Case Studies in Failure Prevention
Q1: How was Flow-Accelerated Corrosion (FAC) identified and mitigated in a power plant's A106B piping?
A1: A power plant noticed increasing iron levels in feedwater. Ultrasonic thickness surveys were conducted, focusing on elbows, tees, and reducers in the A106B condensate and feedwater systems. The surveys revealed significant, localized wall thinning in specific areas, characteristic of FAC. The mitigation strategy was multi-pronged: 1. Short-Term: Schedule replacements for the thinned components, often upgrading them to chrome-moly alloy (e.g., ASTM A335 P11) which is highly resistant to FAC. 2. Long-Term: Reviewed and optimized water chemistry control, specifically maintaining a higher pH (9.6-9.8) with ammonia or other alkalizing agents, which dramatically reduces the solubility of the protective magnetite layer.
Q2: What investigation follows a leak at a A106B socket weld fitting?
A2: A leak at a socket weld fitting triggers a detailed investigation. 1. Visual Inspection: The weld is inspected for obvious defects like cracks or lack of seal weld. 2. Dye Penetrant Testing (PT): Applied to the weld area to reveal fine surface cracks. 3. Root Cause Analysis: Often finds two common issues: a) Improper Gap: A failure to leave a 1/16" gap between the pipe and the socket shoulder before welding, leading to high stress cracks as the weld contracts. b) Poor Weld Procedure: Incorrect welding technique or lack of preheat. The solution involves re-training welders on proper socket weld assembly and procedure, and repairing the faulty weld.
Q3: How can vibration-induced fatigue cracking in A106B support systems be solved?
A3: Cracking found in A106B pipe near pump connections due to vibration is a classic fatigue failure. The solution involves 1. Immediate: Installing a temporary clamp or brace to arrest crack growth and dampen vibration. 2. Investigation: Using vibration analysis tools to measure the frequency and amplitude of the vibration. 3. Permanent Fix: Redesigning the support system. This often involves adding a custom-designed spring hanger or snubber to control movement, installing a sway brace to resist lateral motion, or adding a massive concrete grout block to change the natural frequency of the system and detune it from the vibration source. The cracked section is then cut out and replaced.
Q4: What is the procedure for dealing with discovered graphitization in aged A106B steam lines?
A4: Discovering graphitization (often through metallurgical replication during an outage) in aged A106B steam lines is serious. 1. Assessment: Extensive ultrasonic testing and replication are used to map the extent of the damaged material. 2. Fitness-for-Service (FFS) Analysis: An API 579/ASME FFS-1 assessment is performed by an expert to determine if the pipe can remain in service at reduced pressure/temperature or for a limited time. 3. Action: Based on the FFS, the decision is made to either: a) Monitor: For minor cases, with more frequent inspections. b) Repair/Replace: The affected sections are cut out. The replacement material is often a low-alloy steel like P11 that is resistant to graphitization.
Q5: How is a failure due to oxygen pitting in a closed-loop A106B system corrected?
A5: Correcting oxygen pitting requires addressing both the symptom and the root cause. 1. System Flushing: The system is flushed to remove loose corrosion products. 2. Inspection & Replacement: The most severely pitted sections of A106B pipe are located using internal visual inspection or advanced UT and are replaced. 3. Root Cause Fix: The water treatment program is thoroughly audited. This involves checking the performance of the deaerator, ensuring proper operation of chemical feed pumps for oxygen scavengers (e.g., sodium sulfite), and verifying there are no air leaks on the suction side of pumps. The system is then re-commissioned with close monitoring of oxygen levels until they are consistently at or near zero ppb.
