September 24, 2017

Protecting pipelines from overvoltage interferences

Voltage-controlled short-circuiting device

Pipelines operate under high pressure, meaning that the material used (such as steel) can rapidly reach its maximum lifespan. In extreme cases, heavy corrosion can damage the pipeline and result in leakage that may impact both on the material used and the environment. Thus it is clear that pipelines must be continuously monitored for any kind of deterioration, particularly corrosion, during operation.

Although the coating and painting of a pipeline may help protect against corrosion (also called passive corrosion protection), this is often not sufficient, as small defects within the coating can quickly cause local corrosion effects (electrochemical corrosion). In fact, alternating current (AC) corrosion can occur as a result of AC voltage interference, meaning that active cathodic corrosion protection must be combined with passive corrosion protection.

Cathodic corrosion protection requires that the pipeline be continuously and effectively monitored, so that the voltage level along the pipeline is detected at the relevant test joints and adapted to the intensity of the protective current if needed. This constant measurement of pressure means that sudden pressure drops (or leakage) can quickly be identified.

It is also essential to frequently conduct quantity measurements to compare the quantity at the beginning of the pipeline with that at the end of the pipeline, triggering an alarm should a difference be spotted.

The extended, exposed surface of pipelines can increase the risk of lightning effects and overvoltage interference. Since the pipeline is galvanically connected to the cathodic protection rectifier, lightning and surge protection measures are required to discharge all overvoltages, with the aim of preventing fire or failure of the rectifier.

The widely distributed and highly networked nature of pipelines means that they are often influenced by interference voltages from a variety of sources. These could include:

  • Stray currents caused by electric railways;
  • Electromagnetic fields caused by high-voltage lines;
  • Earth faults and short-circuits caused by high-voltage lines; and
  • Lightning interferences caused by thunderstorms.

Interference voltage denotes non-system voltages that can also occur in the form of transient, temporary or long-duration overvoltage. They can enter a system, for example an insulated pipeline, via galvanic, inductive or capacitive coupling, and are a frequent source of interference or damage to installations and people.

Special surge protection solutions allow for the reduction of these interference voltages down to below defined limit values. DEHN’s long-standing experience in lightning and surge protection for pipelines and its tested products can help limit lightning damage to insulating flanges, cathodic protection and field devices, minimising downtime and the resulting loss of transport capacity and production.

DEHN’s intelligent direct current (DC) decoupling device, VCSD 40 IP65, limits long-duration, temporary and transient overvoltage. Long-duration AC voltages are limited to a pre-set value by the DC decoupling device (voltage-controlled short-circuiting device) without influencing the required DC potential.

Such overvoltage of a given duration or voltage level activates the correct limiting of the VCSD, which is assigned to the relevant overvoltage, and can discharge it to the earth without negatively affecting the cathodic protection potential on the pipeline (d.c. potential).

The effects of dangerously high overvoltage can be reduced to a safe level in the immediate vicinity of the VCSD and the following protection goals can be achieved:

  • Personal protection in the case of temporary and long-duration overvoltage;
  • Protection against AC corrosion; and
  • Protection of devices and components.

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