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Sunday, January 11, 2009

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Earlier posts "Quick Estimation of CO2 Corrosion Rate", "CO2 Corrosion Rate Estimation Using M-506 Model" and "CO2 Corrosion Using Freecorp Model", the DeWaard Milliam model, Norsok M-506 model and FREECORP models have been introduced. All these corrosion estimation models are available FREE for all. Corrosion allowance (CA) is provided in pipeline / piping to cater for required design life. Low corrosion rate (CR) will require low CA. For high CR, higher CA is provided. In many events, corrosion inhibitor (CI) is injected into corrosive in order to reduce CR. Although low CA with CI injection, CI would incur high operation cost. Thus, a life cycle cost (LCC) study will require to be conducted to determining he cost effective option in provision of normal material with CA plus CI or Corrosion Resistance Material (CRA).

For the CA plus CI injection option, one of the factor shall be taken into account in determining CA is the availability of corrosion inhibition. There are many events would lead to corrosion inhibition unavailable i.e. maintenance, nonscheduled trip, deficiency of inhibition, unexpected flow, etc. Following is a simple method in determining the CA by consideration of Availability.

CA = t x [ICR x Av + UCR x (1 - Av)]

where :
CA = Corrosion rate
t = Design life
ICR = Inhibited Corrosion Rate
UCR = Uninhibited Corrosion Rate

Example :
A pipeline is design for 25 years, the corrosion rate is about 3 mm/year. High corrosion allowance (25 x 3 = 75 mm) is required and this has lead to injection of Corrosion Inhibition (CI) to reduce the corrosion rate to 0.3 mm/year. It is expected the availability of corrosion inhibition facilties is about 90%. Calculate CA required with CI injection.

t = 25
ICR = 0.3 mm / year
UCR = 3 mm / year
Av = 90%

CA = t x [ICR x Av + UCR x (1 - Av)]
CA = 25 x [0.3 x 90% + 3.0 x (1-90%)]
CA = 14.25 mm

Required CA is 14.25 mm.

A simple Corrosion Allowance Calculator is ready for download.
Download Corrosion Allowance Calculator (Excel).

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posted by Webworm, 11:23 AM


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