Corrosion in piping

The majority of corrosion in Steel Piping, around 80-85%, typically occurs at pipe supports on straight sections, while about 15-20% happens at elbows.

Corrosion at branch connections, such as tees, is uncommon, as observed by plant maintenance engineers and inspectors.


Table of Contents

1. Corrosion Assessment Procedure

To assess a corroded pipe, the evaluator must first gather specific data:

  • The pipe’s outside diameter (D)
  • Wall thickness (t)
  • Material grade
  • Weld seam type
  • Design pressure
  • Process fluid in the pipe

Then, they need to identify the maximum depth of corrosion pitting (d) or the thinnest remaining wall thickness (t-d) for each corroded area.

The evaluator records the axial and circumferential spread of the corrosion, focusing on the deepest penetration or thinnest wall section. This requires thorough examination to accurately identify the most severe metal loss. Finally, the evaluator calculates the Maximum Allowable Operating Pressure (MAOP) based on the relevant design code.

Based on the above data, the below procedure is widely used for piping assessment;

  1. A pipe segment with corrosion is suitable for continued use if the deepest pit (d) is no more than 20% of the specified wall thickness (t), and the thinnest remaining wall (t-d) is at least 80% of the original thickness.
  2. A segment needs repair or replacement if the deepest corrosion exceeds 80% of the specified wall thickness, or if the remaining wall thickness is less than 20% of the original.
  3. The corroded pipe segment can remain in service if its failure pressure is equal to or greater than the pipe’s original Maximum Allowable Operating Pressure (MAOP).
  4. If the safe pressure of a corroded pipe is below its original Maximum Allowable Operating Pressure (MAOP) but still equal to or above the actual MAOP, the pipe either needs a new safe working pressure determined or requires a suitable repair.
  5. Establishing safe conditions becomes essential when the safe pressure of a corroded pipe falls below the actual MAOP.

2. Classifications of Corrosions of Piping

2.1 External & Internal Corrosions

The impact of material loss due to corrosion is same for both internal and external corrosion, but the methods used to evaluate the damage differ significantly.

For external corrosion, measurement is quite simple. Basic tools like pit gauges and rulers can be used to measure damage. Cleaning the surface of the pipe often makes the extent of damage clearly visible which allows straightforward assessment. The corroded surface can be cleaned, evaluated, and if necessary can be re-painted. Once treated and recoated, it’s generally assumed that external corrosion will halt.

On the other hand, assessing internal corrosion is more complex. It requires ultrasonic devices in most of the cases. A unique consideration for internal corrosion is the use of a corrosion allowance. This allowance involves adding a specified thickness to the measured depth of corrosion, making the evaluation valid for a certain period. Unlike external corrosion, internal corrosion may continue even after initial evaluation and treatment. The corrosion allowance should match the anticipated corrosion rate and the expected duration until the next inspection. This approach acknowledges the ongoing nature of internal corrosion and plans for future integrity management.

2.2 Local & General Corrosions

Local Corrosion involves discrete and relatively isolated areas of metal loss throughout the pipework. The extent of such corrosion—both longitudinally and circumferentially—is easily measured by determining pit depths. Typically a pit gauge is used for measurement. The original surface of the pipe often serves as a reference plane for these measurements.

On the other hand, general or widespread corrosion requires a more complicated measurement study. Places where pipe’s original surface remain intact can be linked with a straight edge in order to allow the measurement of pit depths along this edge. However, if such islands of intact surface is absent, ultrasonic devices should be used to determine wall thickness. Ultrasonic devices requires grinding flat spots on the pipe. This necessary for accurate measurement. It’s important to note that grinding off any metal can reduce the pipe’s failure pressure. Therefore, it is recommended to start grinding at the highest spots to estimate the remaining wall thickness. Repair work is not required if the pit depth does not exceed 20% of the wall thickness.

2.3 Regional Corrosions  

When evaluating the integrity of a pipe with a single corroded pit region, the focus is on assessing the length of the affected area and its maximum depth. However, if there are multiple corroded regions close to each other, their cumulative impact on the pipe’s integrity must be considered.

Burst test data indicate that pipe failures typically originate in the thinnest regions. Therefore, when multiple corrosion profiles are present within a metal loss region, they should be combined into a composite profile for a more accurate assessment. This combination considers the overall effect of the corrosion on the pipe’s strength and durability.

It’s important to note that there are limits to this approach. Corroded regions that are widely separated may not need to be combined for assessment purposes.

The process of combining corroded regions and evaluating their collective impact requires experience and judgment. In situations where experienced personnel are not available, it is recommended to stay on the safe side and make the most conservative estimates regarding the pipe’s integrity. This conservative approach ensures the highest level of safety and reliability in maintaining the structural integrity of the piping system.

3. Conclusion

When evaluating a corroded area of piping, if the determined safe operating pressure exceeds the pipe’s maximum or design pressure, it implies that the pipe can still withstand pressures higher than its original specifications. The piping can be safely returned to service in this case.

On the other hand, if the safe operating pressure is found to be below the maximum or design pressure, it shows a reduction in the pipe’s structural integrity due to corrosion. In such scenarios, the pipe cannot be safely operated at its originally intended pressure. This requires below further actions;

  • Further Evaluation: Conducting a more detailed assessment to understand the extent of corrosion and its impact on the pipe’s overall strength and functionality.
  • Decommissioning: The pipe may need to be taken out of service permanently if the damage is severe and irreparable.
  • Lowering the Operating Pressure: Reducing the pressure at which the pipe is operated to a level that is safe considering its current condition.

Numerous pipe failures in the past have led to the need for assessing flawed pipe. The occurrence of several dramatic explosions resulted in extensive property damage and, in some cases, loss of human lives. There have been more explosions related to piping and pipelines than to any other type of equipment in industrial plants.

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