How often should a petroleum tank be internally inspected?

At least once every 10 years. This interval can be adjusted based on the calculated corrosion rate of the tank bottom and shell (Table 1 & Clause 6.1).

What is the minimum acceptable thickness for a tank shell?

The tank should be considered for retirement if the shell plate thickness is less than 2.5 mm, although a fitness-for-service analysis may permit continued operation (Clause 6.2.2.1).

What does an external inspection involve?

It includes checking the foundation for settlement, visual inspection of shell, nozzles, and roof for damage or corrosion, and ultrasonic thickness gauging at specified intervals (Clause 5).

Are API standards relevant when using IS 9964?

Yes, IS 9964 frequently refers to API standards, especially API 653 (Tank Inspection, Repair, Alteration, and Reconstruction), for detailed procedures and acceptance criteria.

IS 9964

: 2000

Recommendations for maintenance and operation of petroleum storage tanks, Part 1:Preparation of tanks for safe entry and work

CurrentSpecializedGuidelinesBIMFire Safety · Structural Engineering and Structural Sections

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This standard provides recommendations for the preparation of petroleum storage tanks for safe entry, inspection, maintenance, and repair. It details essential safety procedures including tank isolation, emptying, cleaning, gas-freeing, atmospheric testing, and the permit-to-work system to prevent accidents.

Recommendations for maintenance and operation of petroleum storage tanks, Part 1:Preparation of tanks for safe entry and work

Quick Reference Values

Minimum shell thickness for continued service2.5 mm

Maximum interval for internal inspection (default)10 years

Maximum interval for in-service external ultrasonic thickness survey5 years

Maximum interval for external visual inspection1 year

Frequency of routine (in-service) visual inspection1 month

Key Formulas

Corrosion Rate (CR) = (Original Thickness - Measured Thickness) / Time in years

Practical Notes

This code is often used in conjunction with OISD-STD-129 (Inspection of Storage Tanks), which is a mandatory standard for many oil and gas operators in India.

The inspection intervals mentioned are maximums; a Risk-Based Inspection (RBI) approach (as per API 580/581) is often adopted to define more optimized, condition-based intervals.

Comprehensive documentation, including thickness monitoring locations (TMLs), corrosion rate calculations, and settlement records, is crucial for effective long-term asset integrity management.