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 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 For inerting, see Annex F.” o This was not in 5 th 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 working group should review that language to confirm if this is appropriate.
Deflagration Venting Unclear if EN and NFPA 68 are the most appropriate references for hydrocarbon storage tank deflagration venting EN 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 ): “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 : “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 paragraph : “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 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 working group the intended design performance of API-650 frangible roofs. o ISO language to be updated to reflect that expectation
Causes of Tank Internal Deflagration ISO (2008) paragraph 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 ”
Deflagration Prevention Measures ISO (2008) paragraph 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 A bit more investigation is needed to resolve some of the questions. Revisions to some of the ISO language is recommended to clarify the requirements.