Modifying Corrosion Calculations in VMGField
Note: VMG is pleased to announce the renaming of VMG Gasmod to VMGField! VMGField better represents the software’s capabilities and aligns with the vision of the VMG products.
In the Summer 2017 newsletter, evaluating internal corrosion risk in VMGField was introduced. This article expands on that to describe how a user can update the parameters used in the corrosion calculation to align to observed data or account for a mitigation strategy.
The advantages of this include:
- A calculation honoring when mitigation is in place
- Appropriate Risk being estimated
- Proper ranking based on Risk in the Corrosion and Risk Reports
The steps below guide the user through modifying the necessary inputs of the PIMS feature. It assumes a VMGField model exists with the PIMS module active (as per the Summer 2017 newsletter or other).
1. In order to modify the corrosion calculations, the tables used to perform the calculations can be updated. They can be updated through the use of an include file (*.inc). An include file is a text file containing data that is added to or updates the information passed to the simulator. Below is a screenshot of an example .inc file that can easily be created using any text editor like WordPad.
2. Documentation of the tables and the Likelihood calculation is in the VMGField manual starting on P. III-6. The .inc above would do the following:
a. CORT 1 = Corrosion Table 1. It rescales the Likelihood value from the internal range of 0 to 10 (first column) to the data in the second column. In this case it will double the likelihood calculated.
b. CORT 5 = Corrosion Table 5. It refers to the Fs (scale/pitting factor) in the corrosion rate calculation for systems without elemental Sulphur. In this case, it will halve the corrosion rate calculated since the data presented above are half the default values.
c. CORC = Keyword that updates constants used in the corrosion rate calculation (water pH , carbon  and chromium  content of pipe, elemental Sulphur flag [1= on], constant CH in dense dispersion calculation ). In the case above, the only change from default is the elemental Sulphur flag has been set to on.
3. To add a corrosion include file, go to the first timestep of your case, select Data- Include Files- Empirical Include File.
4. Browse to the .inc and select it.
5. Modifying CORT 1 will scale the Likelihood value from the internal range of 1 to 10. In an example file, viewing the Likelihood on the Pipe Cross Section for a pipe leg and the first output timestep, we see the Likelihood is 9 based on the internal table.
6. If we modify CORT 1 to the following and then rerun the simulation, we see that 9 becomes 18 because we scaled the 0-10 to 0-20 so the calculated value gets scaled up by a factor of 2.
7. Repeating the process using the data below shows how the scaling behaves when using more than one, nonlinear interval.
8. The internally calculated Likelihood of 9 becomes 48 since the internal interval from 5-10 is rescaled to 40-50.
9. To modify the corrosion calculation, CORT 5 or CORT 6 can be updated. These tables refer to the Fs (scale/pitting factor) in the calculation. Therefore, if it was desired to halve the calculated corrosion rate due to mitigation strategies, the Fs values in CORT 5 (table used when no elemental Sulphur is present) can be multiplied by 0.5.
10. Inspecting the corrosion rate for the same pipe leg. Unmitigated, we see it ranges from 28.233 to 30.046 mil/yr.
11. Having updated the .inc to include the CORT 5 table defined above, saving it and re-running the simulation, inspection of the Corrosion Rate on the Pipeline Cross section shows the values now range from 14.116 to 15.023, half of the previous case.
12. To update the other constants used in the corrosion rate calculation (water pH, carbon and chromium content of pipe, elemental Sulphur flag, etc), the CORC keyword can be added to the .inc file. Adding the line “CORC = 7 0 0 1 1” will turn on the elemental Sulphur flag. If done, CORT 6 would need to be modified instead of CORT 5 in the previous steps because it provides the table of Fs when Sulphur is present.
Please contact your local VMG office for more information.
Gordon Slemko, P.Eng., VMG Calgary