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Shunting and diversion measures for large external floating roof oil tanks

Shunting and diversion measures for large external floating roof oil tanks

Issue Time:2021-03-30
 Shunting and diversion measures for large external floating roof oil tanks

(1) Standard requirements

1. "9 Techniques for Preventing the Ignition of Static Electricity, Lightning and Stray Currents" API RP2003: 2008

The most effective way to prevent lightning and fire is to use tight seals and properly design shunts. The shunt is to place metal belts at an interval of no more than 3 meters (10 feet) around the tank top to bridge the floating roof and the tank body so as to conduct any lightning-related currents to the ground without igniting it. Sparks will be not generated in the vapor area.

When a weatherproof cover is provided above any type of primary seal, or when a wax scraper or secondary seal is provided, the area between the two seals can contain a flammable vapor-air mixture. In this case, the shunts should be installed to ensure that they directly contact the tank wall above the secondary seal. In any case, the design must ensure that the shunt is in good contact with the tank at the highest point of the tank top (for example, a gap of 51mm is set above).

The shunt can also be placed under the liquid, but due to the difficulty of inspection, this design is not recommended.

2. API545-2009 "Industrial Standard for Lightning Protection of Aboveground Flammable Liquid Storage Tanks"

4.2 External floating roof storage tank

4.2.1.1 Conductive shunt

The shunt is used to conduct the fast and short duration time component of the lightning current.

The shunts are arranged at intervals of no more than 3 meters (10 feet) along the perimeter of the floating roof.

4.2.1.2 Bypass conductor

The bypass conductor is used to conduct the fast and high-to-medium duration components of the lightning current

Direct electrical connection should be used to connect the floating roof of the tank to the tank shell through an appropriate number of bypass conductors.

The maximum end-to-end resistance of each conductor including the joint is 0.03 ohms. The bypass conductor should have the minimum length required for maximum movement of the floating roof. Bypass conductors are evenly arranged along the circumference of the tank at intervals of no more than 30 meters (100 meters), and at least two bypass conductors shall be provided.