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API Publ 2021A-1998: Hazard Evaluation Guidance for Aboveground Storage Tanks Containing Flammable Liquids
A comprehensive technical overview of the scope, methodology, and compliance considerations for fire and explosion hazard assessment in atmospheric storage tanks
Introduction
API Publ 2021A-1998, officially titled Interim Guidance for the Hazard Evaluation of the Potential for a Fire or Explosion in Aboveground Storage Tanks Containing Flammable Liquids, is a critical publication from the American Petroleum Institute (API). This document provides systematic procedures for evaluating the likelihood and consequences of fire and explosion events in atmospheric aboveground storage tanks (ASTs) that store Class I flammable liquids. While it is a publication rather than a consensus standard, it is widely referenced by facility operators, safety engineers, and regulators as a foundational methodology for tank hazard analysis.
Scope and Applicability
API Publ 2021A applies to atmospheric or low-pressure aboveground storage tanks (designed to operate at pressures from atmospheric up to 15 psig) that contain flammable liquids, typically Class I liquids (flash point below 100°F). It covers vertical fixed-roof tanks, internal floating-roof tanks, and external floating-roof tanks. The publication is intended for:
- Hazard identification and risk screening of existing tank farms
- Design review of new tank installations
- Management of change assessments when tank service or configuration changes
The document is scoped to address scenarios such as overfill, roof sinking, tank shell rupture, lightning ignition, and adjacent pool fires. It explicitly excludes pressurized tanks, refrigerated cryogenic storage, and tanks storing non-flammable or combustible liquids above their flash point.
Note: API Publ 2021A is an interim guidance document. For more comprehensive coverage, users should consult API 2021 (on tank fire management) and current editions of API Standard 650, 653, and 2350.
Technical Requirements and Methodology
Hazard Evaluation Process
The core of API Publ 2021A is a structured hazard evaluation methodology that includes the following steps:
- Tank and content characterization – Identify tank type, dimensions, construction material, corrosion history, and liquid physical properties (vapor pressure, flash point, autoignition temperature, flammable range).
- Identification of release sources – Potential loss-of-containment events such as overfilling, corrosion holes, shell weld failure, roof landing (for floating roofs), and piping connection leaks.
- Ignition source assessment – Evaluate nearby hot work, electrical equipment, vehicles, lightning, static discharge, and smoking areas.
- Consequence analysis – Determine the magnitude of potential pool fire, tank fire (rim seal, roof top), or vapor cloud explosion (if confined).
- Risk ranking – Combine likelihood and severity to prioritize mitigation actions.
| Scenario | Trigger Event | Consequence | Risk Level |
|---|---|---|---|
| Overfill with dike fire | Level control failure | Large pool fire | High |
| Roof sinking (internal floating roof) | Pontoon failure or product accumulation | Potential rim-seal fire | Moderate |
| Lightning strike | Electrical discharge | Vapor ignition at vent or rim seal | Variable |
| Adjacent tank domino effect | Radiant heat from nearby fire | Cascading failure | High |
| Corrosion leak during hot work | Undetected release | Vapor cloud ignition | Severe |
Data Requirements
The publication emphasizes the need for accurate process safety information, including:
- Liquid properties (flash point, boiling point, vapor density)
- Tank design datasheets (diameter, height, fill rate, roof type)
- Operational history (turnaround frequency, corrosion rates, previous releases)
- Fire protection systems (foam, water spray, monitors, diking)
Tip: When implementing the API Publ 2021A methodology, ensure your liquid property data comes from reliable sources (e.g., NFPA 30, 49 CFR 173) and is updated whenever tank service changes.
Implementation Highlights
Integrating into a Facility Safety Program
API Publ 2021A is best used as part of a broader risk management framework. It aligns closely with the hazard evaluation step in API RP 750 (Process Hazard Management) and OSHA 1910.119. For an existing tank terminal, the publication helps identify the most vulnerable tanks and those whose failure would have the greatest safety, environmental, or business impact.
Key implementation steps include:
- Prepare a tank inventory sorted by liquid class, age, and inspection records.
- Apply the hazard evaluation to at least the top 20% of high-risk tanks (or as defined by corporate risk criteria).
- Document the evaluation using a worksheet or bow-tie diagram as suggested in the publication.
- Use the results to prioritize secondary containment upgrades, emergency response planning, and foam system installation.
Warning: Do not rely solely on API Publ 2021A for all tank hazards. It does not cover structural failure from seismic loads, wind, or internal overpressure beyond fire exposure. Use additional references such as API Standard 620 and 650 for complete design integrity evaluation.
Compliance and Regulatory Notes
API Publ 2021A is not a mandatory standard; however, it is often cited by regulators (e.g., OSHA, state environmental agencies) as an example of recognized and generally accepted good engineering practices (RAGAGEP) for fire hazard assessment. When a facility experiences a major tank fire, investigators frequently check whether the facility applied a hazard evaluation consistent with this publication.
For companies following API RP 1174 (Onshore Hazardous Liquid Pipeline Emergency Response) or EPA’s Risk Management Program (40 CFR Part 68), using API Publ 2021A can help demonstrate compliance with the hazard evaluation requirement. Audits should verify that evaluations are updated every five years or after significant changes to tank service, capacity, or surrounding population.
Best Practice: To maintain a defensible record, keep a log of all hazard evaluations conducted under API Publ 2021A, including the date, participants, findings, and actions taken. This documentation is invaluable during both internal audits and regulatory inspections.
Critical: API Publ 2021A-1998 is an older publication (1998) and may not reflect current risk assessment methodologies or material science. Always verify that its conclusions are consistent with more recent standards (e.g., API RP 752, 2021 edition) when applied to existing facilities.
Frequently Asked Questions
Q: What is the main difference between API Publ 2021A and API Standard 2021?
A: API 2021 (first published in 1965) is a standard for the management of fires in atmospheric storage tanks, covering fire‐fighting procedures and personnel safety. API Publ 2021A is an interim guidance document that focuses on the prevention side – the hazard evaluation of potential fire or explosion events – and is more analytical in nature. The two documents are complementary.
A: API 2021 (first published in 1965) is a standard for the management of fires in atmospheric storage tanks, covering fire‐fighting procedures and personnel safety. API Publ 2021A is an interim guidance document that focuses on the prevention side – the hazard evaluation of potential fire or explosion events – and is more analytical in nature. The two documents are complementary.
Q: Which types of tank are outside the scope of API Publ 2021A?
A: The publication explicitly excludes: (a) pressurized storage tanks (e.g., propane spheres), (b) refrigerated cryogenic tanks, (c) tanks storing flammable liquids above atmospheric boiling point, and (d) tanks built to API 620 (low‐temperature) or similar alternative designs unless they meet the atmospheric pressure definition. It also does not cover internal vapor cloud explosions within tank vapor spaces when non‐flammable gases are present.
A: The publication explicitly excludes: (a) pressurized storage tanks (e.g., propane spheres), (b) refrigerated cryogenic tanks, (c) tanks storing flammable liquids above atmospheric boiling point, and (d) tanks built to API 620 (low‐temperature) or similar alternative designs unless they meet the atmospheric pressure definition. It also does not cover internal vapor cloud explosions within tank vapor spaces when non‐flammable gases are present.
Q: Is API Publ 2021A still current? Should I use it for new projects?
A: API Publ 2021A was last reaffirmed in 1998. While it is still available from API and commonly referenced in training, many practitioners now use updated resources such as API RP 752 (Management of Process Hazards) and CCPS guidelines for tank hazard analysis. However, the publication remains a valid tool for initial screening hazard evaluations and can be part of a comprehensive risk assessment portfolio. Always check the API website for the latest reaffirmation status.
A: API Publ 2021A was last reaffirmed in 1998. While it is still available from API and commonly referenced in training, many practitioners now use updated resources such as API RP 752 (Management of Process Hazards) and CCPS guidelines for tank hazard analysis. However, the publication remains a valid tool for initial screening hazard evaluations and can be part of a comprehensive risk assessment portfolio. Always check the API website for the latest reaffirmation status.
Q: How do I incorporate the hazard evaluation results into a plant’s safety system?
A: The output of an API Publ 2021A evaluation typically includes a prioritized list of risk‐reducing actions. These should be entered into a facility’s management of change (MOC) or action tracking system. For each high‐risk tank, consider linking the evaluation to operating procedures, inspection frequencies (API 653), and emergency response plans. It is also good practice to review the evaluation during Process Hazard Analysis (PHA) revalidations.
A: The output of an API Publ 2021A evaluation typically includes a prioritized list of risk‐reducing actions. These should be entered into a facility’s management of change (MOC) or action tracking system. For each high‐risk tank, consider linking the evaluation to operating procedures, inspection frequencies (API 653), and emergency response plans. It is also good practice to review the evaluation during Process Hazard Analysis (PHA) revalidations.
© 2026 – Technical overview prepared for informational purposes. Always refer to the current edition of API Publ 2021A-1998 or its successor for official guidance.