1. Introduction
ASTs are critical components of infrastructure in the oil and gas, refining, petrochemical, bulk storage terminals processing & power generation infrastructure. They are designed for storing crude oil, gasoline, solvents, fertilizers, chemicals and other important industrial liquids. Due to the critical role of these facilities and apparatuses, any failure or deterioration on their structural behavior may cause serious safety hazards, environmental pollution, and even catastrophic economic losses.
To address these concerns, the American Petroleum Institute (API) introduced API Standard 653 – Tank Inspection, Repair, Alteration, and Reconstruction. First issued in 1991, API 653 provides a structured framework for ensuring the integrity, reliability, and safe operation of ASTs after they have been placed in service. The standard complements API 650 (Welded Tanks for Oil Storage) and API 620 (Design and Construction of Large, Welded, Low-Pressure Tanks) by focusing on post-construction inspection and maintenance practices.
2. Scope of API 653
- API 653 outlines the requirements for inspecting, repairing, altering, and reconstructing specific categories of storage tanks. Its scope applies to aboveground welded steel tanks that are designed and built in compliance with API 650 or API 12C standards.
- Tanks of any size or capacity used for storing petroleum products, chemicals, or other comparable liquids.
- Atmospheric and low-pressure tanks that are designed for an internal pressure not exceeding 2.5 psig.
- Tanks operating within the temperature limitations established by API 650, unless specially engineered with additional provisions for higher or lower operating ranges.
The standard does not cover:
- Storage tanks fabricated from materials other than carbon steel or stainless steel.
- Vessels designed to contain pressure that are governed by the provisions of the ASME Boiler and Pressure Vessel Code.
- Subsurface storage tanks and compact shop-fabricated vessels that fall outside the scope of API 650 requirements.
3. Definitions and Key Roles
3.1 Authorized Inspector
III-TitleAPI Standard 653 inspector: A person who is certified in accordance with API 653 and conducts the inspection, interprets the results, and initiates recommendations.
3.2 Owner-User
The party offering the for-hire tank which is ultimately responsible for observing API 653 and preserving integrity documents.
3.3 Repair Organization
ADB TankWorks is a licensed contractor or in-house division for tank repairs, modifications, and builds to meet API/ASME code.
4. Inspection Requirements
API 653 specifies multiple types of inspection to ensure comprehensive monitoring of tank conditions.
4.1 External Inspection
- Frequency: At least every 5 years, or sooner based on risk or corrosion rate.
- Scope: Visual evaluation of shell, roof, nozzles, foundations, insulation, and appurtenances.
- Objective: Detect external corrosion, leaks, settlement, coating deterioration, or structural distress.
4.2 Internal Inspection
- Frequency: Every 10 years or based on corrosion rate calculations.
- Scope: Evaluation of tank bottom, shell interior, roof structure, welds, and corrosion protection systems.
- Methods: Visual inspection, ultrasonic thickness testing (UTT), magnetic flux leakage (MFL), and floor scanning.
4.3 Ultrasonic Thickness Testing (UTT)
- Used to measure remaining wall and bottom plate thickness.
- Supports calculation of corrosion rates and remaining life.
4.4 Settlement Surveys
- Identifies differential settlement patterns (uniform, planar, dishing, or edge settlement).
- Excessive settlement can lead to cracking, leakage, and shell-bottom separation.
5. Remaining Life and Corrosion Rate Calculations
A cornerstone of API 653 is the calculation of remaining life and the establishment of inspection intervals.
5.1 Corrosion Rate Formula
mm/y = 87.6 * (W / (D * A * T)):
Where:
- W = weight loss (mg)
- D = density (g/cm³)
- A = area (cm²)
- T = time (hours)
6. Minimum Inspection Intervals
API 653 Prescribed Inspection Intervals
| Inspection Type | Interval (Maximum) | Notes |
| External Visual | 5 years | May be shortened if corrosion rate is high |
| Internal Inspection | 10 years (typical) | Based on corrosion rate or RBI methodology |
| Ultrasonic Thickness | Concurrent with internal or as needed | Required for shell and bottom plates |
| Settlement Survey | 10 years or after major repair | Includes tank foundations |
7. Risk-Based Inspection (RBI)
API 653 permits the use of RBI methodologies to optimize inspection intervals.
7.1 Probability of Failure (PoF)
- Obtained from corrosion rates, weld quality, design details, and tank age.
7.2 Consequence of Failure (CoF)
- Assesses the potential risk for safety, environmental and monetary loss.
7.3 Risk Ranking
- Integrates PoF and CoF in a risk matrix.
- More detailed and more frequent inspections are done on higher-risk tanks.
8. Repair and Alteration Requirements
8.1 Acceptable Repair Methods
- Attach patches or inserts for localized thinning.
- Reinforcement plates for nozzle reinforcement.
- Bottom replacement in case of heavy corrosion.
- Replacements or adjustments shells for integrity.
8.2 Welding Requirements
- Shall conform to ASME Section IX for qualification.
- All welds eligible for NDE techniques (UT, RT, MT, PT).
8.3 Authorized Inspector’s Role
- Examines repair tactics, authorizes steps and confirms the job is finished.
9. Tank Reconstruction
When tanks are dismantled and reassembled or relocated, API 653 provides rules for:
- Re-rating to new design pressures or service conditions.
- Material compatibility checks.
- Foundation design verification.
10. Fitness-for-Service (FFS) Evaluations
When aboveground storage tanks display signs of deterioration, API 653 requires engineers to utilize API 579-1 / ASME FFS-1 for assessing
- Localized wall thinning
- Pitting-type corrosion
- Weld alignment deviations
- Foundation or settlement-related distortions
FFS assessments determine if the tank can continue operating safely or requires repair.
11. Settlement Evaluation
Figure Description – Types of Tank Settlement
- Uniform Settlement: Even downward movement; acceptable within limits.
- Planar Tilt: Uniform tilt; may affect nozzles and piping.
- Dish-Shaped: Center sinks; risk of bottom buckling.
- Edge Settlement: Perimeter settles; induces shell stress.
12. Materials and Weld Inspection
- X Ray Inspection Test (RT): Quality of the weld and sub surface defects.
- Ultrasonic Testing (UT): Detection of Plate thickness and weld defects.
- Magnetic Particle Testing (MT): Surface cracking in ferromagnetic materials.
- Penetrant Test (PT): Surface flaws in any material.
- PMI : positive material identification – confirms alloy.
13. Recordkeeping Requirements
Owner-users must maintain:
- Inspection reports.
- Corrosion rate calculations.
- Settlement surveys.
- Repair and alteration documentation.
- RBI assessments (if used).
These records ensure traceability and regulatory compliance.
14. Regulatory Recognition
- United States: Widely referenced by OSHA and EPA as good engineering practice.
- International: Recognized in Europe, Middle East, and Asia as the benchmark for AST integrity.
- Local Adoption: Many regulatory agencies require API 653 compliance in permitting.
15. Benefits of API 653 Implementation
- Safety: Reduces risk of catastrophic tank failures.
- Environmental Protection: Prevents spills and leaks.
- Cost Savings: Optimized inspection intervals through RBI.
- Global Acceptance: Recognized across multiple jurisdictions.
- Extended Asset Life: Ensures continued service of tanks beyond original design life.
16. Conclusion
API 653 is recognized worldwide as the most up-to-date standard for inspections of in-service storage tanks. With its clear inspection procedures and repair requirements as well as the integration of risk-based inspections methodologies into its framework, the standard is a guide to ensure the safe operation, safety and compliance world-over in critical industrial tank fleets.
Organizations which utilize the benefits of API 653 improve their personnel and environmental safety and economics as well because it is possible to carry out a more efficient inspection planning, minimize downtime periods and provide longer time of service usage to storage tanks.