Corrosion allowance for pressure vessels
The corrosion allowance of a pressure vessel refers to the extra thickness or capacity reserved in the vessel design in order to prevent and mitigate the impact of corrosion on the performance of the pressure vessel when designing and manufacturing the pressure vessel. This design consideration is to ensure that the pressure vessel maintains its structural integrity and functional stability over its service life, even if corrosion occurs.
pressure vessel
Requirements for Selecting Corrosion Allowance
1.General Requirements:
Corrosion allowance should be considered for all parts in contact with the working medium, including the shell, heads, nozzles, manholes (handholes), and internal components, except as noted in point 6 below.
2.Special Provisions or Experience:
If the process system specification or engineering design documents have specific provisions or actual usage experience regarding corrosion allowance, these should be followed.
3.Calculation for Uniform Corrosion:
For vessels with uniform corrosion and reliable data, the corrosion allowance should be calculated as follows:
C2=Design Life×Corrosion Rate
4.Additional Considerations:
The design should also account for erosion, wear, and localized corrosion impacts on the vessel shell or components.
5.Components with Dual Exposure:
For components exposed to different media on both sides, select different corrosion allowances for each side, and sum them to determine the total corrosion allowance for the component.
6.Exceptions:
Corrosion allowance is not required in the following situations:
1).When the medium does not corrode stainless steel (including stainless steel, stainless steel clad plates, and stainless steel overlay).
2).When the base material has reliable corrosion-resistant linings (such as lead, rubber, or plastic linings).
3).For the sealing surface of flanges.
4).For heat exchanger tubes, U-tubes, floating head bolts, longitudinal baffles, tie rods, spacer tubes, baffle plates, and support plates in shell-and-tube heat exchangers.
5).For the external surface of vessels and their external components (such as supports, legs, brackets, and tower top columns) that can be effectively protected against environmental corrosion by painting (excluding skirt supports and anchor bolts).
6).For removable tray plates, packing, etc.
pressure vessel
Values of Corrosion Allowance
Unless otherwise specified by the process system specification or engineering design documents, the corrosion allowance for vessels should be determined as follows:
(1).Shell and Head Corrosion Allowance:
For carbon steel or low-alloy steel vessels with compressed air, steam, or water as the medium, the corrosion allowance should not be less than 1.0 mm.
For other conditions, the corrosion allowance should be selected based on specific requirements or experience.
(2).Higher Corrosion Allowance:
If the corrosion allowance exceeds 6 mm, more corrosion-resistant materials should be used, such as composite steel plates, overlays, or linings.
(3).Determining Corrosion Rate:
The corrosion rate can be determined based on engineering design practice or by consulting relevant corrosion manuals.
By adhering to these guidelines, the integrity and service life of pressure vessels can be ensured, enabling them to withstand the effects of corrosion and operate safely in various industrial environments.
The consideration of corrosion margin is not only related to the economic cost of the pressure vessel, but also a key factor in ensuring that the vessel meets the strength requirements. In actual engineering design, reasonable wall thickness selection must consider corrosion margin to ensure that the pressure vessel can maintain safe operation even if corrosion occurs during its service life.