OISD - GDN - 180 JULY, 1999

LIGHTNING PROTECTION

Prepared by:COMMITTEE ON LIGHTNING PROTECTION

OIL INDUSTRY SAFETY DIRECTORATE2nd Floor, “Kailash”,

26, Kasturba Gandhi Marg,NEW DELHI -110 001.

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NOTE

OISD (Oil Industry Safety Directorate) publications are prepared for use in the Oil and Gas Industry under Ministry of Petroleum & natural Gas. These are the property of Ministry of Petroleum & Natural Gas and shall not be reproduced or copied and loaned or exhibited to others without written consent from OISD.

Though every effort has been made to assure the accuracy and reliability of the data contained in these documents. OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting from their use.

These documents are intended to supplement rather than replace the prevailing statutory requirements.

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FOREWORD

The Oil Industry in India is nearly 100 years old. Due to various collaboration agreements a variety of international codes, standards and practices are in vogue. Standardisation in design philosophies, operating and maintenance practices at a national level was hardly in existence. This lack of uniformity coupled with feedback from some serious accidents that occurred in the recent past in India and abroad, emphasised the need for the industry to review the existing state of art in designing, operating and maintaining oil and gas installations.

With this in view, the Ministry of Petroleum and Natural Gas in 1986 constituted a Safety Council assisted by the Oil Industry Safety Directorate (OISD) staffed from within the industry in formulating and implementing a series of self-regulatory measures aimed at removing obsolescence, standardising and upgrading the existing standards to ensure safer operations. Accordingly OISD constituted a number of functional committees comprising of experts nominated from the industry to draw up standards and guidelines on various subjects.

The present document on “Lightning Protection” was prepared by the Functional Committee on “Lightning Protection”. This document is based on the accumulated knowledge and experience of industry members and the various national and international codes and practices.

It is hoped that provisions of this document if implemented objectively, may go a long way to improve the safety to reduce accidents in Oil and Gas Industry. Users are cautioned that no document can be substitute for the judgment of responsible and experienced engineer.

Suggestions are invited from the users after it is put into practice to improve the document further. Suggestions for amendments, if any, to this standard should be addressed to:

The Co-ordinatorCommittee on “Lightning Protection”

Oil Industry Safety Directorate2nd Floor, “Kailash”,

26, Kasturba Gandhi Marg,NEW DELHI - 110 001.

This document in no way supersedes the statutory regulations of Chief Controller of Explosives (CCE), Factory Inspectorate or any other statutory body, which must be followed as applicable.

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COMMITTEEON

“LIGHTNING PROTECTION”

LIST OF MEMBERS---------------------------------------------------------------------------------------------------------------------------Sl.No. Name Organisation Position in

the Committee---------------------------------------------------------------------------------------------------------------------------

S/Shri

01. V.P. Sharma Engineers India Limited Leader

02. A.K. Roy Indian Oil Corporation Ltd., Member(Refineries Division)

03. S.C. Tyagi Oil & Natural Gas Corporation Ltd., Member

04. H. Prusty Gas Authority of India Ltd., Member

05. P.Kamalasekharan Indian Oil Corporation Ltd., Member(Marketing Division)

06. R. Raghupathy Engineers India Limited Member

07. P.N. Deka Bongaigaon Refinery & Petrochemicals Limited Member

08. J.K. Jha Oil Industry Safety Directorate Member Co-ordinator.

In addition to the above, various other experts in the industry contributed in the preparation, review, and finalisation of this document.

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LIGHTNING PROTECTION

CONTENTS---------------------------------------------------------------------------------------------------------------------------SECTION DESCRIPTION PAGE NO.--------------------------------------------------------------------------------------------------------------------------- 1.0 INTRODUCTION 1

2.0 SCOPE 1

3.0 DEFINITIONS 1

4.0 LIGHTNING PHENOMENON 2

5.0 LIGHTNING PROTECTION CONCEPTS 4

6.0 LIGHTNING PROTECTION SYSTEM 6

7.0 CONTROL OF FUGITIVE EMISSIONS 8

8.0 PLANT BUILDING AND STRUCTURES 12

9.0 PROTECTION OF STORAGE TANKS 16

10.0 REFERENCES 20

11.0 APPENDIX 22

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1.0 INTRODUCTION

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.

2.0 SCOPE

This document explains the lightning discharge phenomena, various lightning concepts and control of fugitive emissions and deals with lightning protection for plant building and structures and Storage associated with an Oil/Gas nstallation as per prevalent national and international standards on the subject and analyses their stipulations and provides minimum requirements to be followed in the Oil Industry. This standard, however, does not include guidelines for protection of electric equipment within or on structures against lightning.

3.0 DEFINITIONS

For the purpose of this standard, the following definitions shall apply.

I) Air Terminal

An air terminal is that component of a lightning protection system that is intended to intercept lightning flashes.

ii) Bonding

An electrical connection between an electrically conductive object and a component of a lightning protection system that is intended to significantly reduce potential differences created by lightning current.

iii) Down Conductors

The conductors which connects air terminals to earth terminations are

called Down Conductors

iv) Earth Terminations

Earthing conductors embedded in the soil and designed for the Safe discharge of lightning currents into the soil are called Earth Terminations

v) 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.

vi) Grounded Terminal:

The portion of a lightning protection system such as ground rod, ground plate, or ground conductor that is installed for the purpose of providing electrical contact with the earth.

vii) Grounded:

A structure is supposed to be adequately grounded if it is connected to earth or to some conducting body that is connected to earth.

viii) Hazardous Area

In accordance with the Petroleum Rules, an area shall be deemed to be a hazardous area, where:

a) Petroleum having flash point below 65o

C or any flammable gas or vapour in a concentration capable of ignition is likely to be present.

b) Petroleum or any flammable liquid having flash point above 65o C is likely to be refined, blended, handled or stored at or above its flash point .

For details, refer OISD Standard 113 on Electrical Area Classification.

ix) Shall

“Shall” indicates a mandatory requirement.

x) Should

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“Should” indicates a requirement which is recommendatory in nature.

xi) 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.

xii) Spark Gap:

As used in this code, the term spark gap means any short air space between two conductors electrically insulated from or remotely electrically connected to each other.

xiii) Striking Distance:

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

xiv) 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.

xv) 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.

xvi) Zone of Protection:

Zone of Protection is the space around a lightning conductor in which the probability of lightning stroke is small.

4.0 LIGHTNING PHENOMENON

Lightning is a natural phenomenon which is said to be formed as a result of a natural build up of electrical charge separation in thunder clouds. In thunder clouds, normally, ice-crystals become positively charged while water droplets become negatively charged. In most of the cases, these particles are so

distributed as to give rise to a negative charge build-up at the base of the cloud.

The negative charge at the cloud-base gives rise to a positive build-up of charge on the earth. The build-up continues till the potential difference between the earth and the cloud base becomes so large that it causes a breakdown of the resistance of air medium, thereby creating a lightning discharge.

The majority of lightning strokes are known to occur with the generation of a negative leader from the cloud to the ground. The leader travels earthwards in discreet steps of approximately 30 to 60 meters each. When this stepped leader is near the ground, its negative charge induces greater amounts of positive charges on the earth, especially on objects projecting above the earth’s surface. These charges attract each other and a cloud bound upward streamer is launched from the ground. The two meet and form a completely conducting path and very high current flows along this path to equalize the potential difference. This is termed as the return stroke. Discharge phenomenon is pictorially depicted in Fig.1.

Some typical values of the stepped leader and return stroke are as follows:-

a) Stepped Leader

- 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

b) Return Stroke

- Peak current of first return Stroke 30 KA

- Speed of Propagation 1/3 Speed of Light

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-

FIGURE 1

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Wave Shape Rise time 2 to 10 micro Sec Total discharge time to 50% 10-250

micro sec.

- Channel Temperature 30,000o K

(For more details on Lightning Phenomenon , reference may be made to the book on “Lightning” by R.H. Golde)

The frequency of lightning differs from region to region. Statistical data for the number of thunderstorms in a given place is compiled and the yearly average is termed as Ceraunic number. The higher this number, the larger is probability of a lightning stroke.

It is estimated that about 2,000 storms exist at any one time in the world, bombarding the earth each year with over 3 billion lightning strokes.

Lightning strokes could result in direct strikes or indirect strikes.

4.1 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.,

4.2 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.

4.3 EFFECTS OF LIGHTNING STROKE

4.3.1 Electrical

As the current is discharged through the resistance of the earth electrode of the lightning protection system, it produces a resistive voltage drop which may momentarily raise the potential of the protective system to high value relative to the true earth. It may also produce around the earth electrode a high potential gradient dangerous to persons and animals.

4.3.2 Side Flashing

The point of strike on the protective system may be raised to a high potential with respect to adjacent metal. There is, therefore a risk of flash over from the protective system to any other metal on or in the structure.

4.3.3 Thermal

Although the lightning discharge current is high, its duration is short, its thermal effect on the protective system is usually negligible. In general the cross section area of lightning conductors is chosen primarily to satisfy the requirement of mechanical strength.

4.3.4 Mechanical

Where a high current is discharged along parallel conductor at close proximity or along a single conductor with sharp bends, considerable mechanical forces are produced.

5.0 LIGHTNING PROTECTION CONCEPTS

Based on the understanding of the lightning discharge phenomenon, following important points need to be noted.

a) The down leader is self triggering and erratic in its downward movement. Its initiation, progress and direction is currently beyond the power of man to control.

b) The final earth strike point is determined by the nature and location of ground points which compete to create upward intercepting leaders.

c) The first upward leader to intercept the downward leader completes the path for the main lightning discharge and usually causes all other down leader

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branches to collapse. Double discharges can occur when two down leaders approach earth with same time and distance parameters. It is not necessary that all up leaders shall intercept a down leader.

5.1

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.

5.2 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.

The down conductor coveys the

lightning energy to the earth in a safe and well defined path thereby preventing damage to the protected structure and avoiding side flashing.

Earth Connection.

The connection to the earth mass shall have a low impedance to prevent the rise of potential of surrounding earth mass and maximize the rate of fall of potential away from the connection point. The earth mass with the earth conductors embedded in the soil provides safe discharge of lightning current into the soil.

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 early streamer emission system employs either a terminal of specific shape (Sphere as in the case of Dynasphere) or enhanced ionizing radioactive air terminal for the generation of ions. Air terminal is connected to a special down conductor attached to an earthing system.

The use of Lasers was proposed in 1974 to discharge thunder storms. 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.

The disadvantages of the Faraday cage are related to its high cost and the fact that it has no impact on the

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electromagnetic pulse related to a close lightning stroke. The major drawbacks to the laser beam are high cost, state of development and the problems of diverting stroke energy to earth without damage to the laser itself.

5.3 Preventive Approach

The preventive approach employs method to prevent build-up of charge in the area to be protected. The system shall be able to reduce the potential between the protected area and the charged clouds, so that the potential difference is not high enough to enable the generation of a leader to the earth within the protected area.

When a thunder cloud passes overhead and the field strength is greater than 2 KV/M2, point discharge currents are generated. Any natural occurring sharp point, such as trees, blades of grass on flat planes or pointed rocks on mountain tops will generate corona discharge. Point discharge currents act to limit the electric strength. Multipoint discharge system consist of three elements as shown in Fig. 2

a) the dissipator or ionizerb) the ground current collectorc) the conductors connecting the

dissipator and the ground current collect.

The configuration depends on the size and height of the structure to the protected, soil conditions, prevailing wind condition, storm patterns, altitude and keraunic number. The basic configuration consists of a conductor with two sharp pointed rods connected at right angles to each other and the right angle rods or spaced along the conductor. The configuration looks like barbed wire. This conductor with multiple sets of rods spaced periodically along the length of conductor is referred to as the dissipating medium. Using this dissipating medium several array configurations can be formed.

It may be noted that as on date multi-point discharge system is not included as an option in any international standards/practice on lightning protection.

6.0 LIGHTNING PROTECTION SYSTEM

6.1 Need For Protection

6.1.1 The necessity of providing lightning protection for structures located in non-hazardous areas shall be assessed by calculating overall risk factor. However, for hazardous areas lightning protection is essential considering the serious consequences of fire/damage due to lightning strokes and shall be provided irrespective of the value of risk factor.

6.1.2 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.

6.2 Zone Of Protection

Lightning protection system shall be designed to protect the structures

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FIGURE 2

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system. The path between air termination and earth shall be such that the lightning

current is discharged to earth without passing through non-conducting parts of the building and also without causing fire, flashover and dangerous potential difference in and around the structure.

Rolling sphere concept is followed to determine the zone of protection of lightning conductor.

In this concept, the zone of protection includes the space not intruded by a rolling sphere having radius equal to the stroking distance when tangent to earth and resting against a lightning protection air termination, all space between the two points of contact and under the sphere are in the zone of protection. A zone of protection is also formed when such a sphere is resting on two or more air terminals and includes the space under the sphere between those terminals. Fig.3 provides a graphic representation of rolling sphere concept

of zone of protection.

Striking distance of 30 Meters may be considered for the protection of structures containing flammable materials and striking distance of 60 meters may be considered for other structures.

Zone of protection for a single mast using 30 meters striking distance is shown in Fig.4 and that for Overhead Ground Wire( Suspended Air Terminals) is shown in Fig 5.

6.3 Lightning Protection of Structures containing flammable materials.

Protection Concept of storage tanks and vessels have been dealt in detail in attached Appendix-I for guidance in designing lightning protection system.

7.0 CONTROL OF FUGITIVE EMISSION

For any fire to take place, three basic conditions i.e presence of oxygen, flammable material and source of ignition must simultaneously exist. The lightning stroke i.e. the return stroke may involve temperatures as high as 30,000o K in the discharge channel. Therefore, if a direct strike of lightning or sparking due to a side flash takes place on a vent of a cone roof tank while a flammable mixture is present, fire is bound to take place. it is therefore, essential that measures to control of fugitive emission from storage tanks are given special consideration.

The design, operating and maintenance practices for a hydrocarbon installation should be given due attention to the need of minimising the possibility of presence of flammable atmospheres. Control of fugitive emissions from vents, proper design and maintenance of seals of floating roof tanks etc. are essential for avoiding damage to petroleum installations due to lightning strokes.

Control of fugitive emissions is important from the view point of prevention of fire, in addition to the benefits of environmental protection and product loss.

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SINGLE MAST

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ZONE OF PROTECTION _DEFINED BY DASHED LINES

Figure 4

ONE OF PROTECTION USINGOVERHEAD GROUND WIRES

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ZONE OF PROTECTION DEFINED BY GROUND WIRE (S)AND DASHED LINES

FIGURE 5

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8.0 PLANT BUILDING AND STRUCTURES

The need for the protection of plant buildings such as substation, control room office buildings, work shop, warehouse etc., and cooling towers shall be assessed taking into consideration the exposure risk and the following factors:

a) Use to which structure is put.b) Nature of its construction.

c) Value of its contents or consequential effects.

d) The location of the structure and

e) The height and the effective collection area of the structure.

The overall risk factor shall be established as per the guidelines of IS:2309 to decide the need for protection.

Structures of exceptional vulnerability by reason of explosive or highly flammable contents need special consideration and every possible protection need to be provided even against the rare occurrence of a lightning discharge.

8.1 General Design Requirements.

A lightning protection system (Conventional Air Terminal System) consists of the following three basic components:

a) Air terminalb) Down conductorc) Earth connection

8.1.1 Air Termination System

The air terminal shall be capable of drawing the lightning discharge to it in preference to vulnerable parts of the protected structure. The air terminations can be of vertical or horizontal type.

Conductors shall be interconnected to form a closed loop.

Vertical air terminations shall be used for very high structures with small base areas e.g. non-conducting chimneys etc. Minimum 2 nos. vertical terminations shall be provided for chimneys.

Vertical air terminations shall project at least 300 mm above the protected structure.

All the vertical air terminations provided on the same structure shall be interconnected.

Where a structure has two elevations; out of which lower is projecting outside and the higher elevation does not protect the lower elevation, separate network shall be provided for lower elevation. Both networks shall be interconnected by connecting the higher elevation down conductor to the lower network.

All the metal piping, railing etc., on the roof shall be bonded to the protective network.

8.1.1.2 Down Conductors

The recommended spacing of down conductors is every 20 M of Perimeter for structures upto 20 M in height and every 10 M of perimeter for structured above 20 M height.

8.1.1.3 EARTH TERMINATION

Each down conductor shall be provided with an earth electrode and all earth electrodes shall be interconnected through underground strip.

Lightning protection earthing system may be bonded to electrical safety earthing system, inside ground.

The use of rod/pipe/strip electrodes is permissible. Their choice will depend upon site conditions, soil resistivity and economic considerations.

The material of earth electrodes shall be galvanised iron.

The whole of lightning protective system including any earth ring shall have a combined resistance to earth not exceeding 10 ohm without taking account of any bonding.

8.1.2 Size and Material of Conductors

The material of air termination network, down conductor and earth termination shall be galvanised iron.

Lightning currents have very short duration, therefore thermal factors are of little consequence in deciding the cross-section of the conductor. The minimum size of the various components of lightning protection system shall be as follows:

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Vertical Air Termination

- 25mm dia., 1000mm long GI Pipe

Horizontal air termination

- 25mm X 3mm GI Strip (or) 40mm X 5mm GI Strip

Down conductors - Same as horizontal air termination.

Earth terminations - 65 mm dia., 3000mm long G.I pipe in Test pit.

All hardwares used for earth connection shall be hot dipped galvanised or Zinc passivated. The amount of galvanising shall be 610 gm/sq.mt as per Industry Practice.

8.2 Protection of Sub-station, Control Room, Pump Houses, UHF / Microwave Tower and similar functional buildings and Structures.

These structures are generally made of insulating materials such as concrete and brick. Protection against direct stroke of lightning is made by properly designed air termination network which may consist of vertical, horizontal conductors or combination of both.

Depending upon the form of the building, required number of down conductors shall be provided as per the guidelines given in IS:2309. Down conductor system shall be routed directly (where practicable) from the air termination to the earth termination network and be symmetrically placed around the outside walls of the structure starting from the corners.

The rolling sphere concept of protection shall be adopted to determine the area/zone of the protection taking into account the possibility of side strikes to the structures.

Any metal in or forming part of the structure on any building services having metallic parts which by design or by chance are in contact with the general mass of the earth should be either isolated from or bonded to the down conductor.

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The same general recommendation applies to all exposed large metal items whether connected to earth or not. ( In this context a large item is considered as one having any single dimension greater than 2 m.) Minor items such as door hinges, metal gutter brackets, reinforcement of small isolated beams may be disregarded.

If portions of a structure vary considerably in height, any necessary air terminations or air termination networks for the lower portions should be bonded to the down conductors of the taller portions in addition to their own down conductors.

Typical examples of air terminations for flat roof, large area roof of various profiles are illustrated in Fig.6 and 7.

All members of UHF/Microwave tower steel structure should be connected with eachother through bolt/nut properly tigthtened at more than one point and whole structure should be electrically continuous and earthed properly,so that the structure can be used as down conductor.

8.3 Protection of Compressor House, Truck Loading, Wagon Loading Structures.

These are in general steel framed building structures, the frame work itself provides an efficient natural network of many paths to earth. The structural steel columns shall be connected to the plant earth grid and as such down conductors are not required. The resistance of the total structure to the general mass of earth shall be measured using earth meggar and it shall be less than 10 ohms.

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A network of horizontal conductors (air termination system) should be fixed to the roof of structure as per the guidelines

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of IS-2309.

Ventilators and other roof projections of non-metallic construction shall be protected by means of air terminal system and connected to the steel frame of structure.

major metal forming part of the structure, including continuous metal reinforcement and services, should be bonded together and connected to the lightning protective system. Such connections should be made in atleast two places and should, wherever possible, be equally spaced around the perimeter of the structure at intervals not exceeding 15 m. For further details refer IS-2309.

Metal inside the structure should be bonded to the lightning protective system.

Vents and exhaust stacks from process plants emitting flammable vapours or dusts should be fitted with flame traps.

8.4 Protection of Stacks.

Non-conducting chimneys whose overall width or diameter at top is upto 1.5m shall be provided with one down conductor, and chimneys with overall width or diameter at top more than 1.5m shall be provided with 2 no. down conductors as shown in Fig.8.

Metal stacks shall be properly earthed at the bottom. No air terminal/down conductors are required if the thickness of sheet steel is more than 4.8 mm.

9.0 PROTECTION OF STORAGE TANKS

9.1 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.

c) 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.

f) 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.

9.2 Normally in hydrocarbon industry two types of above ground storage tanks are in use for storage of flammable liquids at atmospheric pressure:

1) Fixed roof type tanks

2) Floating roof type tanks

Protection against lightning for these two types of storage tanks are illustrated below:

Fixed roof type tanks :

The contents of metallic tanks with steel roofs of riveted, bolted or welded construction with or without supporting members, used for the storage of flammable liquids at atmospheric pressure are considered to be inherently self-protecting against lightning if the following requirements are met:

a) All joints between metallic plates shall be fully riveted, bolted or welded.

b) All pipes entering or connected with the tank shall be metallically connected and properly bonded with the tank at the point of entrance.

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Figure 8 : Lightning protection of RCC Chimneys

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FIGURE – 9 EARTHING OF THE TANKSNOTE: No of earth connections shall be decided based on tank diameter considering that the spacing between any two earth connections shall not exceed thirty meters along the tank perimeter.

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FIGURE – 10 TYPICAL EARTH CONNECTION

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c) All vapour or gas openings shall be closed or provided with flame protection devices when the stored stock may produce a flammable air-vapour mixture under storage conditions.

d) The roof shall have a minimum thickness of 4.8mm.

e) The roof shall be welded, fully bolted or riveted to the tank shell.

Tanks shall be grounded to conduct away the current of direct strokes and to avoid the buildup and potential that may cause sparks to ground. A metal tank shall be grounded by one of the following methods:

a) A tank is connected without insulated joints to a grounded metallic piping system.

b) A vertical cylindrical tank rests on earth or concrete and is at least 6m in diameter or rests on bituminous pavements and is at least 15 M in diameter.

c) By bonding the tank to ground through a minimum of two ground terminals as shown in Fig.9 at maximum 30 M intervals along the perimeter of tank. This also applies to tanks with an insulting membrane beneath the tank.

d) Storage tanks having cathodic protection system require special consideration while designing the grounding system.

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:

a) 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)

b) 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.

c) 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.

d) 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.

10.0 REFERENCES:

The following codes, standards and publications have either been referred to or used in the preparation of this document and the same shall be read in conjunction with this document:

1) OISD Standards/ Recommended Practices such as:

a) OISD- 149; Design Aspects for Safety in Electrical System.

b) OISD- 137 Inspection of Electrical Equipment.

2) BIS (Bureau of Indian Standards) Publication.

a) IS-2309: Protection of Buildings & allied structures against lightning.

3 International Code of Practice / guidelines

a) API Recommended Practice 2003: Protection against ignition arising out of Static, Lightning and Stray Currents -

b) NFPA 780: Standard for the installation of Lightning Protection Systems.

c) BS 6651 Code of Practice for

Protection of Structures against Lightning.

d) Electrical Safety Code - Institute of Petroleum, U.K.

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4. Lightning, Vol.1 & 2 by R.H. Golde. APPENDIX- I

A1. PROTECTION CONCEPTS OF STORAGE TANKS AND VESSELS

The need for the Protection against Lightning for Storage Tanks, Spheres, Bullets containing flammable materials is self evident.

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.

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.

A1.1 Accordingly the following options are available for the augmentation of the lightning protection system.

I) Use of lightning Air Terminals on the tank shell.

ii) Use of lightning protection masts around the storage tanks.

iii) Use of overhead shield wire.

While designing the above options following important aspects need to be kept in mind:

a) Striking distance of 30 m is to be considered for applying the rolling sphere concept of protected zone.

b) Increasing the height of lightning masts above the striking distance (30 m) will not increase the zone of protection. Zones of protection for varying mast height and a striking distance of 30 m are shown in Fig.A1..

c) To prevent side flashes, the minimum distance between a mast or overhead ground wire and the structure to be protected shall be not less than the bonding distance or sideflash distance.

Sideflash distance from a mast can be calculated by the formula:

D = h 6

Where h = height of structure.

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Figure A1

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Sideflash distance from a catenary may be calculated as:

D = I_ 6n

Where I = Length of lightning protection cable between its grounded point and the point under consideration.

n = Between 1 to 2.25 depending upon the number of down conductors and their spacing.

A1.2 Air Terminals installed on Tank Shell:

Protection zone provided by air terminals installed on the tank shall depend upon the tank dia, height of air terminal and spacing of air terminals around the tank perimeter. 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 as shown in Fig.A2

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.

Where the tanks walls are of thickness more than 4.8 mm, separate down conductors, are not required and tank wall itself can be used as down conductor with the proper connection of air terminals to the tank shell. Typical details are given in Fig.A3.

Number of 6 m high air terminals (spaced about 20 m around the perimeter) required for various tank diameters are tabulated below:

TANK DIA NUMBER OF AIR TERMINALS

Upto 12M 3 13 - 21M 4 22 - 32M 5 33 - 38M 6 39 - 45M 7 46 - 51M 8 52 - 57M 9 58 - 63M 10 64 - 71M 11 72 - 79M 12

A1.3 Lightning Masts around Storage Tanks

For a lightning mast to prevent fires due to lightning, following basic factors need to be borne in mind:

a) Lightning 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.

b) 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.

In view of above, lightning mast is required to be located at around 5 to 6 m from the tank shell.

Protection zone of a system of lightning masts with mast height equal to 15 m more than

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AIR TERMINALS ON TANK WALLS

NO. OF AIR TERMINALS

FIGURE A2

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DETAILS OF AIR TERMINALS ON TANKS

FIGURE A3

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the tank height and spacing of about 24 m around the tank is almost same as that provided with the use of lightning conductors installed on the tank shell, the number of lightning masts and the lightening conductors on shell being the same. Protection zone for a 50 m dia tank using lightning masts is illustrated in Fig.A4.

It is thus obvious that both a system of lightning masts around the tank or lightning conductors provided on the tank wall provide similar protection to the tank for preventing fires due to lightning. Lightning masts are much more expensive as compared to the air terminals on the shell while the lightning masts would be easier to install as a retrofit in an operating plant in comparison to the air terminals, since the latter would require welding work on the tank wall.

Lightning mast can be of tubular design or a lattice structure of angle irons. Earthing system of the lightning mast shall have to be bonded to the earthing system of the associated storage tank.

A1.4 Use of Overhead Shield Wire

A 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.

Protection zones using a system overhead earth wires are shown in Fig.A5.

Supporting structures for the overhead earth wires can be either of tubular design or a lattice structure. Height of the structure shall be decided based on the height of the topmost point on the tank roof and the side flash distance. The supporting structure shall also have to be located sufficiently away from the tank so as to prevent any side flash. The earth-wire shall be bonded to the steel supporting structure which can serve as a down conductor. Earthing system of the supporting structure shall be bonded with the tank earthing system.

A1.5

With regard to protection for structures containing flammbale vapours, gases or liquids that can give off flammable vapours, the following principles should be followed:

a) Liquids that can give off flammable vapours 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.

c) Structures and all appurtenances (e.g. dip-gauge hatches, vent valves) shall be maintained in good 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.

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FIGURE A4

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FIGURE A5

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