API API Pressure Vessels & Storage Tank Standards 2 — Questions and Answers
Question 1: API Standard 620 differs from API Standard 650 in that API 620 covers tanks designed for:
- Low-pressure service up to 15 psig, including cryogenic and refrigerated storage (Correct answer)
- High-temperature crude oil storage above 300°F
- Buried or underground petroleum storage
- Fiberglass and composite material tank construction
Correct answer: Low-pressure service up to 15 psig, including cryogenic and refrigerated storage
API Std 620 applies to large welded low-pressure storage tanks (up to 15 psig) including those used for refrigerated or cryogenic service, which API 650 does not cover.
Question 2: Under ASME Section VIII Division 1, the required hydrostatic test pressure for a new pressure vessel is typically:
- 1.3 times the maximum allowable working pressure (MAWP) (Correct answer)
- 2 times the operating pressure
- 1.5 times the design pressure at ambient temperature
- Equal to the relief valve set pressure
Correct answer: 1.3 times the maximum allowable working pressure (MAWP)
ASME VIII Div. 1 requires a hydrostatic test at 1.3× MAWP (adjusted for material strength at test vs. design temperature) to verify structural integrity.
Question 3: API RP 579-1/ASME FFS-1 'Fitness-For-Service' assessments are used to:
- Evaluate whether equipment with known damage or flaws can continue safe operation (Correct answer)
- Certify new pressure vessels for initial commissioning
- Determine insurance replacement value for aging equipment
- Establish maximum allowable operating pressure for new vessel designs
Correct answer: Evaluate whether equipment with known damage or flaws can continue safe operation
API 579/ASME FFS-1 provides assessment procedures to determine if pressure equipment with corrosion, cracks, or other damage remains fit for continued service.
Question 4: In API Std 650, a 'floating roof' tank design is primarily used to:
- Minimize evaporative VOC losses from volatile petroleum products (Correct answer)
- Withstand seismic loading in earthquake-prone regions
- Provide structural support for heavy external loads on the roof
- Facilitate internal inspection without confined space entry
Correct answer: Minimize evaporative VOC losses from volatile petroleum products
Floating roofs rise and fall with the liquid level, eliminating the vapor space above the liquid surface and dramatically reducing evaporative emissions.
Question 5: What is the purpose of an 'annular plate' in an API 650 aboveground storage tank?
- To provide a wider bottom plate at the shell-to-floor junction for stress distribution (Correct answer)
- To support the fixed cone roof rafters at the center column
- To reinforce nozzle openings in the tank shell
- To serve as a corrosion coupon for floor condition monitoring
Correct answer: To provide a wider bottom plate at the shell-to-floor junction for stress distribution
The annular plate (bottom ring plate) is a wider plate at the shell-to-floor junction that distributes the shell load and resists stress concentration at this critical area.
Question 6: According to API 510, the 'remaining life' calculation for a pressure vessel in corrosive service is determined by:
- Dividing the measured corrosion allowance remaining by the corrosion rate (Correct answer)
- Subtracting the vessel's age from its original design life
- Multiplying the wall thickness by the safety factor
- Comparing current MAWP to the original design pressure
Correct answer: Dividing the measured corrosion allowance remaining by the corrosion rate
Remaining life equals the remaining corrosion allowance (measured thickness minus minimum required thickness) divided by the measured or estimated corrosion rate.
API API Pressure Vessels & Storage Tank Standards 2
API Standard 620 differs from API Standard 650 in that API 620 covers tanks designed for: