Presented by the Steel Tank Institute

• Do shop fabricated tanks and field erected tanks have inherent differences due to their size?

• Are lightning strikes on shop built tanks frequent and usual?

• Are lightning strikes on field-erected tanks frequent and usual?

• What does NFPA 780, "Standard for the Installation of Lightning Protection Systems" present?

• Do horizontal cylindrical shop-built tanks have "fixed roofs?"  

• What does NFPA 780 say about self-protecting structures used for the storage of liquids that can produce flammable vapors or used to store flammable gases?

• What does NFPA 780 say about Fixed Roof Tanks and self-protection?

• Does UL 142 have provision for lightning protection?

• Does API 2003, "Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents" suggest that supplemental grounding is necessary for all metallic tanks, equipment and structures?

• What about insulating materials for lightning protection?

• What role does maintenance play in protection from lightning strikes? 

Q: Do shop fabricated tanks and field erected tanks have inherent differences due to their size?
Yes, shop fabricated tanks and field erected tanks have inherent differences due to their size. Most shop fabricated aboveground storage tanks are horizontal and cylindrical in geometry, and less than 3 meters from grade to top of tank.  Typically, vents terminate approximately 4 meters above grade for both aboveground and underground shop built tank installations. Field-erected tanks can be 15 meters or higher above grade.

Q: Are lightning strikes on shop built tanks frequent and usual?
No, lightning strikes on shop built tanks are extremely infrequent and unusual.

Q:Are lightning strikes on field-erected tanks frequent and usual?
Yes, lightning strikes on field-erected tanks are a common occurrence. API found over 70% of the fires at terminal facilities, where field erected tanks were located, were caused by lightning strikes.

Q: What does NFPA 780, "Standard for the Installation of Lightning Protection Systems" present?
NFPA 780, "Standard for the Installation of Lightning Protection Systems" has language for lightning strike protection that is geared towards field-erected tanks for fixed roof tanks.

Q: Do horizontal cylindrical shop-built tanks have "fixed roofs?"  
Horizontal cylindrical shop-built tanks do not have "fixed roofs", so a requirement for the metal thickness of such a tank to exceed 3/16" is not relevant. 

Q: What does NFPA 780 say about self-protecting structures used for the storage of liquids that can produce flammable vapors or used to store flammable gases?
Certain types of structures used for the storage of liquids that can produce flammable vapors or used to store flammable gases are essentially self-protecting against damage from lightning strokes and shall need no additional protection. Metallic structures that are electrically continuous, tightly sealed to prevent the escape of liquids, vapors, or gases, and of adequate thickness to withstand direct strokes in accordance with 6-3.2 shall be considered to be inherently self-protecting.

Q: What does NFPA 780 say about Fixed Roof Tanks and self-protection?
Metallic tanks with steel roofs of riveted, bolted, or welded construction, with or without supporting members that are used for the storage of liquids that give off flammable vapors at atmospheric pressure shall be considered to be protected against lightning (inherently self-protecting) if the following requirements are met:

1. All joints between metallic plates shall be riveted, bolted, or welded.

2. All pipes entering the tank shall be metallically connected to the tank at the point of entrance.

3. All vapor or gas openings shall be closed or provided with flame protection in locations where the stored stock might produce a flammable air-vapor mixture under storage conditions.

4. The roof shall have a minimum thickness of 3/16 in. (4.8 mm).

Q: Does UL 142 have provision for lightning protection?
No, the most common construction standard for aboveground tanks, UL 142, does not have provision for lightning protection. It has been used since 1926.

Q: Does API 2003, "Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents" suggest that supplemental grounding is necessary for all metallic tanks, equipment and structures?
API 2003 establishes that metallic tanks, equipment, and structures commonly found in the petroleum industry that are in direct contact with the ground have proved to be sufficiently well grounded to provide for safe propagation to ground of lightning strokes.  Supplemental grounding by means of driven ground rods neither decreases or increases the probability of being struck, nor does it reduce the possibility of ignition of the contents.  Supplemental grounding is necessary, however, where direct grounding is not provided.

Metallic equipment that does not rest directly on the ground but is connected to a grounded piping system is usually safe for propagation to ground of lightning strokes. Metallic tanks, equipment, and structures that are insulated from ground should be adequately grounded and bonded. 

Q: What about insulating materials for lightning protection?
Structures made of insulating materials such as wood, plastic, brick, tile, or non-reinforced concrete are typically not inherently grounded for lightning protection. 

Q: What role does maintenance play in protection from lightning strikes?  
Metal tanks with fixed metal roofs and horizontal metal tanks maintained in good condition are generally protected from damage by direct-stroke lightning and ignition of its contents, if all metal components are in electrical contact.  Most tank explosions have occurred from openings in the roof, from vents without protection from flashback devices such as pressure vacuum vent valves, and from tank roofs with corrosion holes.

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