Isolating spark gap forpipeline flanges

Equipotential bonding system with lightning protection

Isolating spark gap forpipeline flanges

Risk minimisation plays an important role when planning and operating oil and gas transport systems. Preventing property and environmental damage as well as dangerous situations of any type is the name of the game. One important element in all this is the protection of system components against the influence of transient power surges.

Pipelines are used with increasing frequency to transport oil and gas between production sites, refineries and points of use. Yet the necessary pipeline infrastructure only remains cost-effective if maintenance and repair costs are kept low over many years of operation. In the past, corrosion, in particular, has caused serious problems in metal pipelines. The issue of corrosion protection plays a central role in the planning of new pipelines, though new approaches are likewise being used to strengthen existing systems.

Basic protection against rust generally involves a protective plastic coating. However, there is also a danger of electrochemical corrosion, especially in underground pipeline sections, and even the smallest damaged section or spot can quickly grow in size. The effect can be exacerbated when additional voltage influences from power grids are present. The alternating currents in the damaged areas can accelerate the spread of the corrosion.

Corrosion protection is a sensible method of keeping the expansion of these areas in check. Active cathodic corrosion protection (CCP) systems counteract the harmful ion stream with constant direct current. To accomplish this, the negative pole of the CCP rectifier is put in direct contact with the metal in the pipeline. The positive pole is inserted into the earth as an anode and a DC circuit is completed across the damaged area and the earth.

Since active CCP systems only protect a limited section of the pipeline, the latter is segmented. Sections of a defined length must be electrically isolated. Isolating sets that cover both the pipe elements at the flange joints and the joints themselves are available on the market. These ensure that the CCP systems function correctly on the pipe sections in question and that different sections do not have an impact on each other.

As the pipe sections are laid isolated from the earth potential, special requirements are placed on the lightning protection equipotential bonding system. Earthing the metallic pipeline sections directly would lead to substantial leakage current in the CCP, meaning that the sections must be earthed indirectly.

To this end, isolating spark gaps are used. During normal operation, these high-impedance components act as open switches and ensure electrical isolation between pipe sections and earth potential. If there is a voltage increase, the isolating spark gaps change to very low impedance – and at a defined response voltage, the ‘switch’ is closed.

This sort of voltage increase occurs during direct lightning strikes. However, electromagnetically coupled voltages during lightning strikes in the near vicinity or switching operations and short circuits in parallel high-voltage or railroad lines are also effectively limited. During a direct lightning strike, high currents – up to 200 kA – must be discharged to earth potential. The lightning current is expected to split and go in two different directions, which means that the current path must be able to carry a maximum of 100 kA.

As soon as the isolating spark gap responds and diverts, the lightning current can be discharged to earth via this defined path. This process takes only a few microseconds. After the discharge process, the isolating spark gap returns to its high-impedance state. Isolating spark gap operation is completely maintenance-free. The products are designed to discharge a large number of pulses. However, they should be checked at regular intervals to ensure that they are in their initial high-impedance state.

A special surge protection requirement arises from the limited insulation resistance of the coating. In addition, isolating sets for the flanges have relatively low dielectric strengths, usually in the low kilovolt range. In the GW 24 technical rule issued by the German Technical and Scientific Association for Gas and Water (DVGW), the relationship between flange insulation resistance and isolating spark gap response voltage is described as follows: Class 1 insulating flanges have an AC test voltage of 5 kV while Class 2 insulating flanges have 2.5 kV. It is recommended that the isolating spark gap’s impulse response voltage Uas be selected so that the AC test voltage of the object to be protected has an ‘operational reliability factor’ of 2.

This means that isolating spark gaps with a Uas of 1.25 kV fulfil the requirements of all insulating flange classes. In the European area, the DVGW methods can be found in a recommendation by CeoCor (European Committee for the study of corrosion and protection of pipes and pipeline systems), which has come to be used around the world.

In addition to the response range, the isolating spark gaps have a defined lower exclusion range. Earth currents or nearby high-voltage lines, for instance, can induce a constant 50/60 Hz AC current in the pipeline sections. To keep the isolating spark gap from becoming conductive at each voltage peak and thereby influencing the CCP system, an AC power frequency withstand voltage that is to be maintained is defined. The DVGW addresses this in its GW 24 technical rule with the following recommendation: below 250 V AC and 50/60 Hz, the isolating spark gap must remain non-conducting. This provides a precisely defined, binding framework derived from practical applications for the required response and isolation voltage range.

Isolating spark gaps and accessory materials are usually installed in Ex Zone 1. In the immediate vicinity of gas and oil pipelines, explosive atmosphere must be expected at times. That is why the spark gap was designed with the ‘flameproof’ protection type. The materials and connection technology were developed so that even with the maximum lightning current and explosive ambient atmosphere, no igniting spark occurs that could trigger an explosion. In addition, all materials were designed for long-term operation even in harsh conditions.

The most important facts and figures on the FLT isolating spark gap at a glance:

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Thorsten Heil

Product Marketing Surge Voltage Protection Trabtech,Phoenix Contact