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).
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.
