Piping Piping Inspection and Testing 1 — Questions and Answers
Question 1: What is the most common NDE method used for measuring remaining wall thickness of in-service piping?
- Radiographic Testing (RT)
- Ultrasonic Testing (UT) with thickness gauge (Correct answer)
- Magnetic Particle Testing (MT)
- Visual Inspection (VT)
Correct answer: Ultrasonic Testing (UT) with thickness gauge
Ultrasonic thickness measurement (UTM) using a UT gauge is the standard method for measuring remaining pipe wall thickness from the outside without requiring process shutdown.
Question 2: What is 'injection point corrosion' in process piping?
- Corrosion caused by oxygen injection into steam systems
- Localized and often severe corrosion at or near a point where chemicals are injected into a pipe (Correct answer)
- Internal corrosion in fire water injection lines
- Corrosion due to cathodic protection current injection
Correct answer: Localized and often severe corrosion at or near a point where chemicals are injected into a pipe
Injection points are high-risk areas for localized corrosion because injected chemicals may not be fully mixed with the process stream, creating concentrated corrosive conditions near the injection nozzle.
Question 3: What is 'pitting corrosion' and why is it particularly dangerous in piping?
- Uniform corrosion affecting large pipe surface areas equally
- Localized attack creating small deep holes that can penetrate the pipe wall without significant weight loss or obvious external signs (Correct answer)
- Erosion damage from high-velocity fluid impingement
- Corrosion occurring only on pipe external surfaces
Correct answer: Localized attack creating small deep holes that can penetrate the pipe wall without significant weight loss or obvious external signs
Pitting creates highly localized deep penetrations that can cause through-wall leaks while the average wall thickness appears acceptable, making it difficult to detect with routine thickness surveys.
Question 4: What is the purpose of an acoustic emission (AE) test on piping?
- To measure the sound level inside operating pipelines
- To detect and locate active crack growth, leaks, or structural defects by listening for stress wave emissions during pressurization (Correct answer)
- To verify pipe insulation sound attenuation
- To check for cavitation in pump suction piping
Correct answer: To detect and locate active crack growth, leaks, or structural defects by listening for stress wave emissions during pressurization
Acoustic emission testing listens for high-frequency stress waves generated by active defects (crack growth, leaks) during loading, allowing detection and location of damage without full surface access.
Question 5: What is the 'remaining life' calculation used in piping inspection programs?
- Time until next scheduled maintenance shutdown
- Estimated time until the pipe wall reaches minimum required thickness based on measured corrosion rate (Correct answer)
- The number of pressure cycles remaining before fatigue failure
- Calendar time since the pipe was last replaced
Correct answer: Estimated time until the pipe wall reaches minimum required thickness based on measured corrosion rate
Remaining life = (actual thickness - minimum required thickness) divided by the corrosion rate, providing an estimate of how much service life remains before mandatory retirement.
Question 6: What type of corrosion mechanism causes damage specifically in the heat-affected zones of welds in sensitized stainless steel?
- Uniform corrosion
- Intergranular corrosion (weld decay) (Correct answer)
- Erosion corrosion
- Galvanic corrosion
Correct answer: Intergranular corrosion (weld decay)
Sensitized stainless steel (carbide precipitation at grain boundaries during welding) becomes susceptible to intergranular corrosion in the weld HAZ, known as 'weld decay.'
Piping Piping Inspection and Testing 1
What is the most common NDE method used for measuring remaining wall thickness of in-service piping?