Corrosion Under Insulation (Pipelines) 1

[Inspector_1998]

Recently I have observed CUI at several locations Upstream/downstream of expansion loops of pipelines especially: lower section of pipelines between 4 to 5 clock position) and the length of a corroded area less than a half meter along of the pipelines that marked on the attached image.
the constructed material is Carbon steel and size starting from 8 inches to 18''.
Now, what are the possible solution to stop the further corrosion build up? Since these lines are not coated.
Also, what
thickness measurement carried out and found the remaining thickness as under

18'' Schedule 30 (NT 11.1mm), operating temperature 95C = 5.4mm ( actual remaining thickness) advance CUI stage
12'' Schedule 30 (NT 8.38mm), operating temperature 45C = 5.1mm ( actual remaining thickness) Med.CUI stage.
8'' Schedule 30 (NT 7.04mm), operating temperature 45C = 5.2mm ( actual remaining thickness) Intial CUI stage.

THE INSULATION TYPE: Rockwool insulation.
the affected type has been highlighted in the attachment.

I am looking for the temporary repair since these lines can 't be isolated due operational needed.

thank you in advance

 

[LittleInch]

Clean it, paint it with something like high build PU.

Or epoxy sleeve it or use a couple of clocksprings.

Multiple options - depends on stress on the pipe and how much you want to spend.

Or replace it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Inspector_1998]

LittleInch
thank you so much for prompt reply on my concerns.
Yes, can stop further corrosion occur but what about lowe thickness reading.
do you think GRE or none metallic wrapping can be a good solution to reset the strength of pipelines until the next Shutdown?
How to calculate the stress after thickness reduction if the operation pressure 2.4Bar and temperature 90C

 

[BJI]

Perform a FFS assessment of the LTA in accordance with API579. Not sure about the extent of corrosion but GRP wrapping is probably not required. Do you have the pipe stress results for all loading combinations?

 

[mk3223]

The CUI was resulted from the moisture absorbed by the insulation. Is the insulation material removed in the CUI section to prevent it from getting worse?
It doesn't sound that there is a pipe leak which requires a leak seal repair right away per your description.

 

[LittleInch]

Long axial deep corrosion defects, especially in high bending stress areas like you show might be below the allowable thickness, but you need to assess the damage.

Yes, external sleeves, either steel or fibre glass can make the pipes safe until they can be replaced.

People like this exist to help you ( at a price...)

http://www.furmanite.co.uk/services/composite-repair/low-pressure-composite-repair-furmaglass/

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Lebpower]

CUI could mostly appears on systems with operation temperatures under 100°C. So, on this systems you have to be always inspectioning that the insulation is proper installed, if not it is probably that water or moisture could leak through the insulation and once there the corrosive attack will start.

A worse scenario could be the leak of hydrocarbon inside the insulated pipe which in presence of high temperature could ignite. That important is to inspect insulated pipe.

As they as mentioned above, in this case the best recommedation is to remove all the insulation to check the condition of all the pipe system and check the Thickness of the affected area, then evaluate if it's possible to install a path. then you have to clean and paint the pipe and reinstall the insulation verifying its proper instalation.

Greetings.

 

[Lebpower]

As long as the type of failure is not a crack, i'd recommed a path to avoid downtime of the unit

 

The insulation material already removed on the pipelines and corrosion product some location had been removed and found low thickness for that , I ask the maintenance team to stop the progress in order to avoid hazardous of potential leak .

Now, is there any safe and reliable method to confirm the remaining thickness under corrosion product since these line size above 6 inch above RT film capability?
GRE is a good option but no action will be adopted without knowing the remaining thickness.

 

[LittleInch]

UT probes?

A depth guage?

The corroded material has little strength so removing it does not weaken the pipe but can be alarming when a large chunk falls off in your hand. You really need to reduce pressure or isolate and do this properly or put a large clamp over the corroded bit until you can investigate properly.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BenStewart]

Were the measurements in the opening post before or after descaling?

If post descale, the extent of wall thinning needs to be assessed against the design conditions. As already mentioned above, if tmm-FCA < MAWT then you will need to do a fitness for service assessment.

If before descale, the remaining wall needs to be measured. What are your company's procedural limits for doing live surface preparation? Not an exact science but an estimate of wall loss can be made from factoring down the scab height. Once descaled take measurements. If you're not comfortable doing a live descale it may be an option to locally isolate. RT can still be done on 6" pipe, just make sure enough shots are taken so that the plane of minimum thickness is detected.

Have you got internal wall thinning issues of these lines too? This would need to be accounted for when estimated the remaining wall thickness.

Once the above is done and assuming all is OK (enough wall thickness and no need for a repair/replace) repaint your pipes, then you should consider doing more exploratory inspection for more damage. From your sketch it looks like it's a low point so it makes sense the this is where the water would collect and corrosion occur most aggressively. Look at other low points, areas of damaged or loose insulation/cladding, unsealed areas, etc.

Also, Rockwool isn't the best choice if CUI is a threat. There are numerous companies which offer insulation systems which attempt to prevent water ingress and have shown to be successful in-service.

 

[Chumpes]

Hi
the ASME construction code provides methods for calculation of minimum thickness of pressure items.
do the calculation then compare the result to the thickness you actually measured.
API 579 FFS can help you to make your decision.
regards