Deflagration Venting Final

7/28/2019 Deflagration Venting Final

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ISO-28300 / API-2000Deflagration Venting/Mitigation

Brad Otis

April 28, 2010

2010 API Spring Refining and Equipment Standards Meeting


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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 preventionmeasures

Conclusion


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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)


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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


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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 groupshould review that language to confirm if this is

appropriate.


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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


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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 tointernal 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 onenclosures that have a headhouse or penthouse.


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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.


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API Publication 937 states:API-650 (API, 1993) provides design criteria for fluid storagetanks 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 instorage tank design.

Neither API-650 or API Publications 937 and 937A

reference NFPA-68.


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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 confirmwhether API 650 frangible roof design meets NFPA-68

or if this is an alternative to NFPA-68.


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Deflagration VentingIf 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) indiameter. For explosion venting, See NFPA 68 and orEN 13237

for alternative methods for mitigating tank internal deflagration.

For inerting, see Annex F.


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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


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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, staticdischarge 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 thepotential for a flammable atmosphere inside the tank and

determine whether safeguards are adequate. If explosion venting

is necessary, see 4.2.5.15.


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Deflagration Prevention Measures

ISO-28300 (2008) paragraph 4.5.2 discusses design

options for explosion prevention. Consider modifying the2nd 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 sustainedfire 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.


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Conclusion

A number of issues dealing with tank internaldeflagration 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.

Deflagration Venting Final

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