Std-110, Recommended Practices on Static Electricity
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OISD 110...
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OISD-110 OISD - 110 (Rev.1) FOR RESTRICTED CIRCULATION
RECOMMENDED PRACTICES ON STATIC ELECTRICITY ELECTRICITY
OISD - RECOMMENDED PRACTICE - 110 First Edition, August 1990
Revision 1, August, 1999
Oil Industry Safety Directorate Government of India Ministry of Petroleum & Natural Gas
Comment
OISD-RP-110 First Edition August 1990 Revision 1, August, 1999 FOR RESTRICTED CIRCULATION
RECOMMENDED PRACTICES ON STATIC ELECTRICITY ELECTRICITY
Prepared by
FUNCTIONAL COMMITTEE ON PROCESS DESIGN & OPERATING PHILOSOPHIES
OIL INDUSTRY SAFETY DIRECTORATE 2ND FLOOR, “KAILASH” 26, KASTURBA GANDHI MARG NEW DELHI - 110 001.
OISD-RP-110 First Edition August 1990 Revision 1, August, 1999 FOR RESTRICTED CIRCULATION
RECOMMENDED PRACTICES ON STATIC ELECTRICITY ELECTRICITY
Prepared by
FUNCTIONAL COMMITTEE ON PROCESS DESIGN & OPERATING PHILOSOPHIES
OIL INDUSTRY SAFETY DIRECTORATE 2ND FLOOR, “KAILASH” 26, KASTURBA GANDHI MARG NEW DELHI - 110 001.
NOTE
OIL INDUSTRY SAFETY DIRECTORATE publications are prepared for use in the Oil and gas industry under Ministry of Petroleum and Natural Gas. These are the property of Ministry of Petroleum and 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 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 only to supplement and not replace the prevailing statutory requirements.
Note 1
in superscript indicates the changes / modifications / additions as th approved in 17 Safety Council Meeting held in July, 1999.
II
OISD-110
FOREWORD The Oil Industry in India is 100 years old. As such variety of practices have been in vogue because of collaboration / association with different foreign companies and governments. Standardisation in design philosophies and operating and maintenance practices at a national level was hardly in existence. This, coupled with feed back from some serious accidents that occurred in the recent past in India and abroad, emphasized 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 & Natural Gas, in 1986, constituted a Safety Council assisted by 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 safe operations. Accordingly, OISD constituted a number of Functional Committees of experts nominated from the industry to draw up standards and guidelines on various subjects. The present document on “Recommended practices on Static Electricity” was prepared by the functional committee on “Process Design and Operating Philosophies”. While some of the installations do not have a Work Permit System, a wide variety of practices exist even among those who practice the Work Permit System. 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 standard if implemented objectively, may go a long way to improve the safety and reduce accidents in Oil and Gas Industry. Suggestions are invited from the users for futher improve-ment after it is put into practice. Suggesstions for amendments to this standard should be addressed to The Co-ordinator, Committee on “Process Design and Operating Philosopies”, Oil Industry Safety Directorate, 2nd Floor, “Kailash” 26, Kasturba Gandhi Marg New Delhi-110 001. This document in no way supersedes the statutory regulations of CCE, Factory inspectorate or any other statutory body, which shall be followed as applicable.
III
FUNCTIONAL COMMITTEE ON PROCESS DESIGN AND OPERATING PHILOSOPIES LIST OF MEMBERS
Name
Designation / Organisatio n
Status
S/Shri W.D. Lande
DGM (TECH), HPCL, Visakh Refinery
Member Leader
G. Raghunathan
Chief Manager (Process) HPCL Visakh Refinery
Member
B.K. Sedani
DGM (Elect.) ONGC Bombay
Member
N.N. Gogoi
DGM (LPG, OIL, Duliajan
Member till Oct.87
Shri. A. Sinha
Dy. Planning Manager (B&MIS), OIL Duliajan
Member
S.V. Puthil
Chief Instl.Manager HPCL (Mkt). Bombay
Member till Jan.89
A.M. Pradhan
Sr.Mgr (Safety & Insp.) HPCL, Bombay
Member
S.V. Save
DGM (West Coast Refin) HPCL, Bomaby Refinery
Member
M.A. Sreekumar
Chief Mgr.(TECH) CRL, Cochin
Member
A. Varadarajan
Chief Mgr. (Proc. Devel.) MRL, Madras
Member
B.K. Trehan
Addtl. Director , OISD, New Delhi
Member Till Jan. 89
D.K. Sen
Additional Director OISD New Delhi
Member Coordinator
In addition to the above several experts from industry contributed in the preparation, review and finalisation of the document.
IV
OISD-110
LIST OF PRESENT MEMBERS
1.
Shri. W.D. Lande, GM (Proj.) Member - Leader, OISD Functional Committee Hindustan Petroleum Corporate Ltd., Visakh Refinery, Post Box No.15, VISAKHAPATNAM – 530 001.
6.
Shri. S.V. Save, DGM (West Coast Refinery), Member-OISD Functional Committe Hindustan Petroleum Corporation Ltd., Petroleum House, 17, Jamshedji Tata Road, BOMBAY - 400 020.
2.
Shri. G. Raghunathan, Chief Manager (Process) Member - OISD Functional Committee Hindustan Petroleum Corporate Ltd., Visakh Refinery, Post Box No.15, VISAKHAPATNAM - 530 001.
7.
Shri. M.A. Sreekumar, Chief Mgr.(Tech.) Member-OISD Functional Committee Cochin Refineryes Ltd. Post Bag No.2, Ambalamugal-682 302.
3.
Shri. B.K. Sedani, GM (Elect.) Member-OISD Functional Committee ONGC, Marine Survey, 12th Floor, Express Tower, Nariman Point, BOMBAY - 400 021.
8.
Shri.A. Varadarajan, Chief Mgr (Proc.Devlt.) Member-OISD Functional Committee Madras Refineries Ltd, 480 Anna Salai, MADRAS - 600 035.
4.
Shri.A. Sinha, Dy.Plannin g Manager (B & MIS) Member-OISD Functional Committee Oil Indial Limited, DULIAJAN-786 602 ASSAM
9.
Shri.D.K. Sen, Addl.Director Member-Coordinator OISD Functional Committee, Oil Industry Safety Directorate, 409, New Delhi House, 27, Barakhamba Road, NEW DELHI-110 001.
5.
Shri A.M. Pradhan, Sr. Mgr (Safety & Insp.) Member-OISD Functional Committee Hindustan Petroleum Corporation Ltd., Safety & Inspection Training Centre, 3/4, S.V. Road & Turner Road Junction, Bandra, BOMBAY - 400 050.
V
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6
OISD-110
RECOMMENDED PRACTICES ON STATIC ELECTRICITY CONTENTS. SECTION 1.0
Introduction
1.1
Scope
2.0
Background on Static Electricity
2.1
What is Static Electricity
2.2
Conductivity
2.3
Relaxation Time
3.0
Theory of Static Electricity
3.1
Generation
3.1.1
Generation due to fluid flow
3.1.2
Generation due to settling
3.2
Rate of Generation
3.3
Accumulation
3.4
Conductivity
3.5
Static Discharge
3.6
Sparks and Arcs
3.7
Sparking Potential
3.8
Ignition Energy
4.0
Common Sources of Static Electricity
5.0
Guidelines for Control of Static Electricity
5.1
General
5.2
Spraying, Splashing & Misting
5.3
Agitation and Mixing
5.4
Water
5.5
Flow Velocity
5.5.1
In Tanks
5.5.2
In Pipes
5.6
Filters
5.7
Gauging and Sampling
5.8
Insulated Conductive Objects
5.9
Projections and probes
5.10
Bonding
7
OISD-110 CONTENTS (Cont inu ed) 5.11
Grounding
5.12
Use of Additives
5.13
Internal Coatins
6.0
Specific Guidelines for Control of Static Electricity
6.1
Storage Tanks
6.1.1
General
6.1.2
Sampling of Products
6.2
Tank Trucks, Tank Cars, Fuders
6.2.1
Loading/Unloading Operations in Tank Wagon Gantries
6.2.2
Loading/Unloading Operations in Tanktruck Gantries
6.3
Small Containers (Drums, Cans)
6.4
Leaky LPG Cylinders
6.5
Tank Cleaning
6.6
Synthetic Fiber Cords
6.7
Belt
6.8
Wearing Apparel
6.9
Sand or Shot Blasting
7.0
Effective Bonding/Earthing Systems :
7.1
For Tankwagon Loading/Unloading Gantry
7.2
Tanktruck loading and unloading Gantry
7.3
Barge/Tanker Jetty Operations
7.4
Pipelines/Pumps
7.5
Storage Tanks
7.6
Sampling/Gauging
7.7
Filling small Containers
8.0
Classification of Products
8.1
Non-accumulators
8.2
Accumulators
8.3
Low Vapour-Pressure Products
8.4
Intermediate Vapour-Pressure Products
8.5
High Vapour-Pressure Products
9.0
References Appendix ‘A’ Appendix ‘B’ Appendix ‘C’ Appendix ‘D’
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1
RECOMMENDED PRACTICES ON STATIC ELECTRICITY 1.0
Appendix : A). Now, assume a charged insulated conductor is brought close to a second insulated conductor. Like charges are induced on the opposite end of the second conductor. Unlike charges are induced on the near end of the second conductor, bound to the original charges.
INTRODUCTION It is not possible always in a plant to prevent the formation of explosive mixutre, so a possible source of ignition must be exclude from these areas. Sparks and arcs which result form switches, starters, relays & similar devices have been rendered harmless by explosion-proof installations. However, there exists an ever present fire hazard in the processing industries from ignition with may arise from static sparks.
1.1
If, the opposite end of the second conductor is momentarily grounded, the like charge disappears but the bound unlike charge remains. Then, if the original charge conductor is removed, the second conductor retains the unlike charge which is no longer bound. There is a voltage between the second conductor and ground.
SCOPE The purpose of this document is to assist in reducing fire hazard of static electricity by presenting a discussion of the nature and origin of static charges, the general methods of mitigation and recommendations in certain specific operations for its dissipation. The application is limited to petroleum production, refining and marketing installations.
2.0
BA CKGROUND ON STATIC ELECTRICITY
2.1
WHAT IS STATIC ELECTRICITY ?
Poor conductors behave similarly, but when the charge is in the body of the conductor, more time is required for the transfer. This is important in liquid hydrocarbons because the charge must move out of the liquid’s body to the surface before it can transfer to the inside of the container. 2.3
Relaxation time is a measure of the time it takes charge to leak away form a charged liquid when the liquid fills a metal container connected to ground. The time varies with the product. it is actually the time in seconds to remove 63 percent of the charge.
Static Electricity is a phenomenon of electrification of materials through physical contact and separation and the various effects that result from the positive and negative charges so formed. In general, static electricity results form removal of electrons from the atoms of one material (leaving it with positive charge) and absorption of these electrons on the second material (negative charge) during physical separation of the two materials.
Zero charge is only approached (but not reached) in four or five times the relaxation time, γ (tau). γ is approximately equal to 18 divided by the conductivity of the liquid hydrocarbon in picomhos pr meter.. For example, if a product has a conductivity of 1 picomho per meter, γ is 18 seconds. Thus no charge will be approached in 90 seconds. If the conductivity were raised to 100 picomhos per meter, γ would be only 0.2 second. So practically zero charge condition would remain after 1 second.
Both materials remain charged if they are well insulated electrically. The generation of static electricity cannot be prevented absolutely, because its intrinsic origins are present at every interface. 2.2
RELA XATION TIME
CONDUCTIVITY A charge on one body can induce a charge on a second body that is brought near it (See 1
3.0
THEORY OF STATIC ELECTRICITY
3.1
GENERATION
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2
Generation of electric charge, usually occurs whenever a liquid, for instance a hydrocarbon, flow past a solid or another liquid. The degree of charge generation in the case of oil products is determined not solely by the nature of such liquids or solids but also by the type and concentration of certain trace compounds which are nearly always present in solution in oil products. Static electricity is generated by the separation of like or unlike bodies. Electrostatic charges, positive & negative, always occur in pair and are developed when any tow bodies that have been in contact are separated. The negative charges migrate to one body, leaving the other body with a positive charge. For sufficient charges to be developed, the bodies must become and remain insulated with respect to each other so that the electrons, which have passed over the boundary surface or interface, are trapped when separation occurs. Insulation may occur through complete physical separation o the bodies or because at least one of the bodies is an insulator. Petroleum products which have a low conductivity can serve as an insulators 3.1.1
container or tank, an equal but opposite charge will be induced on the inside surfaces of the tank, Also, a charge of the same sign as the incoming stream will be induced on the outside of the tank. These induced charges arise from charge separation within the tank wall following exposure to the electrostatic field created by the incoming charged liquid stream. 3.1.2
Strong electrostatic fields may also be generated by droplets of sold particles settling in a medium of low conductivity, or by agitation of such particles within the medium. If a liquid in a tank containing ionizable impurities is subject to turbulence, the separation of ions can result in electrostatic charging within the body. Such charging may cause significant variations in voltage within the liquid or on the liquid surface. There is no change in the neutrality of total charge within the tank as long as no charged fluid flows into or out of the tank. 3.2
Generation due to flui d flow : Of most importance in our operations is the contact and separation which takes place in flowing liquids. The liquid, prior to flow, contains equal quantities of ions, positively and negatively charged, and is electrically neutral. However, ions of one sign are preferentially absorbed by the surface of the container or pipe, leaving a surplus of ions of the opposite sign in the liquid at the interface. Upon liquid flow, charging of the liquid occurs because the absorbed icons are separated from the free ions by turbulence. The opposite charge is usually conducted throughout the metallic pipe wall, in the same direction because of the natural attraction between opposite charges. Ioniza- ble impurities, such as water, metal oxide, or chemicals, increase the static generation characteristics. The flow of electricity caused by he entertainment of charged particles in the flowing fluid is known as the streaming current. if this charged stream enters a
Generation due to settling .
RATE OF GENERATION The generating mechanism is related primarily to rat of flow, ionic content, materials turbulence, and surface area of the interface. The rate of electrostatic generation in a pipeline or hose increases with increasing length of pipe or hose to a maximum liming value. The maximum limiting value is related to liquid velocity and conductivity and will be greater for high velocities of liquid flow than for low velocities. The large surface area of filters causes them to be prolific generators of static electricity.
3.3
ACCUMULATION Hazardous electrostatic charges can accumulate only on bodies which are relatively well insulated from each other and from ground. Otherwise, charges leak away and recombine with their counterparts as fast as they are formed. Electrostatic charges can accumulate on the surface of petroleum products which have a sufficiently high resistivity. Humidity has
OISD-110
3
little effect on the migration of charges across hydrocarbon liquid surfaces. The amount of electrostatic charge which may accumulate on an insulated body depends upon: λ
λ
3.4
meter below which static charges may not be dissipated easily by earthing and bonding. An important characteristic in connection with electrostatic hazards is the half-value time of the liquid. This is the time taken for the charge in a liquid, completely filling a closed metal container, to decrease to half its original value. The half value time is inversely proportional to the conductivity and directly proportional to the dielectric constant of the liquid. A residence time (relaxation time) of 3 to 4 tim es the half value time may be assumed to be adequate for charges to “relax”. The Table-I shows the relationship between conductivity’s and half value times of various liquids.
The rate at which the static charge is being generated. The resistance of paths by which the charge leaks off (dissipates).
CONDUCTIVITY The ability of liquid to retain an electrostatic charge is a function of its conductivity. This characteristic may be expressed in terms of conductivity (1 conductivity unit = 1 picomho per meter (or) picosiemens per meter = 10 to the power of minus 14 ohm to the power of minus 1 or in the inverse from as resistivity (1 resistivity unit = 10 to the power of 14 ohm cm). Metals have very high conductivity and oils have low conductivity.
3.5
STATIC DISCHARGE In actual practice, electrostatic charges constantly leak from a charged body because they are always under the attraction of an equal but opposite charge. This leakage characteristic is called relaxation; and, because of this, the most static sparks are produced while the generating mechanism is active. It is possible, however, for charges generated in moving some refined petroleum products to remain for a time after the fluid has stopped because of the insulation qualities of the fluid.
Electrostatic generation is not significant when the conductivity of the liquid exceeds 50 picomhos per meter. Above this value, the charges recombine as fast as they are separated. Thus a conductivity of 50 picomhos per meter is the recommended minimum for the adequate removal of charge from a liquid. However, there is an overall lower limit of connectivity of 10 picomhos per TABLE - I
LIST OF CONDUCTIVITIES & HALF VALUE TIME OF VARIOUS LIQUIDS Liquid
Conductivity (Conductivity units
Conductivity (ohm-1 m-1)
Half Value Time (Sec.)
Highly purified Hydrocarbons
0.001
10 (-15)
12,0000
Light Distillates from refinery operation
0.01 to 10
10(-14) to 10 (-11)
1200 to 12
Shell Jet A-1 with ASA-3
150 to 300
15 X 10 (-11) to 30 X 10 (-1)
0.08 to 0.04
Crude Oil
1000 to 100,000
10 (-9) to 10 (-7)
0.012 to 0.00012
Distilled water
1 X 10 (8)
10 (-4)
12 X 10 (-8)
3
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4
Source : Fire & Safety Manual – Refineries & Petrochemical Panel – National Safety Council
OISD-110
3.6
5
to the rate at which the charge is being placed upon the insulated body and a stabilized condition will be reached.
SPARKS AND ARCS: A spark is essentially a transient phenomenon & can be described as the passage of an electric charge across a gap between tow points not previously in contact. An arc is defined as the flow of electric current that occurs at the instant of separation of two points previously in contact. Electrostatic discharges are usually sparks.
3.7
If this stabilized voltage is below the required sparking potential, no sparking will occur. if the stabilized voltage is above sparking potential, then sparking will occur before stabilization is reached. 3.8
SPARKING POTENTIAL :
IGNITION ENERGY The mere fact that a spark results from high voltage does not mean that ignition of a flammable mixutre will occur. In order to initiate combustion, sufficient energy must be transferred form the spark to the surrounding flammable mixture.
For static electricity to discharge as a spark, the voltage across the spark gap must be above a certain magnitude. In air, at sea level, the minim sparking voltage is approximately 350 volts for the shortest measurable length of gap. Increased gaps require proportionately higher voltages with the actual voltage dependent upon the dielectric strength of the material (or gas) which fills the space in the gap. For air, the dielectric strength is approximately 30,000 volts per cm. Therefore, the voltage across a 1 inch air gap would have to be over 75,000 volts in order for spark discharge to occur.
Experiments under the most favourable conditions have ignited petroleum vapourair mixtures at approximately 0.25 millijoules. The energy requirement increases as the mixture composition approaches the lean or rich sides of the flammable range; it at a minimum where a slightly richer than ideal mixture composition is attained. The energy requirement is also increased by a variety of other factors which tend to decrease the availability of the stored energy to flammable mixture :
In the petroleum industry, these spark gaps will assume many forms and appear at various locations. For example, a spark gap may be formed between a tank vehicle and the overhead filling downspout if they are not bonded together or in metallic contact. In this case, a static potential ditference is developed between the tank vehicle and the downspout due to the static charges generated during the f!ow of product into the compartment.
a)
b)
The potential developed is related to the amount of charge on a body and to the capacitance of this body with respect to its surroundings. Since the capacitance of a body with respect to its surroundings depends upon its size and position, it follows that the same charge will not always result in the same voltage and, hence, sparking may or may not occur.
c)
Under the continuous influence of a charge generating mechanism, the voltage of a an insulated body continues to grow. As the voltage becomes greater, the rate at which charge will leak through the insulation will grow since no insulation is perfect. At some voltage, the leakage of charge will be equal
A portion of the energy will be dissipated in a resistive portion of the discharge circuit and not be available at the spark gap. The electrodes, across which the sparking occurs, will be of a shape and material so that a portion of the energy in the spark will be used to heat the electrodes & will be available in its entirely to heat the material in the gap. This is more pronounced with short gaps and is known as its quenching effect. The spark gap may be so long that the energy is distributed over too great a path length. The energy is not concentrated sufficiently to heat the mixture to ignition temperature.
The typical values of Minimum Ignition Energy (mj), along with the Minimum Experimental Safe Gap (mm) and the quen-ching distance (mm) for some hazardous materials are presented in Table-II. Also, the effect of fuel 5
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concentration on Minimum Spark Ignition Energy is presented in Appendix; B.
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7
TABLE - II MINIMUM IGNITION ENERGY, MAX. EXPERIMENTAL SAFE GAP AND QUENCHING DISTANCE FOR VARIOUS HAZARDOUS MATERIALS.
Chemical
Minimum Spark Ignition Energy (mj)
Maximum Experimental Safe Gap (mm)
Quenching Distance (mm)
Methan e Ethan e Pro pane n-But ane
0.47 0.285 0.305
Isobutane n-Pentane Isopentane n-Hexane
0.52 0.49 0.70 0.29
Cyclohexane n-Heptane Methonal Ethylene
1.38 0.24 0.215 0.096
0.94
1.78 1.78
0.65
1.25
Propylene Benzene Ethylene Dichloride Ethylene Oxide
0.28 0.55
0.91 0.99 1.82 0.59
2.03 1.87
Acetlylene Carbon Monoxide Ammonia Hydrogen
0.02
0.37 0.91 3.18 0.20
0.52
0.20 0.96 1.01
0.55
Hydrogen Sulphide Carbon Disulphide Vinyl Chloride Acetone
1.14 0.91 0.92 1.07
2.16 2.29 1.75 2.41
0.39
2.07 1.52
0.087
>1000 0.02 0.068 0.015 1.15
7
1.18
0.50
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8
4.0
COMMON SOURCES OF STATIC ELECTRICITY Some common sources of static electricity which are experienced in oil industry are as follows: a)
Pulverised materials passing through chutes or pneumatic conveyors, e.g. catalyst handling.
b)
Steam or air/gas flowing from any opening in a pipe or hose, when steam is wet or the air or gas stream contains particulate matter, e.g. steaming of hydrocarbon tanks while cleaning or use of steam eductors for tank degassing/ ventila-tions & use of steam/air lances.
c)
Non-conductive power transmission belts or conveyor belts in motion.
d)
Moving vehicles.
e)
Motion of all sorts that involve changes in relative position of contacting surfaces, usually of dissimilar liquids or solids, e.g. Loose wooden /metallic pieces/ projections in tanks / pipes / vessels, etc.
f)
Hydrocarbon flow through microfilters made of paper/felt elements.
g)
Hydrocarbon liquids flowing at high velocities in pipes/nozzles/fittings, etc.
h)
Spraying/splashing and misting, such as: λ
Free fall of liquid driplets through vapour spaces.
λ
Splash loading of hydrocarbon liquids.
i)
Agitation/mixing & blending including mechanical mixing/agitation with air /steam/gas/ jet nozzles.
j)
Water entrainment, e.g. free presence of water in hydrocarbon products or in tanks.
k)
Switch loading (term used to describe a product being loaded into a tank or compartment which previously held a product of different vapour pressure) can result in ignition when low vapour pressure products are put into a cargo tank containing a flammable vapour from previous usage, e.g. Furnace Oil
loadedinto a tank which last carried gasoline.
5.0
GENERAL GUIDELINES FOR CONTROL OF STATIC ELECTRICITY
5.1
GENERAL : The following is a general discussion of the conditions which must exist in order to have incendiary electrostatic sparking. It also covers the major electrostatic generating & spark promoting mechanisms together with steps which can be taken to prevent gene-ration, accumulation or sparking. This infor-mation forms the basis for establishing the specific guidelines contained in Section 6-0 In order for an electrostatic charge to be a source of ignition, four conditions must be fulfilled : λ
λ
λ
λ
There must be a means of electrostatic charge generation. There must be an accumulator of an electrostatic charge capable of producing an incendiary spark. There must be a means of discharging the accumulated electrostatic charge in the form of an incendiary spark such as a spark gap. There must be a flammable vapour within the spark gap.
Ignition hazards from electrostatic sparks can be eliminated by c ontrolling the above. Since electrostatic accumulation to incendiary potential can only occur with products that have a relatively low conductivity, the following discussion primarily pertain to those liquids which are classified as electro-static accumulators. Likewise, they are confined to situations where flammable vapour-air mixtures might occur. 5.2
SPRAYING, SPLA SHING AND MISTING An electrostatic charge may be generated on liquid droplets when permitted to have a free-fall through a tank vapour space. Also, the charged droplets falling into the oil surface in the tank will increase the bulk liquid electrostatic charge. In addition, the falling or splashing liquid can agitate the liquid in the tank which also can increase the bulk electrostatic charge. If the bulk liquid charge reaches a high enough
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9
potential sparking may occur across the liquid surface or to the shell of the tank. Consequently, spray or splash filling or freefall through a vapour space should be avoided. Further, misting, in addition to creating an electrostatic charge, may create a flammable vapour space over a high flash product which under normal circumstances would be too lean to be hazardous.
The presence of water in hydrocarbons presents several electrostatic generating possibilities. First, water entrained in a hydrocarbon enhances the electrostatic generation properties of the hydrocarbon when moving through pipes, pumps or other equipment. Secondly, a very strong electrostatic field occurs when droplets of water settle out in the hydrocarbon. It should be noted that this settling phenomenon continues for some period after pumping has ceased.
Charged droplets, such as in a heavy mist, my also be subject to electrostatic discharges between clouds or vapour droplets even though the product has a high conductivity. This phenomenon, however, is generally confined to locations where large mist clouds can be formed, such as in a large tank. Thank truck compartments are not sufficiently large to form mist clouds of sufficient size.
Since it is not uncommon to have water in hydrocarbons resulting form such operations as water washing, line flushing, etc., care should be taken to avoid unnecessary mixing. For example, water flushed lines should be drained, and water bottoms in tanks should not be agitated.
Agitation and misting can be avoided by providing a drop piece to the tank bottom when top loading, or reducing velocities until inlet nozzle is well covered to prevent surface agitation. 5.3
5.5
FLOW VELOCITY
5.5.1
In Tanks In keeping with the above discussions of splash filling and agitation, it is obvious that velocities of incoming liquids should be kept low enough to avoid splashing and excessive agitation. Velocities of liquids entering tanks should be held to 1 ft/second) initially until the inlet nozzle is well covered.
AGITA TION AND MIXING The generation of an electrostatic charge in hydrocarbons influenced by movement of the product such as by mechanical mixing or agitation with air, steam, gas or jet nozzles. If such agitation occurs in a liquid hydrocarbon with a substantially low conductivity like ATF, Kerosene, a high electrostatic charge may be built in the bulk liquid. If there is a flammable vapour space above the surface of the liquid, ignition may occur. Consequently, agitation should be avoided where there is a likelihood of flammable vapour space.
Note 1
After the inlet is covered, the velocities of stocks entering tanks should be kept low enough to avoid both breaking the surface and excessive agitation. Velocities upto 4.5 m/second (15 ft/second) may be used for tanks of 45 cubic meter (10,000 gallons) capacity or less & 10.6 m/sec. (35 ft/second) for tanks larger than 45 cubic meter (10,000 gallons capacity. An exception is made for low flash turbo fuels where velocity should be restricted to 6.1 m/second (20 ft/second) regardless of tank size due to the lower conductivity of turbo fuels. However, the flow velocity into the tank should be restricted to 1m/second, if the free water is present in a low conducting stock.
With high velocity jet mixing nozzles, a charge may also result from the stream breaking the liquid surface and coming down as a spray or mist. The latter condition usually exists when the liquid level is low. It is recommended that mixing nozzles be commissioned after ensuring minimum level in the tank to prevent the stream from breaking through the liquid surface. 5.4
5.5.2
WATER :
In Pipes : In pipelines handling non-conductive petroleum products, the flow velocity should be
9
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10
5.6
restricted to the values indicated in Appendix “c”, If water is entrained in the product, the velocity should be limited to 1 m/second. FILTERS :
time should be established on the basis of the maximum flow velocity permitted. Theoretically, a 30 second relaxation time needs only to be provided for products that have low conductivities and can generate flammable aporair mixtures. However, since filters are such prolific electrostatic gene-rators, this precaution is recommended for all services. This is to safe-guard against charge in service, contamination or other abnormal situations. It will also provide protection if a high flash point product, such as kerosene or fuel oil, is loaded into a tank which contains a flammable mixture from previous service. (Loading heating oil into a tank truck which previously handled gasoline)
Because filters and filter separators have a large surface area exposed to fluid flow, they are prolific electrostatic generators. This has been confirmed both by laboratory tests and experience. Micropore paper elements probably generate the highest charges although cloth, felt, chamois and similar non-conductive materials will also generate a high charge. While tests have not been made with metal micropore filter elements., it is suspected that they also would generate a high charge, particularly when they have an appreciable depth or thickness. Deposits left on the filter elements from the fuel may have an increasing effect on their generating capabilities throughout their service life. On the other hand, thin metal screens and perforated metal baskets do not generate high charges. Tests have shown that flow through a 1400 mesh screen did not produce an appreciable electrostatic charge. The high electrostatic charges developed by the flow of fluids through filters can be effectively reduced by permitting sufficient time for charge relaxation to occur. It has been established that a 30 seconds residence time is sufficient to lower the electrostatic charge to a safe level regardless of the fluid conductivity. Consequently, a minimum of 30 seconds holdup time should be built into the piping system between the filter or filter separator and the receiving tank. This holdup may be provi-ded by enlarging or lengthening the piping downstream of the filter or by installing a relaxation tank. If a relaxation tank is provi-ded, it should not have a vapour space and baffles may be required to prevent by pass-ing which would reduce relaxation time. Relaxation time is defined as the time it takes a particle of liquid leaving the filter to reach the receiving tank. This relaxation
5.7
GAUGING AND SAMPLING : Because there may be an electrostatic charge on the hydrocarbon in a tank, the insertion of a metallic or conductive object into the tank before the charge has relaxed may be extremely hazardous. As the conductive gauge or sampling device approaches the product surface, a spark gap can be formed through which an electrostatic discharge might occur. Sparking could also occur as the gauge or sampling device is withdrawing from the liquid. Therefore, metallic or conductive objects such as gauge tapes, sample containers, thermometers, etc. should not be lowered into the tank during, or for a period of time after, all pumping into the tank or circulation within the tank has ceased. For tank trucks, cars or fullers, a 5 minutes waiting period should be observed for all storage tanks, tankers and barges. The intention of the restriction is to avoid the introduction of either conductive probes or insulated conductive objects. Devices which are completely non-conducting and can be so maintained, may be used at any time. These may include glass sample bottles on polypropylene rope and nonconducting gauge tape and rods when operator is earthed through the tank. This restriction does not apply to gauging and sampling in a gauging well or pipe whose bottom end is submerged. However, for practical considerations it is recommended
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to give the above mentioned relaxation time. 5.8
cause it provides a conductive path through which the static charges can recombine. No charge, therefore, can accumulate & no spark can occur. (Bonding of a tank or container has no effect on the liquid bulk charge within the tank or container).
INSULATED CONDUCTIVE OBJ ECTS : An insulated conductive object may accumulate an electrostatic charge when exposed to the stream of a flowing fluid or when exposed to a mist such as a steam cloud. Accumulated charges can be quite large & capable of producing an incendiary spark when a spark gap is formed. Also, a conductive object floating on an oil surface can become charged due to its contact with the oil which may be electro-statically charged due to movement or agitation. if such a floating object approaches a grounded object, such as the tank shell, a spark gap can be formed.
Static bond wires are usually comparatively large because of mechanical considerations; therefore, bond wire remittances are low. Such low resistance’s however, are not needed for static dissipation because electrostatic currents are usually in the order or microamperes (millionths of an ampere). A bond resistance of 1 megohm (1 million ohms) is entirely adequate for these small electrostatic currents since the resultant voltage difference appearing across the bond wire terminals is too low for sparking.
Therefore, care must be taken to prevent an unbonded conductive objective from entering a tank. Likewise, all metallic parts of a fill pipe assembly should form a continuous electrically conductive path downstream from the point of bonding. For example, a metallic coupling on the end of a non-conductive hose can become charged due to the flow of fluid. If the hose is inserted into the dome of a tank truck, sparking might occur between the hose coupling and the shell of the tank, or to the liquid surface.
Bolted connections within the bond wire or at the bond wire terminals are entirely adequate for static dissipation. Soldered or brazed connections are unnecessary. Parts of metallic fill pipe assembly form a continuous electrically conductive path and bond or jumper wires are not needed around flexible joints or swivel joints. Tests and experience have shown that resistances of these joints are low enough to prevent static charge accumulation. Conventional “U” clamps or other equivalent means for supporting riser pipes on metallic loading racks provide an adequate conductive path and permit one end of a bon wire to be fixed to the metallic loading rack rather than directly to the loading piping.
In order to avoid sparking between the metallic coupling on the hose and the shell of the tank or the liquid surface, an external bonding connection between the metallic Note 1 coupling shall be provided. 5.9
5.11
PROJECTIONS AND PROBES :
The earth may be used as part of the grounding system. Where the only gaps over which hazardous static sparks can occur are between an insulated object and grounded object, such as between electrically insulated vessels and grounded piping, the electrical insulation may be by passed by rounding the vessel. This will prevent the accumulation of static charge on the vessel. However, grounding of a container or tank cannot prevent the accumulation of charges on the surface of a liquid in the container if the liquid has a low conductivity.
Conductive projections such as structural members or probes should be avoided in the vapour space of tank. On a rising liquid level, a spark gap can be formed between the projection and product liquid surface. If the product it electrostatically charged, incendiary sparking may occur. 5.10
GROUNDING :
BONDING : Sparking between two conducting bodies can be prevented by an electrical bond attached to both bodies. This bond prevents a difference in potential across the gap be11
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5.12
USE OF ADDITIVES: Earthing alone may be insufficient to remove charges which have been accumulated in a liquid of low conductivity. The most effective method of achieving removal of charges is to increase the conductivity of the liquid to a safe value by means of an anti-static additive. In this way, charges can leak away so rapidly that they can no longer accumulate to a dangerous extend. The effect of adding anti-static additive along with other additives added should be discussed with the manufacturer before a decision on the quantity of additive to be added is made.
OISD-110
5.13
6.0
13
INTERNAL COATING :
—
It is believed that a coat of paint, plastic coating, or layer of aluminum oxide on the inside of cargo or storage tanks does not constitute an electrostatic hazard. Such films are not regards as barriers to the flow of static charges because their resistivity is of the same order of magnitude as the oil.
Avoid high velocity or splash filling, in all types of products, (low vapour pressure, inter-mediate vapour pressure and high vapour pressure) unless the tank is inerted, the product flash point exceeds 54.4 Deg. C (130 Deg.F) and it is not heated to within 6.0 Deg. C (15 Deg. F) of its flash point.
—
Agitation with air, steam gas, jet nozzle or mechanical mixtures should be avoided.
6.1.2
Sampling of Products :
—
Ensure nylon rope/cord is not used for sampling/gauging which is to be lowered into product tanks.
—
Ensure no personnel is allowed on tank roof for gauging / sampling during product transfer unless Dip pipes extend to bottom of tanks. Use only mechanical gauges for ascertaining product transferred during transfer operations otherwise.
—
Ensure gauging/sampling of tank after product transfer is done only after relaxation time of 30 min. unless Dip pipes extend to bottom of tank.
6.2
TANK TRUCKS, TANK CARS, FUDERS :
6.2.1
Loading/unloading operations in Tank Wagon Gantries :
—
Ensure proper structure.
—
Ensure tank wagons are electrically Note 1 bonded to gantry structure.
—
Ensure that the tank cars are fully bonded with the chassis for electrical continuity.
—
Ensure rails on which tank wagons stand are effectively earthed.
—
Ensure rail siding is insulated/ isolated from main running track.
—
Ensure piping / header systems are effectively bonded and in the case of rake, unloading hose point.
SPECIFIC GUIDELINES FOR CONTROL OF STATIC ELECTRICITY
The following is a list of the specific guidelines developed to avoid electrostatic sparking in the presence of a flammable vapour-air mixture. (Refer section 8 for the definition of various product classification, for the understanding / application of following text) 6.1
STORAGE TANKS :
6.1.1
General :
—
Ensure earthing of tanks (Refer Section 7 for details)
—
Ensure no metal objects/apurterances projecting from roof/shell plates which will attract highly charged spots in fuel for dissipation.
—
—
—
—
Ensure reduced rate of flow initially into tank/vessel until fill point/nozzle is completely submerged in fluid (filling rate initially restricted to 1 mtr. per second). Ensure that all tanks are provided with Dip pipes extending to tank bottom. If Dip pipes are not provided, give a relaxation time of 30 minutes before sampling/gauging. Ensure that only gauge tapes with earthing provision are used for gauging. Ensure periodic checking and recording of earthing test for tanks and piping systems are maintained.
13
grounding
of
gantry
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—
Ensure use of continuous bonded decanting hoses.
electrically
—
For tankwagon loading, ensure loading hose is electrically bonded with tank wagon manhole cover.
—
Ensure splash filling is avoided for all white oil products, LDO and low intermediate and high vapour pressure products by filling only wagons fitted with fill pipes.
—
—
Ensure gauging/sampling of tank trucks, tank cars or fuders after product transfer is done only after relaxation time of 10 minutes for ATF or 5 minutes for others, unless Dip pipes extend to bottom of the tank trucks or tank cars.
Loading/Unloading Tanktruck Gantries :
—
Ensure use of electrically continuous hoses having jumper wire between flanges coiled around hose.
—
Ensure electrical bonding of wagon with under carriage for electrical continuity.
—
For transfer mixing operations, ensure pumping rates are reduced to 0.5 metre per second until fill lines/nozzles is completely submerged in product. This is particularly important when mixing gasoline/ kerosene/ HSD/ATF.
—
Ensure gauging/sampling of tank trucks after product transfer is done only after relaxation time of 10 mins for ATF or 5 min for others; unless Dip pipes extend to bottom of the tank truck.
6.3
SMALL CONTAINERS (DRUMS, CANS)
—
Protective bonding is not required if containers are filled through a closed system.
—
Protective bonding is required when fill open containers where the product to be handled has a flash point below 54.5 Deg.C (130 Deg.F) or, in the case of a higher flash point product, when it is heated to within 6.0 Deg. C (15 Deg.F) of its flash point. The purpose is to keep the nozzle and container at the same electrical potential, thus avoiding a possible static spark in the area of a flammable mixture.
—
Provide 30 seconds relaxation between a filter and a container where ‘intermediate vapour pressure products are handled.
—
Small containers made up of plastic or other non-conductive materials should not be used for filling of MS, Naphtha, Kerosene, Diesel etc.
6.4
LEAK Y LPG CYLINDERS
Continuity tests for bonding across the piping Note 1 shall be carried out.
6.2.2
—
—
Operations
In
Switch loading operation to be avoided. In switch loading the high flash product eing loaded into the tank car partially absorbs the vapour from the previous load of low flash product. Thus, in switch loading, the vapour air mixture in the compartment becomes flammable as the tank car is loaded & static sparks can ignite the flammable mixture causing an explosion. For transferring small quantities of product from tank trucks (for correcting dip etc.) do not use plastic bucket or metallic bucket with plastic/plastic coated handles.
Leaky LPG Cylinders should not be turned upside down for speedy evacuation. This can create static charge generation hazard due to two phase flow (liquid and vapour) charge and this charge may lead to fire if this charged stream hits any metallic object, which is not properly earthed. 6.5
TANK CLEANING Introduction of steam into gassy tanks should be avoided. Washing gassy tanks by means of gas oil, or other hydrocarbons using tank cleaning jets should be avoided. Water washing is safe from a static electricity stand-point. However, there should be no insulated conductive objects within the tank.
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Air educators used for gas freeing tanks should be bonded to the tank. 6.6
Accumulation of static charges on the belt can be eliminated through the use of a conductive belt or by making it conductive through use of belt dressings which are available for this purpose.
SYNTHETIC FIBER CORDS Synthetic Fiber Cords can cause static electricity to be generated when they are allowed to run rapidly through an operator’s gloved hand, such as is practised sampling while sampling, dipping, gauging, etc. Since the charge generating cannot be prevented, hazards must be combated by preventing the charge from accumulating to too high a potential. Therefore, if synthetic fiber cords are used, two conditions must be fulfilled : a)
the operator must be adequately earthed (one mega ohm).
b)
the sampling etc. equipment must be of non-conductive material.
These dressings must be renewed frequently to be considered reliable or effective. General practice has been to avoid the use of flat belts in hazardous area. There is less concern with Vee belt drives as they are less likely to develop static charges than flat belt. 6.8
A great may fabrics under favarouble conditions may generate static electricity. This may occur when the fabrics are brought into contact with outer materials and then separated or when rubbed on various substances. Most synthetic fabrics (Nylon, Orlon, Dacron, Rayon, etc) are somewhat more active generators than natural fabrics. Both rubber and leather soled shoes generate static electricity when dragged against dry carpeting or other non-conductive surfaces during period of low humidity. Such potentialities should be recognised and prudence exercised on any occasion when flammable vapours are present.
Natural fibers such as sisal and manila have sufficient conductivity to prevent the operator from becoming charged by handling it. Thus condition (a) becomes unnecessary, and it is recommended that condition (b) remains & should be adhered to. If the sampling, gauging, dipping, etc., equipment is a conductor, the cord must be conductive, e.g. a metal wire. Metal chains should not be used instead. 6.7
WEARING APPAREL :
6.9
SAND OR SHOT BLA STING : In sand or shot blasting operations, static electricity is generated by the sand or shot flowing through the blasting machine and hose. The sweeping effect of the air prevents flammable concentration from occurring within the stream pattern. Bonding should be provided between sand or shot blast nozzle and the work surface. The work surface should be grounded. Sparks have been observed jumping form the rubber hose to grounded objects during sand or shot blasting. This, care should be exercised so that the hoses will not be passed through areas where flammable mixtures exist. The atmosphere around the tank to be blasted and within 15 meters of the sand or shot blasting operation must be gas free. When the tank containing a product has to be sand or shot blasted externally, during the whole period of
BELT : Belt made of rubber, leather or other insulating material, running at moderate or high speeds can generate considerable quantities of static electricity. Generation occurs when the belt separates from the pulley and charges will occur on the pulley (regardless of whether it is conducting or non-conducting) as well as on the belt. if pulley is made of conducting materials, the charge normally will be dissipated through the shaft and bearing to the ground and offer no ignition hazard. In some case however, where the machinery frame is insulated or the bearings are composed of insulating materials such as Nylon, bonding or grounding may be required.
15
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operation there shall be no pumping into or out of the tank in question or those adjacent to it which contain products with a flash point below 51.5 Deg. C. Tanks containing gasoline or any product for which the vapour space tests more than 20 % of the lower explosive limit must be emptied and rendered gas free before sand or shot blasting. If the vapour content of the space above the oil is less than 20% of the lower explosive limit, sand or shot blasting may be done on all external surfaces including the roof. The air intake to the sand or shot blasting equipment must be in an area free from combustible vapours.
7.0
to be used for bonding between tank truck under discharge and receiving tank pipeline. 7.3
BARGE/TANKER JETTY OPERATIONS :
—
6 mm Sq. braided copper wire with crocodile clips on either side are to be used for bonding between barges/tankers under loading/discharge at jetty.
-
Jetty pipeline to be suitably earthed as indicted for tanktruck gantry.
7.4
PIPELINES/PUMPS :
—
Running pipelines are to be bonded with loading gantries by running copper strip jumpers suitably bolted to the flanges.
—
The gantry structure to be suitably earthed in earthing pits of standard specifications (as per electrical installations and number of earthings also to be as per Standards ISNote 1 3043 & IS-7689 and OISD-STD-108).
EFFECTIVE BONDING / EARTHING SYSTEMS: Recommended earthing & bonding systems are given below with specifications:
7.1
FOR TANK WAGON UNLOADING GANTRY:
LOA DING
/
—
Continuity between rail and gantry shall be ensured by checking at a suitable frequency. Note 1
—
The gantry structure to be suitably earthed in earthing pits of standard specifications (as per electrical installations and number of earthings also to be as per standards ISNote 1 3043 & IS-7689 and OISD-108.
—
Tankwagon siding to be insulated from main running track.
7.2
TANKTRUCK LOADING AND UNLOADING GANTRY :
—
For the gantry 6 mm Sq. braided copper wire with one end firmly bolted to the gantry and the other end provided with G.I crocodile clips are to be used, the crocodile clips being attached to the tank-truck under loading or discharging.
—
The gantry to be suitably earthed as indicated for tankwagon gantry.
—
During tanktruck discharge at retail outlets, 6 mm Sq. braided copper wire of suitable length with crocodile clips on either side are
7.5 —
7.6 —
STORAGE TANKS: All storage tanks are to be earthed separately as per electrical specifications "IS-3043-1966, IS-7689 - 1994 and OISDNote 1 STD-108. SAMPL ING /GAUGING : For sampling jars to be inserted into product tanks, use only manila ropes. When the filling nozzle is in electrical contact with the container and will remain so throughout the filling operation, no special bond is required.
7.7
FILLING SMAL L CONTAINERS When the filling nozzle may not be, or remain, in continuous electrical contact with the container, the container shall rest on a metal base-plate while being filled. This baseplate shall be bonded to the supply piping. If the filling nozzle is inherently bonded to the supply piping, as by the use of metallic hose or pipe, no further bond is required. If the filling nozzle is not inherently
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17
bonded to the supply piping, as when a non metallic hose or pipe is used, an additional bond shall be provided between the nozzle and the supply piping.
8.1
Certain petroleum products such as crude oil, residual fuel oil, asphalt (both penetration and cut back), bunker C and residual products with Conradson carbon above 1%, and water soluble products such as alcohol have a high conductivity and, therefore, do not accumulate an electrostatic charge. These liquids are classified as non-accumulators.
Bonding is not needed around flexible metallic joints or swivel joints. 8.0
NON-ACCUMULA TORS :
CLA SSIFICATION OR PRODUCTS. The guidelines covered in Section-6 are based on avoiding an electrostatic discharge in the presence of a flammable vapour. If an electrostatic charge cannot be generated of accumulated, or if a flammable vapour-air condition cannot exist where sparking might occur, the precautions are relatively simple. However, if static electricity generating and accumulating mechanism are present and a flammable vapour-air mixture can exist, then detailed precautions must be observed.
8.2
ACCUMULATORS : Distilled petroleum products, including petroleum solvents, are generally considered electrostatic accumulators since they have a low conductivity. (Refer Section 3.4 Conductivity). Methods for classifying the products and examples in each category are as follows:
8.3
LOW VAPOUR-PRESSURE PRODUCTS: Low vapour-pressure products are those that are handled at a bulk liquid temperature at least 8 Deg. C (15 Deg. F) below their flash point. This classification usually includes those products with flash points above 37.8 Deg. C (100 Deg. F). Products in this classification include heating oil, kerosene, TF-1 or JP-1, diesel oil and special solvents.
Therefore, to apply the guidelines contained in Section-6, it is first necessary to classify the petroleum hydrocarbon or product into one of the categories listed below. For ease of categorising, example have been listed in each case. These examples, however, are not all-inclusive, and it is necessary to classify each product. It must also be pointed out that examples listed are one the basis of normal handling temperature in moderate climatic zones. If products are heated or cooled, the classification may change. Therefore, at specific locations the classification example may change if sub-stantial changes occur in product handling temperature. The chart in Appendix-’D’ provides a means for determining the temperature limits between which a flammable vapour-air mixture can occur. The temperature referred to is the bulk liquid temperature, not atmospheric. Since this charge is on a calculated basis, it is suggested that about a 3 Deg. C (5 Deg. F) safety margin be used when applying it.
These products do not represent a significant electrostatic ignition problem since the environmental condition does not produce a flammable vapour unless they are heated above their flash point, contaminated with high or intermediate vapour-pressure products, or loaded into tanks where a flammable vapour-air mixture might exist from previous usage. An example of the latter case is heating or furnace oil which is loaded into a tank truck which previously contained gasoline. This is commonly called “Switch Loading”. If a low vapour-pressure product is heated, contaminated, or loaded into a tank having flammable vapour-air mixture, it must be field as an i ntermediate vapour pressure.
High vapour pressure products, such as LPG and other compressed or liquefied gases, which are handled in a closed system, are excluded from this classification system.
8.4
INTERMEDIATE PRODUCTS:
VAPOUR-PRESSURE
Intermediate vapour-pressure products are those that are likely to produce a 17
OISD-110
18
flammable vapour-air mixture in the vapour space of a tank. Under normal liquid handling temp-eratures between about 2 Deg. C ( 35 Deg. F) and 37.8 Deg. C (100 Deg. F), flammable liquids having both a Reid Vapour Pressure below 0.34 Kg/cm Sq. abs (5.0 psia) & a flash point below 37.8 Deg. C (100 Deg.F) will fall in this classification. Examples of products in this classification are TF-4 or JP-4, and solvents such as benzol, toluol, and xylol, Contaminated, heated, or “switch loaded’ low vapour-pressure products can be in this classification as well as high vapourpressure products handled below about 2 Deg. C ( 35 Deg.F) 8.5
HIGH VAPOUR - PRESSURE PRODUCTS: High vapour - pressure products are those products whose Reid Vapour pressure is above 0.34 kg/cm Sq. abs (5.0 Psia). Products in this classification are aviation and motor grade gasolines, high vapourpressure naphtha and the like. If a high vapour-pressure product is handled at a bulk temperature below about 2 Deg.C (35 Deg.F), its classification could change to an intermediate vapour pressure product. The charge in Appendix-should be referred to in these cases to determine if a flammable vapour-air mixture will occur. If the bulk temperature of a high vapourpressure product is such that a flammable mixture can occur, it must be classified as an intermediate vapour pressure product.
9.0
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. (I)
N E C 1979, Vol.14
(ii)
Fire Protection Manual for Hydrocarbon Processing Plants by Vervalin.
(iii)
Fire and Safety Manual - Refineries & Petrochemical Panel - National Safety Council
(iv)
N F P A - 1986
(v)
IS - 3043 - 1966
(vi)
IS - 7689 - 1974
(vii)
API - 2003
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APPENDIX : ‘A ’ INDUCED CHARGE AND ITS BEHAVIOUR
Charged Conductor
Uncharged Conductor
Start Induced Opposite Bound Charge
Induced Like Free Charge
Insert Charged Ball Opposite Bound Charge Remains
Like Charge Removed by
Temporary Ground
Ground Free Opposite Charge
Original Charge
Distant
Ground
Voltage
Remove Ball
19
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20
APPENDIX : ‘B ’ EFFECT OF FUEL CONCENTRATION ON MINIMUM SPARK IGNITION ENERGY
e n e i d a t u h 3 , 1 1
e n e l y t e c A
t - c o u l c d o y r C e n a t e c A l y h t E e n l l e y h t t e e c M A
l y l e n h o t y t e h t e M E K
e n a t n e P n
o i t a R e c n e l a v i u q E
e d i x O e n e l y p o r P
e d i x O e n e l y h t E
n e g o r d y H
e n a p o r P
Minimium Spark Ignition Energy Millijoules
e d i f l u s i D n o b r a C
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21
APPENDIX : ‘C’ PIPELINE DIAMETERS VERSUS MAXIMUM FLOW VELOCITY
I TANK BEING LOADED LENGTH OF PIPELINE
EFFECTVELY INFINITE TANK DIMENSIONS (METRES) 61-49-126 ULLAGE (METRES) 03 PRODUCT CONDUCTIVITY 01 500
r e t e m a i D e n i l e p i P
Maximum Velocity (Metres/Sec.
21
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APPENDIX : ‘D’ FLAMMAB ILITY CURVE
a i s P n i e r u s s e r P r u o p a V d i e R
Product Temperature in degrees Fahrenheit (°F)
The Relation ship b etween temperature, Reid Vapour Pressu re, and Flammable limists of petroleum products at sea level. Example : With a product such as Hexane (vapour pressure = 5.0), the vapour space of a tank will be within the flammable limits for product temperatures of about –28° F to + 26°F, or when handling Heptane (vapour pressure = 1.6) at a product temperature of 55°F, the vapour is within flammable limits and care to prevent static discharge should be taken.
OISD-110
23
NOTES
23
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NOTES
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NOTES
25
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