Lightening Protection REFERENCE- OISD-GDN-180

NAGARAJ.H.A

Lightening-Definition

Lightning is a natural phenomenon considered as “Atmospheric Electricity” which develops as a result of natural build-up of electrical charge separation in the storm clouds. Lightning strikes cause enormous loss of life and property all over the world. Hydrocarbon Industry suffers crippling damage, disruption and loss because of this.

Protection to the plant structures, storage tanks and columns in the hydrocarbon industry against strokes of lightning has been a subject of concern and methods of providing lightning protection has been a subject of debate.

Lightening-Data

- Average Current 0.1 to 1 KA 

- Speed of Propagation 2 X 105m/Sec. 

-Length of Steps 30 to 60m apprx. 

- Potential difference between leader & earth.

> 107 to 109 volts  

Return Stroke Peak current of first return Stroke

30 KA  - Speed of Propagation 1/3 Speed of Light

DEFINITIONS

Side Flash: An electric spark, caused by differences of potential, occurring between conductive metal bodies or between such metal bodies and a component of the lightning protection system or ground.

Spark Gap: The term spark gap means any short air space between two conductors electrically insulated from or remotely electrically connected to each other.

Striking Distance: The distance over which the final breakdown of the initial stroke occurs. 

Surge Arrestor: A protective device for limiting surge voltages by discharging or bypassing surge current. It also prevents continued flow of follow current while remaining capable of repeating these functions. 

DEFINITIONS

Vapour Openings: Openings through a tank shell or roof above the surface of the stored hydrocarbon. Such openings may be provided for tank breathing, tank gauging, fire fighting, or other operating purpose.

Flame Protection: Self closing gauge hatches, vapour seals, pressure vacuum breather valves, flame arrestors or other reasonably effective means to minimise the possibility of flame entering the vapour space of a tank.

Lightening Terms

Direct Strikes: Direct-stroke of lightning can cause severe damage to objects that come in its path. The damage is largely caused by heat, mechanical forces and ignition of flammable materials. Typically a direct-stroke on a tree generates so much heat and mechanical force that it tears the limbs off the tree and scatters the bark over a wide area. Humans can suffer heart failure, brain damage, suspension of breathing or paralysis, burns etc.,

  Indirect Strikes: In addition to the direct-stroke, the abrupt change in

the electrical field, caused by a lightning stroke, can cause the lightning conductors to have potentials of mega volts with respect to the true earth, instantaneously. Any nearby metal work which is earthed offers a low impedance path to the stroke and the discharge can flash over to this nearby metal work, when the magnitude of the potential is adequate to breakdown the gap.

Lightening Effects & protection concepts EFFECTS OF LIGHTNING STROKE Electrical Side Flashing Thermal Mechanical LIGHTNING PROTECTION CONCEPTS

Lightning protection concepts are based on either:  Delaying the release of upward streamer for the area to be protected  

(or)  Release of upward streamer from a pre-defined point of the protected structure to divert upon itself the stroke channel and neutralize all the secondary effects.   The first approach is the preventive approach while the second is the remedial or control approach.

protection concepts

Remedial Approach : a) The remedial approach consists of providing safety against lightning strokes by employing means to capture the lightning stroke leader. Protection means in remedial approach are designed:  - To convey the lightning energy to earth via a defined route.  - To ensure low impedance connection to the earth mass. To eliminate the secondary effects. b) The basic components of a lightning protection system in the remedial approach are:   Air Terminal:- The air terminal captures the stepped leader of a

lightning stroke by launching an upward interception streamer.   Down Conductor:- The conductor which connect air termination to

earth termination is called down conductor.

Remedial Approach for Lightening

The remedial approach to lightning protection has four options:- Conventional air terminal (Franklin Rod)- Faraday Cage- Ionizing air terminal- Laser Beam The Franklin rod is a sharp pointed rod designed to intercept the leader of lightning

stroke and to transfer the electric charge to the earth. The Faraday cage consists of metallic material completely surrounding the protected

structure and resulting in its electrostatic shielding. For lightning protection purpose, conductors are spaced in a criss-crossed fashion across the roof structure and sides.

. The laser beam would produce multi-photon ionization. The laser beam could thus intercept a leader as it developed towards the earth, and act as a conductor from the cloud to the ground and then be terminated to a down conductor and the earth mass.

Indian Standard- IS:2309

Indian Standard IS:2309 recommends following factors as a basis of calculating the overall risk factor for assessing the requirement of lightning protection.

  a. Type of Construction. b. Contents of the building and consequential effects due to lightning stroke. c. Degree of isolation. d. Type of terrain where the structure is located. e. Height of the Structure. f. Lightning prevalence. g. Usage of the building.

Depending upon the characteristic features of above factors, overall risk factor shall be calculated as per procedure given in IS-2309.  If the value of risk factor is less than 10--5 (One in Hundred Thousand), then in the absence of any other overriding consideration, protection may not be provided.  If the risk factor is greater than 10--5 (One in Hundred Thousand), then sound reasons shall be necessary to support the decision of not providing the lightning protection.

PROTECTION OF STORAGE TANKS

 Following fundamental principles of lightning protection of the structures and their contents shall be adhered to:-

 a) Flammable liquids shall be stored in essentially gastight structures.  b) Openings where flammable concentrations of vapour or gas can escape to the atmosphere shall be closed or

otherwise protected against the entrance of flame.  Structures and all accessories e.g. dip-gauge hatches, vent valves shall be maintained in good and sound

operating conditions.  d) Flammable air-vapour mixtures shall be prevented to the greatest possible extent from accumulating

outside such structures.   e) Potential spark-gaps between metallic conductors shall be avoided at points where flammable vapours

may escape or accumulate.  Sheet steel of thickness less than 4.8 mm shall not be used as a material of construction for the tanks and

vessels.  A properly designed / constructed gas tight storage tanks considered to be self-protected against lightning,

provided it is properly earthed and bonded. Such a structure may not require any additional means of lightning protection.

PROTECTION OF STORAGE TANKS

Fixed roof type tanks / Floating roof type tanks   Protection against lightning for these two types of storage tanks

are illustrated below:   A properly bonded and earthed metallic storage tank of

adequate shell/roof thickness is self protected against lightning, provided these are tightly sealed to prevent the escape of liquid, vapour, or gas.

It is to be noted that, pressure vessels such as spheres and bullets are gas tight by the design of its construction. Hence, properly earthed spheres and bullets are self protected and do not need additional measures against lightning protection.

PROTECTION CONCEPTS OF STORAGE TANKS AND VESSELS

  Where the probability of flammable atmosphere around the rim of a floating roof tank and around the vent of a fixed roof tank is relatively high as compared to other locations around the tankages, augmentation of lightning protection system may be considered. It may however be borne in mind that the only effective defence against ignition by a direct strike is a tight seal.

  Lightning conductor as part of lightning protection system is incapable of discharging a thunder cloud without a lightning flash. It is thus obvious that in case of a lightning strike, the source of ignition is present due to temperatures of around 30,000o K in the discharge channel of a return stroke or the secondary arc in case of a indirect strike

  The underlying principle for augmenting lightning protection to hydrocarbon storage tanks, is therefore based on shifting the point of strike of lightning stroke to a safe area above the vapour space (or above the classified zones for hazardous atmosphere) so that ignition of vapours probably present above the roof does not take place.

Floating Roof Tanks:

In addition to the concepts followed in case of fixed roof type tanks, following measures are essential in case of floating roof type tanks used for storage of flammable liquids:

Metallic straps (Shunts) at intervals of not over 3 meter length on the circumference of the roof between the floating roof and the metallic shoe that slides on the inside of the shell will permit the charge to drain off without igniting vapour under the seal. Shunt of flexible Type 302, 28 gauge (0.4 mm X 51 mm) wide stainless steel straps or the equivalent current carrying capacity and corrosion resistance are used. (Ref. Fig.10)

Tanks without a vapour space at the seal or with non-conductive seals do not generally require shunts at the seal. However, if shunts are not provided a tight seal must be maintained to prevent accumulation of vapours. Where metallic weather shields cover the seals they shall maintain contact with the shell.

 Where a floating roof tank is equipped with both primary and secondary seals in compliance with environmental regulations, shunts shall be installed so that they directly contact the tank shell above the secondary seal as in case of primary seals explained in “a” above.

All conductive parts of internal floating covers or the floating portions of covered floating roof tanks, ladders should be electrically interconnected and bonded to the tank roof/shell.

Lightening Protection

Air Terminals installed on Tank Shell:A conventional 6 m high air terminal, spaced about 20 m around the periphery protects an area upto 15 to 18 m from the tank shell. This implies that lightning protection using air terminals installed on the tank walls can completely protect a storage tank upto 30 M dia whereas the center portion of the roof remains unprotected for tank diameters more than 30m Lightning Masts around Storage TanksLightning mast located more than 30m away from the tank irrespective of the height of the mast does not provide any improvement to the self protected storage tank. Lightning mast should be located close to the tank and the minimum distance is dictated by the side flash distance and the practical considerations for mast installation.Lightning mast is required to be located at around 5 to 6 m from the tank shell.

Lightening Protection

Use of Overhead Shield WireA system of overhead earth wires can be designed to provide complete protection to a storage tank within the realm of rolling sphere concept based on a striking distance of 30 m. A single earth wire with a minimum clearance of about 8 m above the highest point of the tank can protect a tank of about 6 to 8 m diameter. For tank diameters between 8 to 30 m two parallel earth wires shall be required while for tank diameters between 30 to 80 m three parallel overhead earth wires shall be needed as a minimum.

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