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- Industry context
Last edited 12 Jun 2018
Pipelines are used to convey substances such as water, gas and sewage from one point to another.
The materials used for pipelines vary with the type and purpose of the pipeline. Water pipelines use the largest range of materials, including; ductile iron, steel, concrete, plastic, and so on. Steel and concrete are particularly suited to large diameter pipelines extending over great distances.
The choice of material depends on several factors, including:
- The design of the pipeline.
- Internal and external forces.
- Jointing and laying techniques.
- Frequency of maintenance.
Concrete cylinder pipes are typically made of welded sheet steel with jointing surfaces provided by sprigot and socket rings welded on to each end. The steel cylinder is centrifugally lined with concrete, after testing, followed by a winding of high tensile wire. A dense mortar coating covers the pre-stressing wires.
Pre-stressed non-cylinder concrete pipes are longitudinally and circumferentially pre-stressed. The core of the pipe is formed around longitudinal pre-stressing wires. This provides the stress, when released, to the bonded concrete. In order to withstand pressure and design loads, the core is then circumferentially pre-stressed, with wires covered in protective cement mortar.
Plastic pipes have the advantages of being lightweight, easy to handle, resistant to abrasion and chemicals, and with low frictional flow loss. The disadvantages of plastic pipes are that they have lower tensile strength than metal pipes, and have low resistance to temperature change.
Ductile pipes are normally jointed with push-in joints. Anchored joints are used primarily for gas pipelines. Steel pipelines may be jointed by welding or more conventional methods such as; flanged joints, screwed joints, and proprietary joints with rubber sealing rings. Welding is used where 100% line-tightness is required.
Plastic pipelines are solvent-welded and connected by coupler and rubber rings, or with spigot and Z-socket ends.
Concrete pipelines are joined by a flexible pipe joint comprising a gasket of rubber or synthetic material. Self-centred joints are normally filled with cement mortar or grouted using grout holes incorporated in the pipe collar.
Anchorages or ‘thrust-blocks’ should be provided for all pressure mains fittings at bends, tees and capped-ends, so as to be able to resist the thrust from the effects of internal pressure. Unless restraints are provided joint failure will occur, since no real resistance to ‘blow-out’ is provided by flexible joints.
Pipes are first laid out along the route, in a process also known as ‘stringing’. Slings of canvas or non-abrasive material are used to handle the pipes, which should not be dropped, dragged or rolled.
Special trenching machines or hydraulic backacters are used to carry out excavation for the trenches. Joints in the pipeline may be completed before the pipes are lifted into the trench, depending on length and site location. Alternatively, the pipes are jointed once they have been lowered into position.
The trench is typically at least 300 mm wider than the external diameter of the pipe, with additional space at each joint in order for them to be constructed in-place or inspected. Where trenches are excavated in hard or rocky ground, they should be taken deeper than the required depth and granular material used to bring them level.
Temporarily open pipes should be sealed with caps to prevent ingress of small animals and other objects.
Where pipelines are to be constructed underwater, they can be adopt one of three methods:
- Pulling method: Long lengths of pipe are jointed on the shore and the completed line pulled into water by winches carried on anchorage barges.
- Lay barge method: The pipeline is constructed and lowered to the river or sea bed from a barge. The barge is winched along the pipeline route leaving the completed line in position.
- Floating and sinking method: The pipeline is constructed on shore, fitted with buoyancy tanks and floated into position. Tanks are flooded or deflated as the pipeline is lowered into position by winches on pontoons. A weight coating is used to counter the natural buoyancy of the pipe.
 Coating and linings
Corrosive reactions are inevitable when iron or steel is exposed to moisture. The most cost-effective method of prevention is to coat the steel with an inert substance which effectively separates the steel surface from the corrosive environment and resists the passage of electricity.
 External protection
Bituminous coatings are often used on steel pipes and are applied by dipping in a bath of molten bitumen or painting with a bituminous solution. These are suitable for mildly corrosive conditions only, and for above ground where periodic inspection and maintenance is possible. However, they do not afford lasting protection to external surfaces of pipes laid underground.
Other types of external protection include:
- Bitumen enamel wrapping with a spiral wrap of reinforced glass tissue applied to the outer surface.
- Reinforced bitumen enamel wrapping – the outer wrap has additional reinforcement in the form of a woven glass cloth.
- Tar enamel wrapping – inner spiral wrapping of glass tissue and an outer wrap of coal-tar impregnated glass tissue.
- Plastic cladding (Securiclad) – a seamless plastic sheath of high-density polythene continuously applied.
 Internal protection
Bitumen lining provides heavy-duty protection. After treating the pipe with primer, bitumen is applied centrifugally, with water-spray cooling until the lining has set. Bitumen is suitable for the conveyance of raw and potable waters, seawater, sewage and highly-contaminated effluents. It also provides a very smooth surface with minimum flow resistance.
Plastic lining is produced from thermosetting epoxy-phenolic paint. It is suitable for a wide variety of highly-corrosive substances, including acidulated brines, seawaters, mine waters and acid solutions.
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