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Selection Criteria For Corrosion Allowance Of Pup Joints

What is corrosion allowance?


Corrosion allowance refers to the difference between the wall thickness of the pup joints and the minimum allowable wall thickness. It is an important indicator for measuring the remaining life of the pipeline. Pipelines will suffer from various influences such as corrosion during use. The larger the allowance, the thicker the wall thickness inside the pipeline, the stronger the corrosion resistance, and the longer the operation time.


Selection criteria for corrosion allowance


1. ASME standard


The ASME standard divides corrosion allowance into 5 levels. The higher the level, the stronger the corrosion resistance of the pipeline. The specific standards are as follows:


- Level 1: The minimum acceptable wall thickness is reduced by 5%
- Level 2: The minimum acceptable wall thickness is reduced by 10%
- Level 3: The minimum acceptable wall thickness is reduced by 15%
- Level 4: The minimum acceptable wall thickness is reduced by 20%
- Level 5: The minimum acceptable wall thickness is reduced by 25%


2. NACE standard


The NACE standard divides corrosion allowance into three levels: L1, L2, and L3, of which L3 is the highest level with the largest corrosion allowance. The specific standards are as follows:


- L1: Basic allowance, wall thickness calculation is based on nominal wall thickness and allowable depth of internal corrosion
- L2: Intermediate allowance, which can be calculated according to specific circumstances and verified by inspection
- L3: Advanced allowance, establish pipeline protection measures, and decide according to specific circumstances, inspection and definition


3. GB/T standard


The GB/T standard stipulates that corrosion allowance is divided into three levels: 1, 2, and 3, among which level 1 is the basic allowance and level 3 is the highest allowance. The specific standards are as follows:


- Level 1: The allowable thinning amount is 10% of the minimum wall thickness
- Level 2: The allowable thinning amount is 20% of the minimum wall thickness
- Level 3: The allowable thinning amount is 30% of the minimum wall thickness


Different standards are applicable to different pipeline materials and working environments. Selecting the appropriate allowance level helps to ensure the safety of pipeline operation.


How to determine the corrosion allowance level?


The determination of the corrosion allowance requires comprehensive consideration of factors such as the material, use environment, and operating time of the pipeline. When designing, various possible factors should be fully considered to design a suitable allowance level. It should be pointed out that the higher the margin level, the higher the safety of the pipeline, but the cost of the pipeline also increases accordingly.


How to perform corrosion detection?


Corrosion detection is an important part of ensuring pipeline safety. During the use of pipelines, corrosion detection needs to be carried out regularly to detect and deal with corrosion problems in a timely manner. Commonly used corrosion detection methods include: ultrasonic detection, X-ray detection, coating corrosion detection, etc.


Conclusion


The selection of corrosion margin is an important part of ensuring the safety of pipeline operation. When designing and maintaining pipelines, the appropriate margin level should be selected, the appropriate corrosion detection method should be adopted, corrosion problems should be discovered and dealt with in a timely manner, and the safe operation of the pipeline should be ensured.