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Electric Insulation of Casings
Crossings of buried pipelines under roads and railways, are executed with or without casings. Pipelines can be laid in appropriate types of soil with the help of new and improved pipe pushing techniques without damage to the coating and without affecting the active and passive corrosion protection system.
Casings may effect the cathodic protection of pipelines.
Casings may be necessary for technical reasons. Cathodic corrosion protection for buried steel pipelines and for steel pipes for high voltage cables has proved its worth for a number of years., However, casings may alter its effectiveness. With the metal casings that were formerly used, which have either not been coated or had only a thin coating or bitumen, cathodic protection of the pipeline with an electrically conducting contact between the two pipes was not possible. When using well-coated casing, the pipeline is not cathodically protected within the casing.
The annular space can be filled. As this technique is increasingly being used, it was felt that it was sensible to give some instructions for the filling of the annular space. The annular space is filled to provide corrosion protection for the pipeline and in the inner space of the casing. In addition, durable protection against lifting of the pipeline in the casing shall be achieved.
Risk of Corrosion in Casings
The risk and problems that can arise with casings have been summarised in a study by G. Gamitz. Depending on the type of construction, the casing material and the type of casing used. Plus the type of filling used for the annular space, problems can occur which may jeopardize the pipeline.
- Insufficient cathodic protection
- atmospheric corrosion of the pipeline in the air-filled annular space
- corrosion due to presence of water in the annular space
- AC corrosion on the pipeline in the annular space filled with sand or cement bonded mortar.
Risk Assesment
When there is a metallic contact between the casing and the pipeline, the pipeline is not cathodically protected within the casing.
By potential, resistance, short circuit and current supply measurements this can be checked.
