Lightning is one of the most destructive phenomenon of nature. It causing maximum damage to living beings and to the equipment across the globe. Due to the storage of explosive and hazardous materials in the Oil & Gas Industry, a direct lightning strike or secondary surges could lead to major disaster leading to loss of lives, resources and equipments since the average temperature of it can be around 20,000 degree Celsius. In this article we have explained about design of protection for oil storage tank against lightning strikes given in OSID_GDN 180 and IS/IEC 62305-2 .
The calculation of design of lightning protection can be done by Rolling sphere method.
IS/IEC 62305-2 has mentioned at least level 2 protection is necessary for the structure with risk of explosion.
Lightning protection in oil tanks:
A properly bonded and earthed metallic tank is self protected When the steel tank shell or Roof thickness are of minimum 4.8mm, separate air terminal and down conductor not needed. A separate lightning protection is needed when the steel tank shell or roof are of less than 4.8mm
Lightning protection for storage tanks can be done by three methods
• By using air terminal
• By using Lightning mast
• By using overhead line
Use of Air terminals:
In this method air terminals will be placed on the surface wall of the storage tank. The protection zone provided by the air terminals depends on the diameter of tank, height of air terminals and Distance between the air terminal around the perimeter of the wall. The use of air terminals can be applied to floating roof tanks for reducing the probability of rim fires due to lightning strikes since the space around the rim has relatively higher possibility of flammable atmosphere being present due to leakage from improper sealing.
OSID_GDN 180 has given the calculation of number of air terminal
When the air terminals of height 6m and the distance between the air terminals is 20m will protect area up to 18m from the periphery of the tank with diameter of 30.
Use of lightning mast around the tanks:
The following basic needs to be considered for the protection of storage tank by lightning mast:
1. The Lightning mast should not be placed 30m away from the tank
2. Mast shall be placed 5 to 6m from the tank to reduce the effect of side flashing.
The protection zone given by lightning mast depends on the height of the mast , diameter of tank.
The height of the mast should be greater than height of storage tank.
The number of lightning mast calculation is same as the calculation of air terminal placed on the wall. Earthing system of the lightning mast will be bonded with the earthing system of tank.
Lightning masts are much more expensive as compared to the air terminals on the shell.
Use of overhead line:
To provide complete protection the system of overhead lines can be calculated by using Rolling sphere method. To protect a storage tank with diameter of 6 to 8m can be protected by the single overhead earth wire with with a minimum clearance of about 8 m above the highest point of the tank. For the protection of 8 to 30 diameter tank shall need two parallel overhead earth wire, while 30 to 80 m diameter tank shall need minimum three overhead earth wire. To avoid side flash the minimum distance between supporting mast holding overhead line and tank shall be 6m. For earthing the supporting mast will be bonded with tank’s earthing system.