API API Pressure Vessels & Storage Tank Standards 1 — Questions and Answers
Question 1: API Standard 650 applies to which type of petroleum storage vessels?
- Welded aboveground steel tanks for oil storage at or near atmospheric pressure (Correct answer)
- High-pressure spherical pressure vessels for LPG
- Underground fiberglass reinforced plastic tanks
- Cryogenic storage tanks for LNG service
Correct answer: Welded aboveground steel tanks for oil storage at or near atmospheric pressure
API Std 650 covers the design, fabrication, erection, and testing of welded steel aboveground storage tanks operating at atmospheric or low pressure.
Question 2: What does the 'design pressure' of a pressure vessel represent under ASME/API standards?
- The maximum allowable working pressure (MAWP) used as the basis for vessel design (Correct answer)
- The operating pressure during normal production conditions
- The hydrostatic test pressure applied during commissioning
- The emergency relief valve set pressure
Correct answer: The maximum allowable working pressure (MAWP) used as the basis for vessel design
Design pressure equals or exceeds the maximum allowable working pressure and is the pressure at which vessel wall thickness and component ratings are calculated.
Question 3: According to API Standard 510 (Pressure Vessel Inspection Code), the risk-based inspection (RBI) approach prioritizes inspection resources based on:
- A combination of the likelihood of failure and the consequence of failure (Correct answer)
- Only the age of the vessel since last inspection
- The original fabrication code used to build the vessel
- The number of pressure relief valve activations recorded
Correct answer: A combination of the likelihood of failure and the consequence of failure
API 510 RBI methodology ranks vessels by combining probability of failure (corrosion, damage mechanisms) with consequence of failure (safety, environmental, business impact).
Question 4: In API Std 650 tank design, the 'one-foot rule' is used to calculate shell thickness at:
- One foot above the bottom of each shell course to account for hydrostatic head (Correct answer)
- The top shell course where wind load governs
- The roof-to-shell junction for wind uplift calculations
- The nozzle reinforcement zone at one foot from the centerline
Correct answer: One foot above the bottom of each shell course to account for hydrostatic head
The one-foot rule calculates the required shell thickness at a point one foot above the lower horizontal seam of each shell course, reflecting hydrostatic liquid pressure.
Question 5: Which inspection method is most commonly used to assess the floor thickness of an in-service aboveground storage tank without emptying it?
- Magnetic flux leakage (MFL) floor scanning (Correct answer)
- Radiographic testing (RT) from the roof manway
- Liquid penetrant testing (PT) of the shell
- Visual inspection from inside using a camera drone
Correct answer: Magnetic flux leakage (MFL) floor scanning
MFL floor scanners detect corrosion-induced metal loss in tank floors while the tank is in service or after emptying, without requiring full entry.
Question 6: The 'corrosion allowance' added to calculated pressure vessel wall thickness serves to:
- Extend the operational life of the vessel by providing extra metal for expected corrosion (Correct answer)
- Increase the safety factor for mechanical overpressure events
- Compensate for weld joint efficiency below 1.0
- Account for thermal expansion at elevated operating temperatures
Correct answer: Extend the operational life of the vessel by providing extra metal for expected corrosion
Corrosion allowance provides additional wall thickness beyond the calculated minimum so the vessel remains fit for service throughout its design life as corrosion progresses.
API API Pressure Vessels & Storage Tank Standards 1
API Standard 650 applies to which type of petroleum storage vessels?