fittings such as sluice valves, hydrant tees, end caps etc. usually requires the use of stub flanges complete with backing rings and gaskets. Care should be taken when tightening these flanges to provide even

and balanced torque. Provision should be made where heavy fittings are installed for concrete support both for the weight and to resist the turning moments associated with valves and hydrants.

 

9. Where large diameter fabricated fittings are installed on the main, similar concrete support may be necessary to counteract the imbalance of forces under working conditions.

 

10. PVC pipes and fittings may be partially or completely surrounded by concrete but the pipe should be protected by a heavy duty polyethylene membrane to avoid possible damage during pouring or compaction and to prevent high localised stresses.

 

11. After completion of an installation, pipe work and fittings should be inspected and made ready for testing to ensure the safety of the system. If the system is a large one it should be made ready to be tested in sections of convenient length.

 

12. The degree to which the trench is backfilled prior to testing will be influenced by:

• The prevailing site and/or traffic conditions.

• The potential risk for flotation.

• The unbalanced forces due to configuration and imposed test pressure.

Where practical it is advisable to consider leaving at least the mechanical joints exposed through-out the testing.

 

13. As part of the preparation for the hydrostatic pressure test, all anchorages and struts should be checked to ensure they are adequate to with-stand the excess pressure and it is advisable to re-tighten all bolted and flanged joints and to check that all intermediate control valves are open.

 

14. Complete and accurate records should be taken of the installation. It is useful for records to be taken before the pipes are buried whilst memories

are fresh and key elements are still visible.Photographic records of important or complex feature should be considered.

 

15. The marker tape, where used, should be laid along the line of the main and connected at each end to either a sluice valve or hydrant. The recommended position of the tape is 350mm below the surface directly above the crown of the pipe