Uncommon Failure in Refinery Tower Collapse
by John E. Bringas, P.Eng., Codes and Standards Training Institute (CASTI), www.casti.ca
The Failure
When the aviation fuel processing tower collapsed on April 2, 2019 at the Imperial Oil refinery in Sarnia, Ontario, it was very fortunate that no one was injured and no releases to air or water occurred. To Imperial Oil’s credit, on June 6, 2019, they posted a message on their Facebook page explaining why the tower collapsed.
https://www.facebook.com/ImperialSarnia/posts/844063822616092
In short, this posting mentioned that: “The Imperial Oil investigation identified that a design change made to the tower during the last maintenance period had unknowingly increased the risk of pyrophorics.” Consequently, the probability and consequences of pyrophoric materials developing from operation and leading to a failure during a shutdown period were not sufficiently established in the design change.
When the tower was opened for inspection, it allowed the ambient air (oxygen) to enter the tower which caused the pyrophoric materials that had unknowingly built-up during operation to auto-ignite, producing high temperatures that the tower was not designed to support and the tower collapsed.
Because this failure mechanism does not occur during operation, as most do, it falls into a unique category of “shutdown” mechanisms, which for example, also includes polythionic stress corrosion cracking.
There were many news reports of the failure, although the following The Sarnia Journal news report includes this photo of the collapsed tower.
https://thesarniajournal.ca/design-flaw-byproduct-caused-tower-collapse-imperial/
Learning More About Pyrophoric Damage
The potential of pyrophoric material related failure in refinery equipment is well-known and commonly addressed with good engineering design practices. Likewise, these types of failures are not common. In fact, after 41 years of engineering and inspection experience, I have never personally seen this type of failure, so I wanted to learn more about it and started researching the literature.
Having taught a course covering API RP 571 Damage Mechanisms Affecting Fixed Equipment in the Refining Industry since the first edition in 2003, I knew that pyrophoric material damage was not addressed in it. So my next step was to review API 579-1/ASME FFS-1 Fitness-For-Service standard where pyrophoric material damage is not specifically addressed and is only briefly mentioned in 9.5.2.2 d) with a reference to hazardous materials with auto-ignition temperatures.
My next step was to review API RP 580 Risk-based Inspection, however, pyrophoric material damage is also not covered in it.
Going down my check list, I went to API RP 581 Risk-based Inspection Methodology and jackpot! I was pleasantly surprised to find references to pyrophoric materials and auto-ignition temperatures. For example, 4.8.6 establishes the consequence area calculations that yield significantly different results depending on whether the auto-ignition “not likely consequence” equations are used or “auto-ignition likely consequence” area equations are used.
We will certainly include this case study in the next API 571 Damage Mechanism course offered by Codes and Standards Training Institute (CASTI), along with many other case studies of damage mechanisms.
Helpful References
Other API RP 581 references to pyrophoric material, auto-ignition temperature, or related topics include:
4.1.2 Fluid Properties
4.1.7 Calculation of Release Phase
4.2.3 Level 2 Consequence of Failure
4.8.2.2 Development of Generic Equations
4.8.6 Blending of Results Based on AIT
5.8.1.2 Probability of Ignition Given a Release
Figure 4.2 –Level 1 COF Release Event Tree
Table 4.1 – List of Representative Fluids Available for Level 1 Consequence Analysis (Pyrophoric Materials)
Table 4.2 – Properties of the Representative Fluids Used in Level 1 Consequence Analysis (Pyrophoric Materials)
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- Pyrophoric materials, by definition, auto-ignite and therefore, a very low value for the AIT is assumed.
Table 4.8 – Component Damage Flammable Consequence Equation Constants
Table 4.9 – Personnel Injury Flammable Consequence Equation Constants
Table 5.3 – Event Probabilities (Probability of Immediate Ignition, Given Ignition)
Annex 3.A—Basis for Consequence Methodology (Examples)
3.A.3.5.3 Basis for Flammable Consequence Area Tables
3.A.3.5.3.2 Predicting Probabilities of Flammable Outcomes
Other API RP references are as follows.
API RP 575 Inspection Practices for Atmospheric and Low-Pressure Storage Tanks
8.4.2 Preliminary Visual Inspection
Inspectors should also be alert to accumulation of dry pyrophoric material (self-igniting when exposed to ambient conditions) during inspection. These accumulations may occur on the tank bottom, in the seal rim space areas, or on the top of rafters. Such accumulations that cannot be cleaned out prior to inspection should be kept moist to reduce the potential for ignition. See API 2015 and API 2016 for more information on controlling pyrophoric deposits.
API Standard 2000 Venting Atmospheric and Low-pressure Storage Tanks
3.5 Considerations for Tanks with Potentially Flammable Atmospheres
3.5.1 General
Depending on the process, operating conditions, and/or relieving conditions, the vapor space in the tank can be flammable. Ignition of the vapor 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, 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.
3.5.2 Design Options for Explosion Prevention
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- b) Inert-gas Blanketing—An effective means of reducing the likelihood of a flammable atmosphere inside a tank, when engineered and maintained properly. Note that inerting can introduce an asphyxiation risk and in sour services can promote the formation of pyrophoric deposits.
API RP 576 Inspection of Pressure-relieving Devices
Some precautions to follow when inspecting valves exposed to hazardous materials include the following.
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- Evaluate the potential for the valve to contain pyrophoric [e.g. iron sulfide (FeS)] or reactive materials, and determine the appropriate precautions for the material involved.