2010 API Spring Refining and Equipment Standards Meeting
ISO-28300 / API-2000
Deflagration Venting/Mitigation
Brad Otis
April 28, 2010
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Agenda
 Four Areas of Proposed Work
o Clarify “Emergency Venting”
o Review deflagration venting and applicable industry
standards
o Understand API-650 frangible roof performance
o Clarify tank deflagration causes and prevention
measures
 Conclusion
Emergency Venting
 Section 3.4 states:
“venting required when an abnormal condition, such as
ruptured internal heating coils or an external fire, exists either
inside or outside a tank.”
o An external fire inside the tank??
o Other abnormal conditions should be referenced
 Proposed 3.4:
“venting required when an abnormal condition exists, such as
external fire or other overpressure circumstances (see 4.2.5)”
Deflagration Venting
 Section 4.2.5.15 states:
“Tank contents can ignite, producing an internal deflagration
with overpressures that develop more rapidly than some
venting devices can handle. For explosion venting, see NFPA
68 and EN 13237. For inerting, see Annex F.”
o This was not in 5th edition of API-2000
Deflagration Venting
 Placement of this paragraph under the section “other
overpressure circumstances” makes it mandatory to
consider and evaluate storage tank internal deflagration
overpressure protection (or prevention).
o Recommendation: The ISO-28300 working group
should review that language to confirm if this is
appropriate.
Deflagration Venting
 Unclear if EN-13237 and NFPA 68 are the most
appropriate references for hydrocarbon storage tank
deflagration venting
 EN-13237 was not reviewed due to insufficient time
 NFPA 68 was reviewed in detail and the following
issues were identified:
o Clearly covers deflagration venting
o Covers hydrocarbon storage tanks but it does not
provide direct guidance
Deflagration Venting
Closest guidance is section 8.6 “bins, hoppers and silos”
(NFPA 68-2002):
“The entire enclosure top can be made to vent deflagrations. In
such cases, design and operating conditions (internal and external
pressure, wind loads, and snow loads) can cause the mass of the
roof to exceed that prescribed for deflagration vent closure. Roof
panels are to be as lightweight as possible and are not to be
attached to internal roof supports. API 650, Welded Steel Tanks for
Oil Storage, should be referenced for guidelines for the design of a
frangible, welded roof joint. Although frangible roof design in
accordance with API 650 is not intended to serve as deflagration
venting, experience shows that such roofs have successfully
vented deflagrations. A frangible roof design is not recommended
for use as the inner roof on enclosures that have a headhouse or
penthouse.”
Deflagration Venting
 The latest (2007) edition of NFPA 68 does NOT include
the previous language in red.
 It states in 8.6.3.4:
“The remaining portions of the enclosure, including anchoring,
shall be designed to resist the calculated Pred, based on the
vent area provided.“
 Extensive work was done to assess frangible roof
designs (see API Publications 937 and 937A)
 It is not clear whether the API-650 frangible roof design
complies with the NFPA-68 criteria or whether it should
comply.
Deflagration Venting
 API Publication 937 states:
“API-650 (API, 1993) provides design criteria for fluid storage
tanks with frangible roof joints. The rules are intended for fluid
storage tanks used to store flammable liquids which could be
subjected to sudden over-pressurization. Over-pressurization
can occur due to the ignition of flammable vapors and can
exceed the capabilities of the pressure relief vents specified in
storage tank design.”
 Neither API-650 or API Publications 937 and 937A
reference NFPA-68.
Deflagration Venting
 It appears that this was intentional and that API-650
frangible roof design may be an alternative to NFPA-68
design.
 Recommendation: the API Sub-Committee on
Atmospheric Storage Tanks (SCAST) should confirm
whether API 650 frangible roof design meets NFPA-68
or if this is an alternative to NFPA-68.
Deflagration Venting
If an alternative, then consider the following revision to
ISO-28300 paragraph 4.2.5.15:
“Tank contents can ignite, producing an internal deflagration with
overpressures that develop more rapidly than some venting
devices can handle. If a tank vapor space is flammable and this is
ignited, the resulting gas expansion can exceed the capabilities of
storage tank pressure relief vents. Experience has shown that a
fixed-roof tank with a weak (frangible) roof-to-shell attachment,
such as that described in API Std 650 will be sufficient to vent an
internal deflagration. If this approach is chosen to mitigate an
internal deflagration scenario, care should be taken to ensure that
the current requirements for a frangible roof-to-shell attachment
are met, particularly for tanks smaller than 15 m (50 ft) in
diameter. For explosion venting, See NFPA 68 and or EN 13237
for alternative methods for mitigating tank internal deflagration.
For inerting, see Annex F.”
API 650 Frangible Roof Performance
 What is considered good frangible roof performance?
o Tank shell/bottom seam fails?
o Tank shell/roof seam tears but roof is held?
o Tank shell/roof seam fails but roof is thrown?
 Video Examples
 Recommendations
o API SCAST summarize to the ISO-28300 working
group the intended design performance of API-650
frangible roofs.
o ISO-28300 language to be updated to reflect that
expectation
Causes of Tank Internal Deflagration
ISO-28300 (2008) paragraph 4.5.1 is not clear how fire
exposure can result in deflagration. Consider:
“Depending on the process, and operating conditions, and/or
relieving conditions, the vapour space in the tank can be
flammable. Ignition of the vapour space while within the
flammable region likely leads to tank roof damage and/or loss of
containment. Ignition sources include, but are not limited to, static
discharge inside the tank due to splash filling or improper level
gauging, pyrophoric materials on the inside surfaces of the tank,
external hot work on the tank, fire exposure of the tank tank or
tank fittings above the auto-ignition temperature due to external
fire exposure, or flame propagation through a tank opening or
vent caused by a lightning strike or external fire. Consider the
potential for a flammable atmosphere inside the tank and
determine whether safeguards are adequate. If explosion venting
is necessary, see 4.2.5.15.”
Deflagration Prevention Measures
ISO-28300 (2008) paragraph 4.5.2 discusses design
options for explosion prevention. Consider modifying the
2nd sentence as follows:
“…The user is cautioned that the use of a flame arrester within the
tank's relief path introduces the risk of tank damage from
overpressure or vacuum due to plugging if the arrester is not
maintained properly. The user is also cautioned that a sustained
fire on the outlet of the flame arrestor or on other parts of the
tank/fittings may result in temperature high enough to ignite
internal flammable vapors….”
Conclusion
 A number of issues dealing with tank internal
deflagration exist in the current edition of ISO-28300.
 A bit more investigation is needed to resolve some of
the questions.
 Revisions to some of the ISO-28300 language is
recommended to clarify the requirements.