NDT Leak Testing Techniques 4 — Questions and Answers
Question 1: Which leak testing method is BEST suited for detecting leaks in underground pipelines without excavation?
- Bubble soap solution testing
- Acoustic emission or acoustic leak detection (Correct answer)
- Hydrostatic pressure testing
- Dye penetrant inspection
Correct answer: Acoustic emission or acoustic leak detection
Acoustic emission and acoustic leak detection methods can locate leak sources in buried pipelines by detecting the sound generated by escaping fluid.
Question 2: In a vacuum bell jar leak test using a helium mass spectrometer, what is the test configuration?
- Helium is sprayed outside while the part is at atmospheric pressure inside
- The part is placed in a vacuum chamber; helium is applied outside, and the detector monitors inside (Correct answer)
- The part is immersed in a water bath and pressurized with helium
- The part is pressurized internally with helium while a sniffer scans the exterior
Correct answer: The part is placed in a vacuum chamber; helium is applied outside, and the detector monitors inside
In the vacuum bell jar method, the part interior is evacuated and connected to the detector, and helium applied outside permeates through leaks to be detected.
Question 3: What is the significance of the 'leak rate' unit 'Pa·m³/s' (or atm·cc/s) in leak testing?
- It describes the velocity of gas through a leak path
- It quantifies the volumetric flow of gas through a leak at a given pressure differential (Correct answer)
- It measures the cross-sectional area of a leak
- It indicates the chemical composition of escaping gas
Correct answer: It quantifies the volumetric flow of gas through a leak at a given pressure differential
Leak rate units like Pa·m³/s express the volume of gas passing through a leak per unit time at a specified pressure, quantifying how much gas is lost.
Question 4: What condition must be met before performing a pneumatic pressure leak test on a pressure vessel?
- The vessel must be filled with a dye solution first
- A risk assessment must confirm pneumatic testing is safe, as compressed gas stores more energy than liquid (Correct answer)
- The vessel must always be tested at twice the maximum allowable working pressure
- The vessel must be cooled to below 0°C before pressurization
Correct answer: A risk assessment must confirm pneumatic testing is safe, as compressed gas stores more energy than liquid
Because compressed gas stores significantly more energy than liquid, a pneumatic test requires a safety risk assessment before proceeding.
Question 5: A refrigerant system leak is most commonly detected using which instrument?
- Ultrasonic flaw detector
- Electronic halogen (refrigerant) leak detector (Correct answer)
- Magnetic flux leakage scanner
- Eddy current tester
Correct answer: Electronic halogen (refrigerant) leak detector
Electronic halogen or refrigerant-specific detectors sense refrigerant molecules escaping from a leak and are standard tools in HVAC/R leak testing.
Question 6: Which factor most significantly affects the sensitivity of a bubble leak test?
- The color of the bubble solution
- The surface tension and viscosity of the bubble solution (Correct answer)
- The ambient humidity at the test location
- The material of the component being tested
Correct answer: The surface tension and viscosity of the bubble solution
Lower surface tension solutions produce smaller, more visible bubbles from tiny leaks, directly affecting the sensitivity of the test.
Question 7: What is the purpose of a 'soak time' or 'stabilization time' in a pressure decay leak test?
- To allow personnel to leave the area for safety during the test
- To allow temperature and elastic effects to stabilize so pressure changes reflect only actual leaks (Correct answer)
- To pressurize the system to the required test level incrementally
- To allow dye penetrant solution to enter any leaks
Correct answer: To allow temperature and elastic effects to stabilize so pressure changes reflect only actual leaks
Soak time allows thermal equilibration and elastic expansion of the test part to stabilize, so subsequent pressure decay is attributable to leakage rather than these effects.
NDT Leak Testing Techniques 4
Which leak testing method is BEST suited for detecting leaks in underground pipelines without excavation?