Agenda Item:
650-735
Title:
AST Grounding RequirementsRESPONSE
QUESTION
May
only
the
materials
listed
in
If
materials
other
than
those
listed
inofis
Are
the
maximum
allowable
stresses
to
Must
the
joint
efficiencies
of
API
620
the
radiography
examinations
maximum
design
temperature
API
650
F.7.3
mandates
that
selfShould
the
load
combinations
ofAPI
API
Date:
Must
roofs
subject
toof
partial
vacuum
Rev
1for
– January,
2011
be
supplied
for
design
guidance
umbrella
Dothe
the
requirements
API
650
F.7.2
Agenda Item:
Rev
2 – January,
2012
620
Section
4
and
Appendices
Q,
R,
API
620
are
to
be
permitted,
how
are
be
used
in
roof
design
those
of
API
Table
5-2
be
applied
toF.7.3
the
roof?
Section
5.26.4
be620
used
for
not
limited
to
250
degrees
Fahrenheit
supporting
roof
thickness
be
not
less
620-228
be
applied
to
roof
loading
in
excess
of
1"
water
column
self-supported
cone
roofs
subject
to forCraig Meier, P.E.
roofs
be
provided
in
620?
for
compliance
with
API
5.12.4
Contact:
Name:
and
S
be
used
in
API
650
F.7.3
roofs?
the
design
allowables
to
be
620?
applicable
roof
butt
joints
to response
achieve
and
less
suggested
by
the
than
that
required
API
650
3.10.5
calculations?
also
subject
toby
an
internal
design
API
620
5.10.3.5?
Although
umbrella
roofs
(API
650
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roofs aorBaker Consulting Group, Inc.
Company:
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higher
joint
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to
the that
second
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ofAPI
Inquiry
6503.10.6.
Does
this
mean
that
the
pressure
ofthe
F.1.3
be
designed
according
Inquiry
620-I-17/02
indicates
thatFigure
the
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and
V.7.3,
among
Phone:
others)
are 405-334-7657
mean
details
of
620
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isAPI
one620-I-05/02
toE-mail:
modify
the
potential
non-compliance
agreement
to
API
620
for
the
external
pressure
rules
given
infollowed
620
5.10.3.5
are ofcraig.meier@bakercgi.com
permitted,
Inquiry
says
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are
tohow
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for
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properties
API
620?
API
3.10.2.8
and
theinNotes
located
loading
well?
inapplicable
for
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there
are
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roofas
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Purpose:
To make
supplementary
AST grounding requirements consistent
between
industry
documents.
immediately
after
3.10.5
and
3.10.6
are
roofs.
umbrella
roofs in API 620 and that
inapplicable
particular
usage?
such designs in
arethis
covered
by API
620
Source:
Craig Meier
1.1. This may be too vague.
Revision:
2
Impact:
Consistent safety practices and regulatory environment. Cost savings
for larger tanks that do not require supplemental grounding.
Background:
AST grounding requirements are not consistent in the following
documents:
API RP 545 – Recommended Practice for Lightning Protection of
Aboveground Storage Tanks for Flammable and Combustible Liquids,
1st Edition, October 2009.
API Standard 2003 – Protection Against Ignitions Arising Out of Static,
Lightning and Stray Currents, 7th Edition January 2008.
NFPA 780 – Standard for the Installation of Lightning Protection
Systems, 2008 Edition.
API Standard 650 – Welded Tanks for Oil Storage, 11th Edition
w/Addendum2, May 1, 2010
A summary of the differences are as follows (see Appendix to this ballot
for more detail):
•
API 545 says that tank size and foundation type are not
relevant in lightning protection grounding at the bottom of the
tank.
•
NFPA 780 has different grounding requirements based on size
and foundation type.
•
API 2003 does not speak to tank size, but does mention that
when a tank is not grounded (such as with a nonconductive
membrane and not connected to a grounded piping system)
supplemental grounding is required to prevent damage to
insulating materials and foundation.
•
API 650 does not require supplementary grounding, but has
requirements when they are used.
The philosophy is to make API 650 consistent with NFPA 780. API 2003 lightning protection
content has been/will be replaced by API 545. Suggest that API 545 is made consistent with
NFPA 780.
PROPOSED CHANGES
5.8.11.3: If required by the Purchaser, g Grounding lugs shall be provided as required by Table
5-16. Additionally, if required by the Purchaser, grounding lugs shall be supplied in the quantity
specified on the Data Sheet, Table 4,. All lugs provided shall comply with Figure 5-23 and The
lugs shall be equally spaced around the base of the tank. Provide a minimum of four lugs. The
suggested maximum lug spacing is 30 m (100ft). Lugs shall be provided in accordance with
Table 5-16.
5.8.11.4: The effect of ground rods or similar devices on dike containment liner performance
should be considered.
Table 5-16: Supplemental Grounding (This is a new table; re-number subsequent tables)
Connection to
Grounded Piping
Tank
System without
Diameter Insulating Joints
All
Yes
All
No
Foundation
Type
All
All
NonConducting
Membrane
Under Tank
N/A
Yes
Supplemental
Grounding
None
30m (100 ft ) Max
Intervals, 2 Minimum
None
> 6m (20
ft)
< 6m (20
ft)
No
Earthen or Concrete Pad
No
No
Earthen or Concrete Pad
No
30m (100 ft ) Max
Intervals, 2 Minimum
> 15m
(50 ft)
< 15m
(50 ft)
No
Bituminous/Asphalt
No
None
No
Bituminous/Asphalt
No
30m (100 ft ) Max
Intervals, 2 Minimum
Note:
Tanks that rest directly on a foundation of soil earth, asphalt or concrete are inherently
grounded for purposes of dissipation of electrostatic charges. The addition of ground rods or
similar devices will not reduce the hazard associated with electrostatic charges in the stored
product. API RP-2003, API RP 545 and NFPA-780 contain additional information about tank
grounding issues as well as comments about lightning protection.
Ballot Appendix
API 545 Content
A.2.1: “Flat-bottom tanks resting on the ground need not be grounded by the use of external
grounding rods for the purpose of lightning protection.”
A.2.2: “The occurrences of incendiary sparks, rim-seal fires, etc., are not dependent on tank
grounding resistance or tank dimensions. This is because the tank will inevitably have ground
conductivity through its massive steel structure in contact with the ground. There will also be
additional grounding through the many pipes and cables that connect to each tank.” “A tank is
considered adequately grounded if the tank bottom is resting on the ground or foundation. This
applies whether or not there is an elastomeric liner in or under the tank bottom.”
A.2.3: “Lightning safety for tanks is not dependent on tank grounding.”
NFPA 780 Content
Note that 7.4.1.7 (will be renumbered from 7.4.1.4 in the upcoming 2011 version)
7.4.1.7.1 Tanks shall be grounded to conduct away the current of direct strokes and the buildup
and potential that causes sparks to ground.
7.4.1.7.2 A metal tank shall be grounded by one of the following methods:
(1) A tank shall be connected without insulated joints to a grounded metallic piping
system.
(2) A vertical cylindrical tank shall rest on earth or concrete and be at least 6 m (20 ft) in
diameter, or rest on bituminous pavement and be at least 15 m (50 ft) in diameter.
(3) A tank shall be bonded to ground through a minimum of two grounding electrodes, as
described in Section 4.13 (at least 10 ft underground surface, etc.), at 30 m (100 ft) max
intervals along the perimeter of the tank.
(4) A tank using an insulating membrane beneath the tank shall be grounded per (3).
API 2003 Content (Understanding is that the lightning portion of the document has been
superseded by API 545)
4.5.3 Grounding (This is in the static electricity portion of the document for AST’s).
Last Sentence:
“The addition of grounding rods and similar grounding systems will not reduce the hazard
associated with electrostatic charges in the fluid. However additional grounding may be required
for electrical safety (NFPA 70) or lightning protection (see Section 5 and NFPA 780).”
4.5.8.1 Open Floating-roof Tanks
Note: Shunts are believed to reduce the probability of lightning-induced secondary current flow
but are unlikely to provide protection against seal fires caused by a direct lightning strike.
5.4.1 Inherent Grounding (This is in the lightning protection portion of the document for AST’s)
Metallic tanks, equipment and structures found in the petroleum industry that are in direct
contact with the ground (i.e., no nonconducting membranes) have proved to be sufficiently well
grounded to provide for safe propagation to ground of lightning strokes. Supplemental
grounding by means of driven ground rods neither decreases nor increases the probability of
being struck, nor does it reduce the possibility of ignition of the contents. Supplemental
grounding is necessary, however, where direct grounding is not provided.
Metallic equipment that does not rest directly on the ground but is connected to a grounded
piping system is usually safe for propagation to ground of lightning strokes. Such equipment
may require supplemental grounding to prevent foundation damage.
Metallic tanks, equipment, and structures that are insulated from ground should be adequately
grounded and bonded. Such connections, when properly designed, provide a means of
propagating the discharge to ground without causing damage to insulating materials that may be
in the direct path of the stroke.
API 650 Content
5.8.11.3: If required by the Purchaser, grounding lugs shall be provided in the quantity specified
on the Data Sheet, Table 4, and comply with Figure 5-23. The lugs shall be equally spaced
around the base of the tank. Provide a minimum of four lugs. The suggested maximum lug
spacing is 30 m (100ft).
Note: Tanks that rest directly on a foundation of soil, asphalt or concrete are inherently
grounded for purposes of dissipation of electrostatic charges. The addition of ground rods or
similar devices will not reduce the hazard associated with electrostatic charges in the stored
product. API RP-2003 and NFPA-780 contain additional information about tank grounding
issues as well as comments about lightning protection.
Additional Background
Interpretations of an email from John M. Tobias, PhD, PE
Chair, NFPA 780 Standard for the Installation of Lightning Protection Systems
•
Low resistance from the tank to the ground is desirable because it prevents development
of higher voltage during a lightning event minimizing arc development.
•
The need for ground rods at smaller tank diameters is based on the concept of a
“breakpoint” where the resistances developed by two ground rod electrodes and a plate of
metal approach being equal:
Resistance of 2 ground rods = Resistance of the tank bottom to ground
•
In other words, the resistance of the tank bottom must be lower than the resistance of 2
ground rods, with appropriate safety factors applied.
•
The increased size for an asphalt foundation takes into consideration the higher resistivity
of asphalt compared to earth or concrete.