API 653 Pressure Testing 3 — Questions and Answers
Question 1: Under API 653, a pneumatic pressure test may be used instead of a hydrostatic test only when:
- The tank is too large to fill economically
- The owner/operator can demonstrate that water would damage the tank or process (Correct answer)
- The tank is scheduled for decommissioning within 5 years
- Shell thickness is greater than 1 inch
Correct answer: The owner/operator can demonstrate that water would damage the tank or process
API 653 permits pneumatic testing only when the owner/operator documents that introduction of water is not practical due to process contamination or structural concerns.
Question 2: What is the primary safety hazard associated with pneumatic pressure testing of storage tanks compared to hydrostatic testing?
- Higher risk of corrosion on internal surfaces
- Energy stored in compressed gas greatly exceeds that of liquid under the same pressure (Correct answer)
- Pneumatic tests require higher absolute pressure levels
- Air leaks are harder to detect visually than water leaks
Correct answer: Energy stored in compressed gas greatly exceeds that of liquid under the same pressure
Compressed gas stores far more energy per unit volume than liquid under the same pressure, making pneumatic test failures catastrophically more energetic than hydrostatic failures.
Question 3: According to API 653, what document must the owner/operator prepare before conducting a hydrostatic test on a reconstructed tank?
- A material test report (MTR) for all new plate
- An authorized inspection agency approval letter
- A written test procedure reviewed by the engineer of record (Correct answer)
- An API 510 pressure vessel inspection report
Correct answer: A written test procedure reviewed by the engineer of record
API 653 requires a written hydrostatic test procedure to be prepared and reviewed by the engineer of record prior to testing.
Question 4: When a tank bottom is tested using a vacuum box test per API 653, what is the minimum vacuum level (negative gauge pressure) that must be achieved?
- 1 psi (7 kPa)
- 2 psi (14 kPa)
- 3 psi (21 kPa) (Correct answer)
- 5 psi (35 kPa)
Correct answer: 3 psi (21 kPa)
API 653 requires a minimum vacuum of 3 psi (21 kPa) for a vacuum box test to be considered effective for detecting bottom weld leaks.
Question 5: For a reconstructed tank, API 653 requires the hydrostatic test to be witnessed by:
- The tank owner's safety officer only
- An authorized inspector (AI) (Correct answer)
- A certified weld inspector (CWI)
- The local fire marshal
Correct answer: An authorized inspector (AI)
API 653 requires an authorized inspector (AI) as defined by the standard to witness the hydrostatic test on a reconstructed aboveground storage tank.
Question 6: What is the purpose of monitoring foundation settlement during the hydrostatic fill of a repaired or reconstructed tank per API 653?
- To confirm that soil compaction meets ASTM standards
- To detect differential settlement that could overstress the bottom and shell (Correct answer)
- To verify that the anchor bolt torque is adequate
- To measure uplift forces for wind loading calculations
Correct answer: To detect differential settlement that could overstress the bottom and shell
Settlement monitoring during filling allows early detection of non-uniform foundation movement that could lead to bottom distortion or shell cracking.
Question 7: Which of the following statements about the API 653 alternative leak test for tank bottoms using MFL (Magnetic Flux Leakage) is correct?
- MFL can replace visual inspection but not vacuum box testing
- MFL is an acceptable method for detecting bottom pitting and wall loss but must be supplemented by a sensitive leak test (Correct answer)
- MFL alone satisfies all API 653 bottom inspection requirements
- MFL is only approved for tanks with bottoms thicker than 0.5 in
Correct answer: MFL is an acceptable method for detecting bottom pitting and wall loss but must be supplemented by a sensitive leak test
API 653 accepts MFL for bottom thickness/corrosion scanning, but a sensitive leak test (such as vacuum box) is still required to verify weld integrity.
API 653 Pressure Testing 3
Under API 653, a pneumatic pressure test may be used instead of a hydrostatic test only when: