Design of Protection for Oil Storage Tank Against Lightning Strike

Introduction:

Lightning is one of the most destructive phenomena of nature. Due to the storage of explosive and hazardous materials in the Oil & Gas Industry, not only the direct lightning strike but also the increase in temperature on the components due to the flow of lightning impulse current will lead to a huge disaster. 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 and the same is explained in 1.15. 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 storage tank made of steel having a wall thickness greater than 4.8mm then the tank can be considered to be self-protected from lightning strikes but the proper bonding to the earth should be ensured.

The 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 around the perimeter of the surface wall of the storage tank. The protection zone provided by the air terminals depends on

• diameter of tank,

• height of air terminals and

• distance between the air terminal

  OSID GDN 180 has given the calculation for finding the number of air terminals needed for protecting a storage tank of diameter 'D' when the air terminals are placed 20m apart along the perimeter of the tank.

  No of air terminal = (π* D)/20

  D- Diameter of storage tank

  Example:

  Diameter of tank: 30 m

  No of air terminals = (π*30)/20

  No of air terminals = 5

When the air terminals of height 6m are placed at 20m apart, they will protect area up to 18m from the periphery of the tank and the center portion of the tank will remain unprotected.

Use of lightning mast around the tanks:

The height of the mast should be greater than height of storage tank. The following basic needs to be considered for the protection of storage tank by lightning mast:

1. Mast shall be placed 5 to 6m from the tank to reduce the effect of side flashing.
2. The Lightning mast should not be placed 30m away from the 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:

The air terminal in the form of overhead lines can be used to provide complete protection to the storage tanks. The position and the number of overhead lines can be calculated by using Rolling sphere method. To avoid side flash the minimum distance of 6m between supporting mast holding overhead line and tank shall be maintained. For the overhead protective lines, we have to maintain a clearance of about 8m from the top most point of the storage tank.

The number of overhead lines needed for protecting the storage tanks of different sizes as specified by OISD GDN 180 are as follows.

The earthing of the supporting mast shall be bonded with tank's earthing system.