When pipelines are laid, the pipes are positioned end to end, and welded in place – often using specialised flanges. Wherever there is a join in a pipe, the risk of corrosion is introduced. This can be lessened by the use of specialised alloys, and bending pipes rather than using angled pipe fittings.
New alloys are constantly being sought which are stronger and more corrosion resistant than those already on the market. However, there is often a payoff. Some alloys are immensely stress resistant, but don’t bend easily, for example. Others are highly malleable, but corrode.
Pipe bending is the most efficient way of switching direction in a pipeline without the use of pipe fittings. Specialised bending machines ensure the pipe retains its strength and contours. However, the lengths of pipe still have to be welded together. To reduce corrosion, low carbon steels are used. However, these have low ‘weldability.’ The best alloys are Duplex and Super Duplex stainless steels, which are corrosion resistant and weld well at low temperatures. At high temperatures there is a risk of crystallisation, which destroys the anti-corrosion effect. To avoid this, pipe flanges are often used across the joint.
Flanges are extensively used in undersea pipelines, where pressure stresses and corrosion are major problems. The pipes can be manufactured in prefabricated lengths on shore, using pipe flanges, before installing underwater. This keeps joint welding to a minimum.
We at Chemipetro supply flanges in Duplex and Super Duplex for a wide range of uses, including undersea gas and oil pipelines.