TML

[Corgas]

Recently an inspector (API 570 certified) told me that there's no need to place a tmL on the inside of radii of elbows. According to his opinion tml should only be placed on outdide radii of elbows.

He also mentioned that this criteria comes from a software that he's using.

any thoughts?

 

[rustbuster]

It depends on the expected damage mechanism. Where flow accelerated corrosion is a concern, there may be eddy currents that promote damage on the inside radius and down stream.

What software is he using? what is his corrosion background?

I would stick to measuring both where you have access.

Good Luck

 

[Corgas]

rustbuster:

software I think is ultrapipe. I just finished reading API 570 and in 5.3.6.d) At any point of flow direction change, such as the inside and outside radii of elbows.

This is recommended where erosion/corrosion is expected.

His particular inspection plan does not include any tml on inside radii of elbows, only on outside.

Is it common practice just to UT outside radii of elbows?

 

[SJones]

If you are considering particulate erosion as the degradation mechanism, flow studies show that momentum will keep the particles destined for the extrados. If you are expecting a degradation mechanism such as amine corrosion, where fluid turbulence strips a protective film, then you would probably need to check both sides.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[rustbuster]

Corgas,

As Sjones puts it, if the damage expected is erosion (particulate's) go with the OD. I have seen corrosion on or just downstram of elbows on the inside radius of amine, steam condensate, produced water, and various others.

For a good monitoring program you should first define the damage expected. Treat each commodity / service as unique corrosion circuits / loops and assess the type(s) of damage expected.

From there you can build sound inspection plans.

Good Luck

 

[apiguy]

Being a 570 inspector myself, here is what we typically do,

During the initial data collection phase, we take readings completely around the pipe typically on 4 quadrants. After a corrosion rate is established, then we evaluate the revelance of the inside radius readings and through an RBI analysis, elect to maintain or delete this data recording point. Many different schools of thought out there on this one, but typically, the OD is thinner by nature of manufacturing. Bent pipe and other weird critters have as much as 18% thinning in the outside radius while the ID stays fairly constant. When in doubt, shoot a profile radiograph looking for sublte density changes in both planes.

The APIGUY....