Estimating the wall thickness loss of the pipeline 1

[gogu0451]

I have a pigging sample. It has 5000 ppm wt Iron content. I am not sure what kind of iron (ferric or ferrous). I know that the total sample obtained is 100 lbs sludge. Let’s say I know the approximate length of the pipeline is 20 miles. Is there any way we can find out the approximate wall thickness loss of the pipeline. Assume that the iron content is responsible for the corrosion taking place inside the pipeline. Let’s say pipeline diameter is 20 inch and wall thickness is 0.531 in. You can assume any constants like density.

Thank you very much for your response in advance.

 

[SJones]

The biggest assumption that you will make is that the iron came uniformly from the total inside surface area of the pipeline. The second biggest assumption will be that there was no iron in the fluid before it entered the pipeline. Have a look at NACE RP0192-98.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[kenvlach]

Aren't you also assuming that pigging removed 100% of the corrosion?

After the big assumptions, the math is almost trivial:
5000 ppm Fe x 100 pounds = 0.5 pounds Fe,
[÷] by 20 miles = 0.025#/mile

0.025#/(0.284#/cu in) = 0.088 cu in Fe/mile
Take ID of pipe as 20" for simplicity, so inside SA = 3,981,000 sq in/mile,
The thickness of Fe removed is
0.088 cu in/3,981,000 sq in = 2.2 x 10^-8 inch
= 5.6 x 10^-10 meters.

As diameter of Fe atom is 2.48 x 10^-10 meters,
it seems that the pigging recovered about 2 monolayers of Fe atoms.
There's more oxidized Fe left on the inner surface of the pipe.

 

[rconner]

I'm rough guessing you had maybe 12 million pounds of steel in the original pipeline. A hundred pounds of iron, if you assumed it came from same, would be about one thousandth of a percent of that total. [However, I'm not sure you can read much into this, one way or the other.]

 

[0707]

The way to estimate wall thick loss of your piping is to take wall thickness measurements thought ultrasonics with intelligent pigging.

Another possibility, maybe less accurate than intelligent pigging, is to use Long Range Guided Waves ultrasonics.

I suppose that through iron analysis extrapolations it will be very difficult for you to estimate corrosion loss.

 

[rconner]

I would only add that any method is dependent on the accuracy and or precision of the instruments/measurements, and if repeated measurements over successive intervals of time with accurate intruments are not available (from which e.g. to determine say a "corrosion rate"), also of course dependable knowledge of the actual original thickness of the pipe at the specific measurement locations. I suspect any assumptions concerning original thickness should be duly noted/considered in such cases.

 

[0707]

To make an accurate corrosion rate on your piping system through iron losses you have to install several removable corrosion coupons in representative corrosion locations end check them for iron losses each three or six months till you have confidence on your corrosion rates accuracy.

 

[BenThayer]

you don't happen to work for BP, do you?

 

[stanweld]

Why does one assume that all the corrosion products ended up in the sludge? Corrosion products will also be carried by the fluid in the pipe throughout its operating life and the rate at which corrosion will occur should rarely if ever be considered constant.

 

[0707]

BenThayer

I do not work for BP. I work for GALP