United States
Environmental Protection
Agency
Office of Solid Waste
and Emergency Response
(5104)
EPA 55Q-F-9?-QQ2b
May 199?
f/EPA
CATASTROPHIC FAILURE OF
STORAGE TANKS
The Environmental Protection Agency (EPA) is issuing this Alert as part of its ongoing effort to
protect human health and the environment by preventing chemical accidents. Under CERCLA,
section 104(e) and Clean Air Act CCAA), EPA has authority to conduct chemical accident
investigations. Additionally, in January 1995, the Administration asked the Occupational Safety
and Health Administration fOSHA) and EPA to jointly undertake investigations to determine
the root causefsl of chemical accidents and to issue public reports containing recommendations
to prevent similar accidents. EPA has created a chemical accident investigation team to work
jointly with OSHA in these efforts. Prior to the release of a full report, EPA intends to publish
Alerts as promptly as possible to increase awareness of possible hazards. Alerts may also be
issued when EPA becomes aware of a significant hazard. It is important that facilities, SERCs,
LEPCs, emergency responders and others review this information and take appropriate steps
to minimize risk.
PROBLEM
Catastrophic failures of
aboveground, atmospheric
storage tanks can occur when
flammable vapors in the tank explode
and break either the shell-to-bottom or
side seam. These failures have caused
the tanks to rip open and, in some cases,
hurled the tanks through the air. A
properly designed and maintained
storage tank will break along the shell-
to-top seam. Then, the fire would more
likely be limited to the damaged tank
and the contents would not be spilled.
This alert describes the types of tanks
that may be prone to catastrophic failure
and maintenance practices that can help
prevent the accidents.
Several accidents have occurred
within the last few years in which
storage tanks have failed
catastrophically when the flammable
vapors inside an atmospheric tank
exploded. The tank was either propelled
upward from its base (shell-to-bottom
seam failed) or split along the side seam.
As a result, workers were killed or
injured and the contents were released
into the environment.
Three specific incidents demonstrate the
potential dangers posed to workers, the
public, and the environment when these
storage tanks fail catastrophically. In
these incidents, the shell-to-bottom seam
failed after an explosion and the tank
was propelled upward. All occurred in
older, atmospheric steel storage tanks.
Often workers were performing tank
maintenance or other activities that
introduced an ignition source. The
vapors were ignited either inside the
tank or outside and then flashed back
into the tank.
In a 1995 incident, during a welding
operation on the outside of a tank, the
combustible vapor inside two large, 30-ft
diameter by 30-ft. high, storage tanks
exploded and propelled the tanks
upward one landing more than 50 feet
away. The flammable liquid inside was
instantly released and ignited, resulting
in a massive fire that caused five deaths
and serious injuries.
In a 1992 incident, while workers were
welding the outside of a tank empty of
liquid, the residual vapor in the storage
tank exploded and propelled the tank
upward and into an adjacent river. Three
workers were killed and one was injured.
In a 1994 incident, during a grinding
operation on a tank holding petroleum-
Chemical Emergency Preparedness and Prevention Office
i Printed on recycled paper
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Catastrophic Failure of Storage Tanks Caused by Vapor Explosion
May 1997
based sludge, the tank was propelled upward,
injuring 17 workers and spilling its contents over
a containment berm into a nearby river.
T
ank design and inspection/maintenance
practices are factors directly related to
catastrophic tank failure.
Tank
Historically, accidents where the shell-to-bottom
seam fails are more common among older
storage tanks. Steel storage tanks built before
1950 generally do not conform to current
industry standards for explosion and fire
venting. Atmospheric tanks used for storage of
flammable and combustible liquids should be
designed to fail along the shell-to-roof seam
when an explosion occurs in the tank. This
prevents the tank from propelling upward or
splitting along the side. Several organizations
have developed standards and specifications for
storage tank design. Published standards relevant
to this design feature include API-650,"Welded
Steel Tanks for Oil Storage" issued by the
American Petroleum Institute (API). Additional
codes and standards, published by API and
other organizations, address tank design,
construction, venting, and safe welding and are
listed at the end of this alert.
Poor inspection, maintenance,
Tanks that are poorly maintained, rarely
inspected, or repaired without attention to
design, risk catastrophic failure in the event of a
vapor explosion. Either weakening of the shell-
to-bottom seam through corrosion or
strengthening the shell-to-roof seam relative to
the shell-to-bottom seam will increase the
vulnerability of the tank to failure along the
shell-to-bottom seam. The practice of placing
gravel and spill absorbants around the base of
the tank, may increase the likelihood of bottom
corrosion. Given years of this practice, the
bottom of some tanks, especially older ones, may
be below ground level, thereby trapping
moisture along the tank bottom. This can
weaken the bottom and the shell-to-bottom
seam. Alternatively, changes to the roof seam
such as modifications to or replacement of the
roof, or attachments to the roof, could make the
roof-to-shell seam stronger relative to the shell-
to-bottom seam.
Other hazards that can contribute to a tank
explosion and possible consequences are:
Combustible
Generation of combustible vapors is a hazard
not only for the storage of pure flammable
liquids but also for the storage of any sludge or
mixture where a combustible component is
present or can be produced by reaction. Sludge
(slop tanks) and mixture (e.g., oil/water) tanks
may be particularly vulnerable because they are
sometimes open to the air; explosive
atmospheres may form inside and outside the
tank. Facilities may not always recognize this
hazard. In addition, even tanks appearing to be
empty may pose a hazard if they still contain
combustible vapors.
In the cited cases, the potential for combustible
vapors was not clearly recognized and materials
were stored in tanks that were not equipped with
flame arresters to prevent external fire from
reaching the vapor space inside the tank or with
vapor control devices to limit vapor emissions
from the tank.
Ignition
When combustible vapors escape from their
containment and mix with air in the presence of
an ignition source, combustion may occur. To
minimize this hazard, all possible ignition
sources must be isolated from potential
combustible vapors, e.g., welding equipment or
other maintenance equipment that can spark or
arc, sources of static electricity, lightning, "hot
work" in adjacent areas, and any electrical
equipment in the vicinity of tanks that does not
conform to National Fire Protection Association
(NFPA)-70, "National Electric Code."
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Catastrophic Failure of Storage Tanks Caused by Vapor Explosion
May 1997
Proximity to
environment
The danger posed by these tanks is often
increased when the location of the tank does not
conform with current minimum spacing
requirements. Sections 2-3.2 to 2-3.3 of NFPA-30
discuss minimum spacing. For mitigating
consequences to workers, the environment, and
other tanks, proper secondary containment
(diking) should be considered for containment.
Facilities should evaluate their storage tanks
for potential to catastrophically fail and
identify factors that could cause storage
tank explosion. Some of the factors to look for
include, but are not limited to, the following:
4> Atmospheric storage tanks that do not meet
API-650 or other applicable code(s) and
contain flammable liquids or liquids that
may produce combustible vapor.
4 Tanks with corrosion around the base and/or
steel tanks whose base is in direct contact
with ground and exposed to moisture.
4 Tanks or associated structures (e.g., pipes)
with weakened or defective welds.
4 Tanks used to store mixtures containing
water and flammables where the water
phase is at the tank bottom and may con-
tribute to internal bottom corrosion.
4 Tanks containing combustible vapor and
not equipped with flame arrestors or vapor
control devices to limit emissions.
4- Possible ignition sources near tanks
containing combustible vapor.
Storage tanks should comply with all
regulations, industry codes and standards,
including inspection and maintenance
requirements to keep tanks in proper condition.
Facilities with storage tanks that can contain
flammable vapors should review their
equipment and operations. Areas to review
should include, but not be limited to, the
foil owing:
1) Of
API and other organizations have standards and
codes that address recommended practices for
tank design and construction. It is imperative
to evaluate whether the liquids or certain
components of liquid mixtures may generate
combustible vapors. Design measures include
fire protection, flame arresters, emergency
venting (such as part of theAPl-650), prevention
of flash back (for tanks containing flammable
liquids), and proper berming or diking.
2)
of
API-653 has tank inspection guidelines and
procedures for periodic inspections and testing,
especially for older tanks. These procedures call
for written documentation of inspections by API
Certified Tank Inspectors. Measures to review
include procedures for pressure testing, welding
inspections, and checks for corrosion or metal
fatigue. API-650 specifies welding procedures
and welding qualifications as well as joint
inspection (e.g., radiograph and magnetic
particle examination). Programs for tank
inspection and maintenance should be
developed in accordance with these standards.
3) Hot-work
Both the Occupational Safety and Health
Administration's (OSHA) regulations concerning
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Catastrophic Failure of Storage Tanks Caused by Vapor Explosion
May 1997
hot work and NFPA's standards on welding
should be reviewed for compliance. Hazard
reduction measures include proper hot-work
procedures such as obtaining a hot work permit,
having a fire watch and fire extinguishing
equipment present, and proper testing of
atmosphere for explosivity; covering and sealing
all drains, vents, manways, and open flanges;
sealing all sewers (to prevent gas or vapor
migration); and training workers and providing
them with appropriate protective equipment.
4)
Both OSHA regulations and NFPA standards
should be reviewed for compliance. Hazard
reduction measures may include: having all
electrical equipment in a hazardous
environment conform with the requirements of
the National Electric Code (NFPA-70),
grounding tanks to dissipate static charge, using
only "non-spark producing" tools and
equipment in flammable atmospheres, and
taking care to not create sufficient heat or sparks
to cause ignition of flammable vapors.
The above information is for general
guidance only. References with
information about the hazards of
catastrophic failures and methods of minimizing
them are listed below. Regulations potentially
applicable to storage tanks and codes and
standards that may be relevant are included.
For more information consult the following:
Section 112(r) of the Clean Air Act focuses on
prevention of chemical accidents. It imposes on
facilities with regulated substances or other extremely
hazardous substances a general duty to prevent and
mitigate accidental releases. Accident prevention
activities include identifying hazards and operating
a safe facility.
EPA's Risk Management Program (RMP) Rule [40
CFR 68] is intended to prevent and mitigate
accidental releases of listed toxic and flammable
substances. Requirements under the RMP rule
include development of a hazard assessment, a
prevention program, and an emergency response
program.
EPA has tank inspection regulations under the Spill
Prevention Countermeasure and Control Plan and
Oil Pollution Control Act of 1990 [40 CFR119],
The Occupational Safety and Health
Administration (OSHA) has the Process Safety
Management Standard [29 CFR 1910.119], which
includes regulations on tank inspection, fire
prevention, and conduct during hot-work;
regulations concerning the storage of flammable
and combustible liquids [29 CFR 1910.106];
regulations concerning fire protection and
prevention during welding, brazing, and cutting
[29 CFR 1910.252] and regulations covering the
duties and responsibilities of afire watch 129 CFR
Part 126].
Occupational Safety and Health Administration
Phone: (202) 219-8151 - Public Information
Web site: http://www.osha.gov
Codes
The American Petroleum Institute (API) has tank
standards and guidelines on safe welding.
American Petroleum Institute
1220 L St NW
Washington DC 20005
Phone: (202) 682-8000
Web site: http://www.api.org
Relevant API standards include:
API Standard 620 Design and Construction
of Large, Welded, Low-Pressure Storage Tanks,
ninth edition, February 1996 (includes
Addendum 1, December 1996).
[API Standard 650 comes from] Welded Steel Tanks
for Oil Storage, ninth edition, May 1993
(includes Addendum 1, December 1994;
Addendum 2, December 1995; and Addendum
3, December 1996).
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Catastrophic Failure of Storage Tanks Caused by Vapor Explosion
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API Recommended Practice (RP) 651
Cathodic Protection of Aboveground Petroleum
Storage Tanks, first edition, April 1991.
API RP 652 Lining of Aboveground
Petroleum Storage Tank Bottoms, first edition,
April 1991.
API Standard 653 Tank Inspection, Repair,
Alteration, and Reconstruction, second edition,
December 1995 (includes Addendum 1,
December 1996).
API Standard 2000 Venting Atmospheric and
Low-Pressure Storage Tanks: Nonrefrigerated
and Refrigerated, fourth edition, September
1992.
API RP 2003 Protection Against Ignitions
Arising Out of Static, Lightning, and Stray
Current, fifth edition, December 1991.
American Society of Mechanical Engineers
1828 L St NW, Suite 906
Washington DC 20036
Phone: 1 (800) 843-2863 or (202) 785-3756
Publications and membership 1 (800) 843-2763
Codes and standards (212) 705-8500
Accreditation and certification programs (212)
705-8581
Web site: http://www.asme.org
The American Society of Nondestructive Testing
(ASNT) certifies welding and non-destructive
examination (NDE) and non-destructive testing
(NDT) inspectors,
American Society of Nondestructive Testing
P.O. Box 28518
1711 Arlingate Lane
Columbus, OH 43228
Phone: 1 (800) 222-2768 or (614) 274-6003
Web site: http://www.asnt.org
API PUBL 2210 Flame Arrestors for Vents of
Tanks Storing Petroleum Products, second
edition, 1982.
API RP 2350 Overfill Protection for Petroleum
Storage Tanks, first edition, March 1987.
The American National Standards Institute (ANSI)
has the B-31,3 Refinery Piping Code and other
standards and codes.
The American Welding Society (AWS) certifies
ivelding inspectors with the, designation AWS QC-1
(Quality Control) Welding Inspector and has
guidelines on safe ivelding.
American Welding Society
550 NW Lejeune Rd
Miami, FL 33126
Phone: 1 (800) 443-9353 or (305) 443-9353
Web site: http://www.amweld.org
American National Standards Institute
655 15th St NW
Washington DC 20005
Phone: (202) 639-4090 or
11 West 42nd St
New York, NY 10036
Phone: (212) 642-4900
Web site: http://www.ansi.org
The America?!- Society of Mechanical Engineers
(ASME) has the Pressure Vessel Code and other codes
relevant to tanks and storage ivssels.
The National Fire Protection Association (NFPA) has
lightning and flammable/combustible liquid codes.
National Fire Protection Association
1 Batterymarch Park
P.O. Box 9101
Quincy, MA 02269-9101
Phone: (617) 770-3000
Customer Sendee: 1 (800) 344-3555
Web site: http://www.nfpa.org
Relevant NFPA codes include:
NFPA 30 Flammable and Combustible Liquid
Code, 1996 edition.
NFPA 51 Design and Installation of Oxygen-
Fuel Gas Systems for Welding, Cutting, and
Allied Processes, 1992.
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Catastrophic Failure of Storage Tanks Caused by Vapor Explosion
May 1997
NFPA 51B Fire Prevention in Use of Cutting
and Welding Processes, 1994.
NFPA 70 National Electric Code. 1996.
NFPA 77 Static Electricity. 1993.
NFPA 780 Lightning Protection Code. 1995.
Underwriters Laboratories Inc. (UL) has standards
for product safety,
Underwriters Laboratories Inc.
333 Pfingsten Rd
Northbrook, IL 60062
Phone: (847) 272-8800
Web site: http://www.ul.com
Relevant UL standards include:
UL-142 Standard for Steel Aboveground
Tanks for Flammable and Combustible Liquids,
1993.
FOR MORE INFORMATION,,.
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MONDAY-FRIDAY, 9 AM TO 6 PM, EASTERN TIME
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NOTICE
The statements in this document are intended solely as guidance. This document does not substitute for EPA's or other
agency regulations, nor is it a regulation itself. Site-specific application of the guidance may vary depending on process
activities, and may not apply to a given situation. EPA may revoke, modify, or suspend this guidance in the future, as
appropriate.
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