Sr. Number : OISD/DOC/2018/01
Page No. I
OISD-RP-110
First Edition - Aug, 1990
First Amendment - Aug, 1999
First Revision - Oct, 2018
FOR RESTRICTED CIRCULATION ONLY
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RECOMMENDED PRACTICES
ON
STATIC ELECTRICITY
Prepared by:
COMMITTEE ON STATIC ELECTRICITY
Oil Industry Safety Directorate
Government of India
Ministry of Petroleum & Natural Gas
8th Floor, OIDB Bhavan, Plot No. 2, Sector – 73, Noida – 201301 (U.P.)
Website: www.oisd.gov.in
Tele: 0120-2593800, Fax: 0120-2593802
Sr. Number : OISD/DOC/2018/01
Page No. II
PREAMBLE
Indian petroleum industry is the energy lifeline of the nation and its continuous performance
is essential for sovereignty and prosperity of the country. As the industry essentially deals
with inherently inflammable substances throughout its value chain – upstream, midstream
and downstream – Safety is of paramount importance to this industry as only safe
performance at all times can ensure optimum ROI of these national assets and resources
including sustainability.
While statutory organizations were in place all along to oversee safety aspects of Indian
petroleum industry, Oil Industry Safety Directorate (OISD) was set up in 1986 by
Ministry of Petroleum and Natural Gas, Government of India as a knowledge centre for
formulation of constantly updated world-scale standards for design, layout and operation of
various equipment, facility and activities involved in this industry. Moreover, OISD was also
given responsibility of monitoring implementation status of these standards through safety
audits.
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In more than three decades of its existence, OISD has developed a rigorous, multi-layer,
iterative and participative process of development of standards – starting with research by
in-house experts and iterating through seeking & validating inputs from all stake-holders –
operators, designers, national level knowledge authorities and public at large – with a
feedback loop of constant updation based on ground level experience obtained through
audits, incident analysis and environment scanning.
The participative process followed in standard formulation has resulted in excellent level
of compliance by the industry culminating in a safer environment in the industry. OISD –
except in the Upstream Petroleum Sector – is still a regulatory (and not a statutory) body but
that has not affected implementation of the OISD standards. It also goes to prove the old
adage that self- regulation is the best regulation. The quality and relevance of OISD
standards had been further endorsed by their adoption in various statutory rules of the land.
Petroleum industry in India is significantly globalized at present in terms of technology
content requiring its operation to keep pace with the relevant world scale standards &
practices. This matches the OISD philosophy of continuous improvement keeping pace with
the global developments in its target environment. To this end, OISD keeps track of
changes through participation as member in large number of International and national
level Knowledge Organizations – both in the field of standard development and
implementation & monitoring in addition to updation of internal knowledge base through
continuous research and application surveillance, thereby ensuring that this OISD Standard,
along with all other extant ones, remains relevant, updated and effective on a real time basis
in the applicable areas.
Together we strive to achieve NIL incidents in the entire Hydrocarbon Value Chain. This,
besides other issues, calls for total engagement from all levels of the stake holder
organizations, which we, at OISD, fervently look forward to.
Jai Hind!!!
Executive Director
Oil Industry Safety Directorate
Sr. Number : OISD/DOC/2018/01
Page No. IV
FOREWORD
The Oil Industry in India is over 100 years old. As such, various practices have been in vogue
because of collaboration/ association with different foreign companies and governments.
Standardization in design philosophies, operating and maintenance practices remained a grey
area. This coupled with feedback from some serious accidents that occurred in the past in India
and abroad, emphasized the need for the industry to review the existing state-of-the-art in
designing, operating and maintaining of 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, standardizing 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.
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The original document on “Recommended Practices on Static Electricity” was prepared by the
functional committee in Aug 1990 and later amended in Aug 1999. This document was prepared
based on the accumulated knowledge and experience of industry members and the various
standards, codes and practices. In view of revision in national/ international standards like API
2003, NFPA 77, IS 7689 etc., various sections of this standard have been revised and new
sections added.
Note 1
in superscript indicates the changes / modifications / additions as approved in 17 th Safety
Council Meeting held in July, 1999.
The figures and annexures used in the document are representative in nature.
We, at OISD, are confident that the provisions of this standard, when implemented in totality,
would go a long way in ensuring safe operation of the target group of locations.
Needless to mention, this standard, as always would be reviewed based on field level
experience, incident analysis and environment scanning. Suggestions from all stake holders may
be forwarded to OISD.
Sr. Number : OISD/DOC/2018/01
Page No. V
NOTE
Oil Industry Safety Directorate (OISD) 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 only to supplement and not to replace the prevailing statutory
requirements of PESO, DGMS, Factory Inspectorate or any other Government body which
must be followed as applicable.
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Where ever Acts/ Rules/ Regulation and National/ International Standards are mentioned in the
standard, same relates to in-vogue version of such documents.
Sr. Number : OISD/DOC/2018/01
Page No. VI
FUNCTIONAL COMMITTEE
RECOMMENDED PRACTICES
ON
STATIC ELECTRICITY
LIST OF MEMBERS
Name
Designation / Organisation
Status
S/Shri
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
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W.D. Lande
In addition to the above several experts from industry contributed in the preparation, review and finalisation of the
document.
Sr. Number : OISD/DOC/2018/01
Page No. VII
FUNCTIONAL COMMITTEE
RECOMMENDED PRACTICES
ON
STATIC ELECTRICITY
LIST OF MEMBERS
(2018)
Sl. No. Name
Shri Rahul Prashant
Shri Amit Kumar
Shri MK Sahu
Shri Ravi Ayyagari
Shri Amol Joshi
Shri Dushyaant Kumar
Shri Ashish Saxena
Shri Sanna Devanna
Shri S K Sinha
Shri DD Sarkar
Shri Deepankar Sarma
Shri Mukul Singh
Shri Rana Raghubir Singh
Shri M M Prasad
Shri A Ramachandran
17
18
Shri Parmod kumar
Shri M. Vamshi Krishna
IOCL
ONGC
EIL
RIL
Essar
GAIL
IOCL
MRPL
IOCL
BPCL
OIL
IOCL
HPCL
HPCL
Cairn, Oil and gas
Vedanta Ltd.
OISD
OISD
Position in the
Committee
Leader
Member
Member
Member
Member
Member
Member
Member
Member
Member
Member
Member
Member
Member
Member
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1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Organization
Member
Member Coordinator
In addition to the above several experts from industry contributed in the preparation, review and finalisation of the
document.
Sr. Number : OISD/DOC/2018/01
Page No. VIII
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
CONTENTS
SL. NO.
DESCRIPTION
PAGE NO.
Introduction
1
1.1
Scope
1
2.0
Theory of Static Electricity
1
2.1
Background
1
2.2
Generation
2
2.3
Rate of Generation
3
2.4
Accumulation
3
2.5
Conductivity
3
2.6
Static Discharge
4
2.7
Sparks and Arcs
5
Sparking Potential
5
Ignition Energy
5
Common Sources of Static Electricity
7
Guidelines for Control of Static Electricity
7
General
7
Spraying, Splashing & Misting
8
Agitation and Mixing
8
Water
9
Flow Velocity
9
Filters
10
Gauging and Sampling
11
Insulated Conductive Objects
11
Projections and probes
12
Bonding
12
2.8
2.9
3.0
4.0
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
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1.0
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Page No. IX
CONTENTS (Continued)
DESCRIPTION
4.11
Grounding
13
4.12
Use of Additives
13
4.13
4.14
4.15
5.0
Internal Coating
Powder and Dust
Vacuum Truck operation (Gully Sucker)
Specific Guidelines for Control of Static Electricity
14
14
15
15
5.1
Storage Tanks
16
5.2
Tank Trucks and Tank Wagon
17
5.3
Small Containers (Drums, Cans)
18
5.4
Leaky LPG Cylinders
18
5.5
Tank Cleaning
18
5.6
Belt
19
5.7
Wearing Apparel
19
5.8
Sand or Shot Blasting
19
6.0
Effective Bonding/Earthing Systems
20
Tankwagon Loading/Unloading Gantry
20
Tanktruck loading/ unloading Gantry
20
Barge/Tanker Jetty Operations
20
Pipelines/Pumps
20
Storage Tanks
21
Filling small Containers
21
Classification of Products
21
Non-accumulators
21
Accumulators
22
7.2.1
Low Vapour-Pressure Products
22
7.2.2
Intermediate Vapour-Pressure Products
22
7.2.3
High Vapour-Pressure Products
22
8.0
References
23
Appendix „A‟
24
Appendix „B‟
25
Appendix „C‟
26
Appendix „D‟
27
6.1
6.2
6.3
6.4
6.5
6.6
7.0
7.1
7.2
PAGE NO.
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SL. NO.
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OISD – RP – 110
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RECOMMENDED PRACTICES ON STATIC ELECTRICITY
1.0
INTRODUCTION
It is not possible always in a plant to prevent the formation of explosive mixture, so any possible source of
ignition must be excluded from these areas. Sparks and arcs which result from switches, starters, relays &
similar devices in hazardous area are rendered harmless by explosion-proof installations. However, there
exists an ever present fire hazard in the processing industries from ignition which may arise from static
sparks.
1.1
SCOPE
The purpose of this document is to assist oil and gas industry in reducing fire hazard due to static
electricity by presenting a discussion on the nature and origin of static charges, the general methods of
mitigation and recommendations in certain specific operations for its dissipation.
2.1
THEORY OF STATIC ELECTRICITY
Background
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2.0
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 from removal of electrons from the atom of one material (leaving it with positive charge)
and absorption of electrons on the second material (negative charge) during physical separation of the two
materials (Appendix: A)
Both materials remain charged if they are well insulated electrically. The generation of static electricity can
not be prevented absolutely, because its intrinsic origins are present at every interface.
In insulating materials, electrons are tightly bound to nuclei of the atom and are not free to move. Non
conductive glass, rubbers, plastic resins, dry gases, paper, petroleum fluids are such examples.
A static electric charge will accumulate when separation rate of charges is more than the combining rate of
charges. When recombining of charge takes place through electrical resistive path, the process proceeds
at a finite rate, called charge relaxation time or decay time. It is expressed as follows:
- (t/Ʈ)
Q = Q0 e
Where Q = remaining charge
Q0 = initial charge
t = elapsed time
Tau (Ʈ) = relaxation time constant, which is equal to R x C (resistance and capacitance of the material)
Relaxation time is a measure of the time it takes for the charge to leak away from 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.
Zero charge is only approached (but not reached) in four or five times the relaxation time, Ʈ (tau). Ʈ is
approximately equal to 18 divided by the conductivity of the liquid hydrocarbon in picomhos per meter.
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 2
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
For example, if a product has a conductivity of 1 picomho per meter, Ʈ is 18 seconds. Thus zero charge
will be approached in 90 seconds. If the conductivity were raised to 100 picomhos per meter, Ʈ would be
only 0.2 second. So practically zero charge condition would remain after 1 second.
2.2
GENERATION
Generation of static electric charge usually occurs whenever a liquid, for instance a hydrocarbon, flows
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.
2.2.1
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Static electricity is generated by the separation of like or unlike bodies. Electro-static charges, positive &
negative, always occur in pair and are developed when any two 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 of the bodies or because
at least one of the bodies is an insulator. Petroleum products which have a low conductivity can serve as
insulators
Generation due to fluid 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 ions are separated from the free ions by turbulence.
Figure below show how the charges are mixed with the liquid and carried downstream.
Fig: charge generation due to fluid flow
The opposite charge is usually conducted throughout the metallic pipe wall, in the same direction
because of the natural attraction between opposite charges. Ionizable impurities, such as water, metal
oxide, or chemicals, increase the static generation characteristics.
The flow of electricity caused by the entrainment of charged particles in the flowing fluid is known as the
streaming current. If this charged stream enters a 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. If the
tank is grounded, this charge on the outside surface will flow to ground.
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
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Page No. 3
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
2.2.2
Generation due to settling.
Strong electrostatic fields may also be generated by droplets of solid 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.
2.3
RATE OF GENERATION
2.4
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The generating mechanism is related primarily to rate 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 limiting 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.
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. In cases where charge dissipates by moving across the surface of
equipment or other solid bodies, humidity can have a significant effect by changing the conductivity of the
surface. During periods of normal humidity (50 percent or more), an invisible film of water provides an
electrical leakage path over most solid insulators.
Where charges dissipate by moving through liquids or solids or over the surface of a liquid, humidity has
no effect. The amount of electrostatic charge which may accumulate on an insulated body depends upon:


2.5
The rate at which the static charge is being generated.
The resistance of paths by which the charge leaks off (dissipates).
CONDUCTIVITY
The capability of a substance to transmit electrostatic charges is called 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 12 ohm to the power of minus 1 per meter or in the inverse from as resistivity (1 resistivity
unit = 10 to the power of 12 ohm cm). Metals have very high conductivity and oils have low conductivity.
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 conductivity of 10 picomhos per meter below which static
charges may not be dissipated easily by earthing and bonding.
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 4
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
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 of
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 times the half value time
may be assumed to be adequate for charges to “relax”. The Table-I shows the relationship between
conductivities and Relaxation time of various liquids. Refer NFPA 77 for conductivity and relaxation time of
additional liquids.
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.
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2.6
TABLE - I
LIST OF CONDUCTIVITIES & RELAXATION TIME OF VARIOUS LIQUIDS
Liquid
Conductivity
(pS/m)
Relaxation time constant
(sec)
Benzene
Gasoline/ MS
Diesel
Xylene
Toluene
Lube Oil (base)
Lube oil (blended)
Crude oil
Gasoline (straight run)/
Naphtha
Kerosene
Methyl isobutyl ketone
0.005
10 to 3000
0.5 to <50
0.1
<1
0.1 to 1000
50 to 1000
>1000
~ 0.1
~100 (dissipation)
1.8 to 0.006
36 to >0.36
~100
21
180 to 0.018
0.36 to 0.018
<0.018
~100 (dissipation)
1 to 50
0.39 to 19
6
<5.2 x 10
-5
>2.2 x 10
Table 1: Conductivity and relaxation time constants of typical liquids – for additional liquids refer NFPA 77
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
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Page No. 5
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
2.7
SPARKS AND ARCS:
A spark is essentially a transient phenomenon & can be described as the passage of an electric charge
across a gap between two 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.
A spark results from the sudden breakdown of the insulating strength of a dielectric (such as air) that
separates two electrodes of different potentials. This breakdown produces a transient flow of electricity
across the spark gap and is accompanied by a flash of light, which indicates a high temperature. In
contrast to a spark, an arc is a low-voltage, high-current electrical discharge that occurs at the instant two
points, through which a large current is flowing, are separated.
2.8
SPARKING POTENTIAL:
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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 minimum sparking voltage is approximately 350 volts for the shortest
measurable length of gap of 0.01 mm. 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.
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 difference is developed
between the tank vehicle and the downspout due to the static charges generated during the flow of
product into the compartment.
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.
Under the continuous influence of a charge generating mechanism, the voltage of 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 to
the rate at which the charge is being placed upon the insulated body and a stabilized condition will be
reached.
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.
2.9
IGNITION ENERGY
The mere fact that a spark results from high voltage does not mean that ignition of a flammable mixture
will occur. In order to initiate combustion, sufficient energy must be transferred from the spark to the
surrounding flammable mixture.
Experiments under the most favourable conditions have ignited petroleum vapour-air 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.
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
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Page No. 6
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
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:
a)
b)
c)
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 not be available in its entirety 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.
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Any combustible material should be evaluated for its potential as an ignitable atmosphere in the
presence of discharges of static electricity. This evaluation requires determining the Minimum ignition
energy of the material. Table-II provides typical gases and vapours and the lowest values of their
minimum ignition energies (MIEs), stoichiometric composition and flammable limits. Refer NFPA 77 for
MIE related data of additional Gas or Vapours. The effect of fuel concentration on Minimum Spark
Ignition Energy is presented in Appendix: B.
TABLE - II
Gas or Vapour
@ STP
Lowest MIE
(mJ)
Stoichiometric
Mixture
(% by volume)
Flammable
limits
(% by volume)
Acetone
Acetylene
Ammonia
Benzene
Butane
Carbon disulphide
Ethane
Ethanol
Ethylene oxide
Hydrogen
Hydrogen sulphide
0.19
0.017 @ 8.5%
680
0.2 @ 4.7%
0.25 @4.7%
0.009@7.8%
0.23@6.5%
0.23
0.065@10.8%
0.016@28%
0.068
4.97
7.72
21.8
2.72
3.12
6.53
5.64
6.53
7.72
29.5
-
2.6-12.8
2.5-100
15-28
1.3-8
1.6-8.4
1-50
3-12.5
3.3-19
3-100
4-75
4-44
Methane
0.21@8.5%
9.47
5-15
N-Pentene
0.24
2.55
1.5-7.8
Propane
0.24
4.02
2.1-9.5
Propylene
0.18
-
2-11
Toluene
0.24@4.1%
2.27
1.27-7
Xylene
0.2
1.96
1-7
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
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Page No. 7
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
3.0
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/ ventilations & 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 micro-filters made of paper/felt elements.
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g) Hydrocarbon liquids flowing at high velocities in pipes/nozzles/fittings, etc.
h) Spraying/splashing and misting, such as:

Free fall of liquid droplets through vapour spaces.

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
loaded into a tank which last carried gasoline.
l)
Powder and dust
m) Loading/unloading operations of Tank Trucks/Tank Wagons
n) Cylinder Bottling operations & Evacuation process
o) Synthetic / Nylon clothes
p) Gauging & Sampling
q) Two phase flow (liquid & gas)
4.0
GENERAL GUIDELINES FOR CONTROL OF STATIC ELECTRICITY
4.1
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 generation, accumulation or sparking. This
information forms the basis for establishing the specific guidelines contained in Section 5.0
In order for an electrostatic charge to be a source of ignition, four conditions must be fulfilled:
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 8
RECOMMENDED PRACTICES ON STATIC ELECTRICITY




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 static electricity can be controlled by the following methods:
1) Removing the ignitible mixture from the area where static electricity could cause an ignitioncapable discharge
2) Reducing charge generation, charge accumulation, or both by means of process or product
modifications
3) Neutralizing the charges, the primary methods of which are grounding isolated conductors and
air ionization
4.2
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4) Operating outside the flammable range
SPRAYING, SPLASHING 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
potential sparking may occur across the liquid surface or to the shell of the tank.
Consequently, spray or splash filling or free-fall 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.
Charged droplets, such as in a heavy mist, may 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. Tank truck
compartments are not sufficiently large to form mist clouds of sufficient size.
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.
4.3
AGITATION AND MIXING
The generation of an electrostatic charge in hydrocarbons is 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
hydro-carbon 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.
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.
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 9
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
4.4
WATER:
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.
Since it is not uncommon to have water in hydrocarbons resulting from 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.
4.5
FLOW VELOCITY
4.5.1
In Tanks
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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 below 1 m/s (3 ft/ second) initially until the fill pipe is submerged up to a height of
twice the diameter of the pipe or 61 cm, whichever is less. For easy reference, permissible flow rates for
initial filling @1 m/s are given below:
Size (in mm) of Inlet Pipe
300
250
200
Max. Flow (cum/ hr)
246
168
109
150
100
80
59
27
25.5
After the low initial filling rate period, a maximum flow can be established in accordance with the following:
1. For uncontaminated single-phase liquids, the inlet flow velocity should be restricted to 1 m/s until the
fill pipe has been submerged to a depth of twice the inlet pipe diameter. Fill rate can then be increased
up to 6 m/s.
2. For multiphase or contaminated liquids and where it cannot be ensured that water bottoms will not be
disturbed, the inlet flow velocity should be restricted to 1 m/s during the entire fill cycle.
In case of a floating roof, observe the 1 m/s velocity limitation until the roof become buoyant. As industry
practice, the floatation of the tank roof is generally achieved by using water as filling media. A flow rate
limit of 6 m/s is to be maintained for floating roof tanks to avoid damaging the roof by too rapid movement.
When the material is static accumulator and contains a dispersed phase, such as entrained water droplets,
the inlet flow velocity should be restricted to 1 m/s throughout the filling operation.
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 10
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
4.5.2
In pipelines
In pipelines handling non-conductive petroleum products, the flow velocity should be restricted to the
values indicated in Appendix „C‟. If Water is entrained in the product, the velocity should be limited to 1
m/s.
4.5.3
Loading of Tank Truck/ Tank Wagon
No tank vehicle shall be loaded at a rate exceeding 1 m/s at the delivery end of the filling pipe until the
filling pipe is completely submerged in petroleum and thereafter the loading rate may be gradually
increased but it shall at no time exceed 6 m/s at the delivery end of the filling pipe.
FILTERS:
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.
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4.6
Deposits left on the filter elements from the fuel may have an increasing effect on their generating
capabilities throughout their service life.
If, after filtration, the liquid is discharged from the pipe into a container where the possibility of a
flammable mixture exists, specific precautions may need to be taken. When the pore or screen size of
the filter is larger than 300 microns (less than 50 mesh/in.), it is unlikely that hazardous levels of
electrostatic charge will be generated in the filter/screen. Therefore, no specific provision for downstream
relaxation is necessary.
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 provided by enlarging or
lengthening the piping downstream of the filter or by installing a relaxation tank. If a relaxation tank is
provided, it should not have a vapour space and baffles may be required to prevent by passing 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 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 vapour-air mixtures. However, since filters are such prolific
electrostatic generators, 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)
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 11
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
4.7
GAUGING AND SAMPLING:
Sampling and gauging devices shall be either completely conductive or completely nonconductive.
Conductive sampling and gauging devices shall not be used with a nonconductive lowering device like
cable, dry clean natural fiber rope etc.
Conductive sampling and gauging devices (including the sampling container and lowering device) shall
be properly bonded to the tank. Such bonding should be accomplished by use of a bonding cable or by
maintaining continuous metal-to-metal contact between the lowering device and the tank hatch.
Totally nonconductive hand gauging or sampling devices are those that do not contain any metallic
components, such as weights, caps or labels. Totally nonconductive hand gauging or sampling devices
might not retain the necessary high level of nonconductivity because of environmental factors (e.g.
moisture or contamination), therefore, the appropriate relaxation time is recommended when
nonconductive devices are used.
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Clean dry natural fiber rope (e.g. manila, sisal etc.) can be nonconductive when used in low humidity
conditions. When used, natural fiber ropes should maintain contact with the tank hatch because they
might be conductive if not kept clean and dry (free of moisture). Synthetic fiber (e.g. nylon and
polypropylene etc.) rope is not recommended because when synthetic ropes rapidly slip through gloved
hands for appreciable distances, such as into large tanks, an insulated person can become charged.
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
sufficient period of time (relaxation time) after all pumping into the tank or circulation within the tank has
ceased.
4.8
INSULATED CONDUCTIVE OBJECTS:
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 electrostatically 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.
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 12
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RECOMMENDED PRACTICES ON STATIC ELECTRICITY
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.
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 couplings shall be provided. Note 1
4.9
PROJECTIONS AND PROBES:
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 is electrostatically charged, incendiary sparking may occur. Following are some of
the examples of spark promoters:
1. Gauge rods or side wall probes which are not connected to the bottom
2. Sample container or thermometers which are allowed into the vapour space.
4.10
BONDING:
Bonding is used to minimize the potential difference between conductive objects, even where the resulting
system is not grounded. 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 because it provides a
conductive path through which the static charges can recombine. No charge, therefore, can accumulate &
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 13
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
no spark can occur. (Bonding of a tank or container has no effect on the liquid bulk charge within the tank
or container).
Static bond wires are usually comparatively large because of mechanical considerations; therefore, bond
wire resistance are low. Such low resistances however, are not needed for static dissipation because
electrostatic currents are usually in the order or microamperes (millionths of an ampere).
Bolted connections, stranded or braided wires, are adequate for static dissipation. Even though parts of
metallic fill pipe assembly form a continuous electrically conductive path, bond or jumper wires are needed
around flexible joints or swivel joints. All joints in pipelines, valves, plants, storage tanks and associated
facilities and equipment for petroleum shall be made electrically continuous by bonding or otherwise. 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 bond wire to be fixed to the
metallic loading rack rather than directly to the loading piping.
4.11
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Bonding should not compromise sections of pipe that are supposed to be isolated. For example, insulating
joints provided in pipelines protected with cathodic protection systems.
GROUNDING:
Grounding (i.e. earthing) equalizes the potential difference between the objects and the earth. 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 bypassed by grounding 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.
For earth resistance values, OISD-STD-137 shall be referred.
4.12
USE OF ADDITIVES:
Earthing alone may be insufficient to remove charges which have been accumulated in a liquid of low
conductivity. A non-conductive material can be made sufficiently conductive either by adding conductive
ingredients to its composition or by applying hygroscopic agents. Antistatic additives can also be mixed
with liquid and particulate streams for faster charge relaxation. However, it may be noted that increasing
the conductivity of the material will not eliminate hazards if the materials remains isolated from ground. 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 hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 14
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
4.13
INTERNAL COATING:
It is believed that a coat of paint, plastic coating, or layer of aluminium oxide on the inside of cargo or
storage tanks does not constitute an electrostatic hazard. Such films are not regarded as barriers to the
flow of static charges because their resistivity is of the same order of magnitude as the oil or because of
small bare areas (holidays) in the coating. Metal containers with internal coatings or linings up to 0.080
in. (2 mm) in thickness may be treated as an uncoated metal container.
POWDER AND DUST HANDLING
Powders include pellets, granules, dust particles, and other particulate solids. Pellets have diameters
greater than 2 mm, granules have diameters between 420 μm and 2 mm, and dusts have diameters of
420 μm or less. It should be noted that aggregates of pellets and granules often contain a significant
amount of dust. The movement of powders in industrial operations commonly generates static electricity.
The accumulation of these charges and their subsequent discharge can lead to fires and explosions.
Static charge can be expected any time a powder comes into contact with another surface, such as in
sieving, pouring, scrolling, grinding, micronizing, sliding, and pneumatic conveying. Electrostatic charge
accumulation in particulate solids can be minimized by decreasing the powder‟s bulk resistivity by
increasing the moisture content of the powder, by reducing conveying speed or throughput, or by
substituting processes that result in less particle contact per unit time (e.g., gravity transfer vs. pneumatic
conveying). The use of ionization to reduce accumulated charge is also effective for some applications.
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4.14
Fabric filters
As dusts are drawn or blown into a filtration system, they necessarily carry with them a static electric
charge, the magnitude of which depends on the characteristics of the dust and the process. The charge
remains on the dust and accumulates on the surfaces of the filters within the housing. It is therefore
important to keep all conductive equipment grounded to prevent the induction of this accumulated charge
onto conductive components that could have inadvertently become ungrounded. Such induction is
particularly true in the case of cage assemblies on which the filters are mounted. Filters and cages
should be engineered so that a positive ground connection is always ensured during maintenance. The
resistance between the cage and ground should be less than 10 ohms.
Pneumatic Transport Systems
Pneumatic transport of powdered material through pipes or ducts can produce a static electric charge on
both the product being transported and the conduit.
 Pipes and ducts should be metal and should be grounded.
 Equipment to which the conduits connect should be metal and grounded to dissipate the charge
impressed on it by the transport of the material.
 Where the use of pipe-joining methods or installation of piping components results in an interruption
of continuity of the ground path, jumper cable to be provided.
 Nonconductive pipe or ductwork should not be used.
 Short lengths of transparent plastic should not be used as flow visualizers.
Flexible Hose
Significant static charge can be generated where nonconductive hoses are used for solids transport.
 Non-conductive hoses should be used only in areas that are not hazardous.
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 15
RECOMMENDED PRACTICES ON STATIC ELECTRICITY

Conductive or static-dissipative hoses should be used for transport of combustible dust and in
situations where a flammable external atmosphere might exist. Such hoses should be adequately
connected to conductive end fittings and properly grounded.
Flexible Boots and Socks
Flexible boots and socks are commonly used for gravity transfer operations.
 Where combustible dusts are handled, the end-to-end resistance of boots and socks should be less
than 106 ohms, as measured using a megohmmeter.
 Flexible connections should not be depended on for a bond or ground connection between process
equipment. Separate bonding or grounding connections should be used.
4.15
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Bulk storage
Where powders are moved into bulk storage (e.g., silos, rail cars, trucks), the powder is compacted by
the force of gravity. The compaction process is accompanied by bulking brush discharge which can result
in surface flashes up to 1 meter in length. Use of nonconductive containers for storage of combustible
conductive powders should be avoided for bulk storage. Where this is not possible, grounding of the
powder should be provided, for instance, by inserting a grounded rod into the container prior to filling.
VACUUM TRUCK OPERATION (Gully Sucker)
Explosions and fires could occur during vacuum truck operations because of the following risk factors:
a) Ignition of a flammable atmosphere in the container being vacuumed;
b) Ignition of a flammable atmosphere generated in the areas around the vacuum truck
c) Ignition of a flammable atmosphere inside the vacuum truck.
Following precautions should be taken during vacuum truck (gully sucker) operations.
1. Vacuum hose should be checked for electrical continuity before use. Hoses constructed of conductive
material or thick-walled hoses with imbedded conductive wire should be used.
2. Thin wall, plastic hose with a spiral wound conductive metal spring backbone should not be used
because of the possibility of electrical breakdown through the thin plastic next to the metal spiral
3. The complete system needs to be bonded so that there is a continuous conductive path from the truck
through the hose and nozzle to the tank.
4. Portable, nonconductive containers should not be used as an intermediate collection vessel during
vacuum truck operations because of charge accumulation on the container. Only conductive
containers should be used and bonded to the hose nozzle and the container being drained.
5.0
SPECIFIC GUIDELINES FOR CONTROL OF STATIC ELECTRICITY
The following is a list of the specific guide-lines developed to avoid electrostatic sparking in the presence
of a flammable vapour-air mixture. (Refer section 7.0 for the definition of various product classifications,
for the understanding / application of following text)
Guidelines to avoid generation of static electricity for various equipment are as under:
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 16
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
5.1
STORAGE TANKS:
5.1.1
General:

Ensure earthing of tanks

Ensure no metal objects/appurtenances 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 before sampling/gauging.
 Ensure that only gauge tapes with earthing provision are used for gauging.
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 Conductive sampling and gauging devices (including the sampling container and lowering device)
should be properly bonded to the tank compartment or truck.
5.1.2

Ensure periodic checking and recording of earthing test for tanks and piping systems are maintained
in line with OISD-STD-137.

Avoid high velocity or splash filling, in all types of products, (low vapour pressure, intermediate
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 high speed mechanical mixtures should be avoided.
Sampling of Products:

Ensure synthetic 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.

A 30 minute waiting period is recommended before gauging or sampling large storage tanks, greater
than 37850 Litres if a gauge well is not being used. If a gauge well is not being used, and the tank or
vessel is smaller, the recommended waiting period before gauging or sampling can be lower: 5
minutes for tanks between 18925 Litres and 37850 Litres; 1 minute for tanks less than 18925 Litres.
Longer waiting periods may be appropriate for ultra-low conductivity liquids, such as very clean
solvents and chemical-grade hydrocarbons.
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 17
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
5.2
TANK TRUCKS AND TANK WAGON GANTRY
5.2.1
Loading/unloading operations in Tank Wagon Gantries:

Ensure proper earthing of gantry structure.

Ensure tank wagons are electrically bonded to gantry structure.


Ensure electrical bonding of wagon with under carriage 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 including unloading hose point (in case of rake) are effectively
bonded.

Ensure use of continuous electrically bonded decanting hoses.

For tank wagon 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 & high vapour
pressure products by filling only wagons fitted with fill pipes.

Ensure gauging /sampling of tank wagon/ tank truck 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
wagon

Continuity tests for bonding across the piping shall be carried out. Note 1
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Note 1
5.2.2
Loading/Unloading Operations in Tank truck Gantry:

Ensure use of electrically continuous hoses having jumper wire between flanges, coiled around hose.

Ensure that the Tank Truck is fully bonded with the chassis 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

Switch loading operation to be avoided. In switch loading the high flash product being loaded into the
tank truck 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 truck 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.
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 18
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
5.4
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.

Ensure earthing of Tank Truck body.
SMALL CONTAINERS (DRUMS, CANS)

Protective bonding is not required if containers are filled through a closed system.

Protective bonding is required when filling 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.
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5.3


Metallic cans may be safely filled through metallic spouts provided they are maintained in contact with
the can throughout the filling operation. In some cases, it is difficult to guarantee that the required
level of contact will be maintained throughout the entire filling operation. In these cases, bonding wires
should be provided between the fill pipe and the can.

Small containers made up of plastic or other non-conductive materials should not be used for filling of
MS, Naphtha, Kerosene, Diesel etc.
LEAKY LPG CYLINDERS
During evacuation of leaky LPG cylinders, it is to be ensured that the equipment such as evacuation gun
and evacuation stand used for the purpose is properly earthed.
5.5
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 using sprays should be done only in an inerted or nonflammable atmosphere.
Air eductors used for gas freeing tanks should be bonded to the tank.
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 19
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
5.6
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 cases however, where
the machinery frame is insulated or the bearings are composed of insulating materials such as Nylon,
bonding or grounding may be required.
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
5.7
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Using a conductive belt can eliminate accumulation of static charges on the belt. The nonstatic generating
characteristics of conductive belts apply to new clean belts. The user must assume responsibility for
establishing an effective preventive maintenance program to ensure continued safe operation. Another
means of preventing the accumulation of static charges on the belt is by making the belt conductive with
belt dressings. These dressings must be renewed frequently to be reliable or effective.
WEARING APPAREL:
Many fabrics under favourable 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.
Under favorable conditions, many fabrics can generate static electricity. Static discharges directly from
clothing are highly unlikely to ignite ordinary hydrocarbon gases in the air. However, clothing can be a
significant contributor to body charging as a result of its removal or movement relative to other clothing
(e.g. wearing very loose coveralls). This possibility should be recognized and prudence exercised on any
occasion when flammable vapors/gases are present. As a minimum precaution, clothing must not be
removed in a potentially flammable atmosphere, loose clothing should be avoided, and hydrocarbonsaturated clothing should not be removed until personnel involved are adequately grounded.
5.8
SAND OR SHOT BLASTING:
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. Thus, 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
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
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
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Page No. 20
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
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.
6.0
EFFECTIVE BONDING / EARTHING SYSTEMS:
Recommended earthing & bonding systems are given below with specifications:
6.1
TANK WAGONS LOADING / UNLOADING GANTRY:

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 IS-3043 & IS-7689 and OISD-108)
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Note1

6.2
6.3
6.4
Tank Wagon siding to be insulated from main running track.
TANK TRUCK LOADING/ 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.

During tank truck discharge at retail outlets, minimum 6 mm Sq. braided copper wire of suitable length
with crocodile clips on either side are to be used for bonding between tank truck under discharge and
receiving tank pipeline.
BARGE/TANKER JETTY OPERATIONS:

Minimum 6 mm Sq. braided copper wires 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.
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 earthing pits also to be as per Standards IS-3043 & IS-7689 and OISDNote 1
STD-108).
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
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Page No. 21
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
6.5
STORAGE TANKS:

6.6
All storage tanks are to be earthed separately as per electrical specifications "IS-3043-1966, IS-7689 1994 and OISD-STD-108. Note 1
FILLING SMALL CONTAINERS
When the filling nozzle may or may not 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 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.
CLASSIFICATION OF PRODUCTS.
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7.0
The guidelines covered in Section-5 are based on avoiding an electrostatic discharge in the presence of a
flammable vapour. If an electrostatic charge cannot be generated or 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 mechanisms are present and a flammable
vapour-air mixture can exist, then detailed precautions must be observed.
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, examples 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 on 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 substantial 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 chart is on a calculated basis, it is
suggested that about a 3 Deg C (5 Deg F) safety margin be used when applying it.
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.
7.1
NON- ACCUMULATORS:
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.
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 22
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
7.2
ACCUMULATORS:
Distilled petroleum products, including petroleum solvents, are generally considered electrostatic
accumulators since they have a low conductivity. (Refer Section 2.4 Conductivity). Methods for
classifying the products and examples in each category are as follows:
7.2.1
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, diesel oil,
special solvents etc.
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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 filled as an intermediate vapour pressure.
7.2.2
INTERMEDIATE VAPOUR-PRESSURE PRODUCTS:
Intermediate vapour-pressure products are those that are likely to produce a flammable vapour-air
mixture in the vapour space of a tank. Under normal liquid handling temperatures 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 vapour-pressure products handled below about 2 Deg. C (35 Deg F)
7.2.3
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-D should
be referred to in these cases to determine if a flammable vapour-air mixture will occur.
If the bulk temperature of a high vapour-pressure product is such that a flammable mixture can occur, it
must be classified as an intermediate vapour pressure product.
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
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Page No. 23
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
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. NFPA – 77 “Recommended Practices on Static Electricity”
v. IS - 3043 “Code of Practice for Earthing”
vi. IS - 7689 “Guide on control of undesirable Static Electricity”
vii. API – 2003 “Protection against ignitions arising out of static, lightning, and stray currents”
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8.0
viii. The Petroleum Rules, 2002
ix. L. G. Britton, “Using Material Data in Static Hazard Assessment”
x. L. G. Britton, “Using Heats of Oxidation to Evaluate Flammability Hazards”
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 24
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
APPENDIX: ‘A’
INDUCED CHARGE AND ITS BEHAVIOUR
Charged
Conductor
Uncharg
ed
Conducto
Start
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Induced
Opposite
Bound Charge
Induced Like
Free Charge
Insert Charged Ball
Opposite
Bound
Charge
Like Charge
Removed by
Ground
Ground
Temporary
Ground
Free Opposite
Charge
Original
Distant
Ground
Voltage
Remove
Ball
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
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Page No. 25
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
APPENDIX: ‘B’
Flammable limit
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Stoichiometric
EFFECT OF FUEL CONCENTRATION ON MINIMUM SPARK IGNITION ENERGY
Lowest
Minimum
Ignition
Energy
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
Sr. Number : OISD/DOC/2018/01
OISD – RP – 110
Page No. 26
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
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APPENDIX: ‘C’
“OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”
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OISD – RP – 110
Page No. 27
RECOMMENDED PRACTICES ON STATIC ELECTRICITY
APPENDIX: ‘D’
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Reid Vapour Pressure in Psia
FLAMMABILITY CURVE
Product Temperature in degrees Fahrenheit (°F)
The Relationship between temperature, Reid Vapour Pressure, and
Flammable limits 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 hereby expressly disclaims any liability or responsibility for loss or damage resulting
from the use of OISD Standards/Guidelines.”