How to calculate the corrosion allowance

Calculating the corrosion allowance involves considering factors that can affect the rate of corrosion and adding a margin of safety to the original thickness of the piping. The corrosion allowance is typically specified during the design phase of a piping system to ensure its longevity and safety.

How you can calculate the corrosion allowance

Understand the Corrosion Environment: Assess the environmental conditions that the piping system will be exposed to, such as the type of fluid being transported, temperature, humidity, pH, presence of contaminants, and other corrosive agents.

Identify the Material’s Corrosion Resistance: Different materials exhibit varying degrees of corrosion resistance. Select a material that is suitable for the specific corrosive environment and consider its corrosion rate data.

Determine the Design Life: Decide on the desired design life for the piping system. This is the intended service life for the pipe before any significant replacement or repair is required.

Select the Corrosion Allowance Value: The corrosion allowance is typically expressed in millimeters (mm) or inches (in). The specific value chosen depends on the severity of the corrosion environment, the material’s corrosion rate, and the desired design life.

Calculate the Corrosion Allowance: Use the following formula to calculate the corrosion allowance:

Corrosion Allowance = (Corrosion Rate × Design Life)

Add the Corrosion Allowance to the Original Wall Thickness: Once you have calculated the corrosion allowance, add this value to the original wall thickness of the piping material to obtain the final required wall thickness.

Final Wall Thickness = Original Wall Thickness + Corrosion Allowance

The corrosion allowance ensures that even if corrosion occurs over the piping system’s service life, there will be enough material remaining to maintain structural integrity and prevent leaks or failures.

Design Life

Design Life (DL)                 = 20 years for Equipment

= 10 years for piping, or

= Per Client’s requirements

  • Corrosion Rate (CR) Estimation

CR = mils per year (mpy) or millimeters per year (mm/yr

  • Corrosion Allowance (CA)

CA = 1/16”, 1/8”, 3/16” or 1/4”

= 1.6 mm, 3.2 mm, 4.8 mm or 6.4 mm

Maximum CA = 1/4” or 6.4 mm

(Upgrade material if required CA is > ¼” or 6.4 mm)

CA = CR x DL

Corrosion Mechanisms in Petroleum & Chemical Industries

  • Amine Corrosion
  • Ammonium Bisulfide Corrosion
  • Ammonium Chloride Corrosion
  • Atmospheric Corrosion
  • Boiler Feed Water /Condensate Corrosion
  • Carburization
  • Caustic Corrosion
  • CO2 Corrosion
  • Cooling Water Corrosion
  • Corrosion Under-Insulation (CUI)
  • Erosion-Corrosion
  • Flue Gas Dew Point Corrosion
  • Fuel Ash Corrosion
  •  Galvanic Corrosion
  • Graphitic Corrosion of Cast Iron
  • High Temperature H2/H2S Corrosion
  • High Temperature Hydrogen Attack

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