API RP 545-2009 pdf download.Recommended Practice for Lightning Protection of Aboveground Storage Tanks for Flammable or Combustible Liquids.
3 Terms and Definitions For the purposes of this document, the following definitions apply. 3.1 action integral The joule or ohmic heating energy dissipated per unit resistance at the lightning attachment point. The action integral is measured in A 2 s (amperes-squared seconds), which is the same as JΩ –1 (joules per ohm) (Rakov and Uman, Lightning: Physics and Effects, p. 277). 3.2 bonding An electrical connection between two electrically conductive objects that is intended to significantly reduce potential differences. 3.3 bypass conductor A conductive cable that provides a direct electrical connection between the tank shell and the tank floating roof. 3.4 external floating roof tank EFRT An aboveground tank with a floating roof, which has no fixed roof and has an open top. 3.5 flash A complete discharge of the cell between the thundercloud and ground (as it applies to cloud-to-ground lightning) (Uman, The Lightning Discharge, p. 10). 3.6 grounded (grounding) Connected (connecting) to ground or to a conductive body that extends the ground connection (NFPA 780). 3.7 internal floating-roof tank IFRT An aboveground fixed roof tank with a floating roof inside the tank. 3.8 release prevention barrier A release prevention barrier includes steel bottoms, synthetic materials, clay liners, and all other barriers or combination of barriers placed in the bottom of or under an aboveground storage tank, which have the following functions: a) preventing the escape of contaminated material, and b) containing or channeling released material for leak detection.
3.9 shunt A short conductor that is electrically connected to the tank floating roof and contacts the tank shell. 3.10 striking distance The distance over which the final breakdown of the initial lightning stroke occurs. 3.11 stroke One current component of a lightning flash. The number of strokes per flash is typically three to four, but may be as low as one or as high as 30 (Uman, All About Lightning, p. 41). 4 Protection of Specific Types of Tanks 4.1 Fixed-roof Tanks (Metallic) and Tanks with Internal Floating Roofs For fixed roof tanks (metallic cone or dome) and internal floating-roof tanks (IFRTs), there is a possibility of flammable vapors being present at atmospheric vents. If present, flammable vapors can be ignited by a lightning flash. Shunts or bypass conductors are not required for lightning protection. Bonding techniques to prevent static discharge between the floating roof and shell are addressed in API 650, Appendix H. Tanks handling low vapor pressures or in-service with properly maintained floating roofs with tight-fitting seals are not likely to have flammable vapors at atmospheric vents unless it is being refilled from empty. In these cases, no further lightning protection is required (see Annex B). 4.2 External Floating Roof Tanks 4.2.1 Bonding Between Floating Roof and Shell 220.127.116.11 Shunts for Conduction 18.104.22.168.1 General Shunts are used for conduction of fast and intermediate duration components of lightning-stroke current. 22.214.171.124.2 Number and Placement The shunt to shell contact point shall be submerged at least 0.3 m (1 ft) below the surface of the liquid product. The shunt shall have as short and direct a path as possible from the conductive floating roof to the tank shell. The shunts shall be spaced at intervals no greater than 3 m (10 ft) around the perimeter of the floating roof. When retrofitting existing tanks with submerged shunts, the abovedeck shunts shall be removed.
126.96.36.199.4 Durability The shunts and termination connections shall be of sufficient flexibility, cross-sectional area, and corrosion resistance to have a minimum service life of 30 years. 188.8.131.52 Bypass Conductors 184.108.40.206.1 General Bypass conductors are used for conduction of the intermediate and long duration component of lightning-stroke current. 220.127.116.11.2 Number, Length and Electrical Resistance The tank floating roof shall be bonded to the tank shell by direct electrical connection through an appropriate number of bypass conductors. Each conductor, including connections, shall have a maximum end-to-end electrical resistance of 0.03 Ω. The bypass conductors shall be of the minimum length necessary to permit full movement of the floating roof. Bypass conductors should be evenly spaced not more than every 30 m (100 ft) around the tank circumference with a minimum of two.