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Sunday, May 16, 2010

Earlier post “Seawater Treatment & Injection For Well Maintenance & Increase Productivity” discussed about the water source and its associated water injection treatment. Seawater will be filtered with coarse and fine filters to remove particles smaller than 2 micron with 98% removal efficiency. Filtered seawater will be deaerated to bring the Oxygen concentration down to 20 ppb. Oxygen scavenger is then injected to further reduce the Oxygen concentration down to 10 ppb in order to reduce corrosion to cost effective level. Chlorine is injected to injection water to avoid bacteria and alga growth.



Residue Oxygen in injection may still result corrosion in the pipeline. How much it affect the corrosion of Carbon steel pipeline ? Present of Chlorine is known further increase corrosivity of injection water. How this impact the corrosion water ? Increase of water velocity in pipeline will increase flow induced erosion of pipeline. Simultaneous erosion (flow induced) - corrosion (Oxygen & Chlorine) present in the pipeline. How water velocity affect corrosion ? What is the water velocity limit ? This post will present a water velocity limit with varies of oxygen concentration (and Chlorine content) in water .

Oxygen corrosion in water injection pipeline is controlled by diffusion rate of Oxygen into Cathodic with following reaction :


Steel is corroded on the Anodic side with following reaction :




Water Injection Velocity Limit
Chlorine Free
water injection velocity limit (Vmax)of Carbon steel pipeline is :



With Chlorine concentration of 0.5 ppm
water injection velocity limit (Vmax)of Carbon steel pipeline is :

where
CA = Corrosion Allowance (mm)
Y = Life span (years)
CO2 = Oxegen concentration (ppb)
T = Seawater temperature (degC)
ρw = Seawater density (kg/m3)

Example
Refer following figures for water injection velocity limit (m/s) versus oxygen level (ppb) with Chlorine free and 0.5 ppm Chlorine.


Above figures was based on

Corrosion Allowance, CA = 3.0mm
Life span (years), Y = 20 years
Seawater temperature, T = 25 degC
Seawater density, ρw = 1030 kg/m3

At 20 ppb of Oxygen concentration, the velocity limit is 5.7 m/s (Chlorine free) and 2.6 m/s (Chlorine = 0.5 ppm).

At 10 ppb of Oxygen concentration, the velocity limit is 12.2 m/s (Chlorine free) and 5.6 m/s (Chlorine = 0.5 ppm).


Ref :
1)    J.W. Oldfiedl, G.L. Swales, B.Todd, “ Corrosion of Metals in Deaerated Seawater”, Proc. Of the 2nd Corrosion Conference, held Jan, 1981
2)    J.M. Drugli, T. Rogne, “ Effect of Oxygen and Chlorine Content on the Corrosion Rate of Carbon Steel Welds in Injection Water” CORROSION/93 paper no 65
3)    Mamdouh M. Salama, “Erosion Velocity Limits for Water Injection Systems”, 1993

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posted by Webworm, 6:37 AM

1 Comments:

Blogger tushar said...

Do anybody can let me know what is velocity limit for carbon steel PE lined pipe for produced water servive? Is there any specific formula to calculate the same?

May 18, 2010 at 8:14 AM  

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