Corrosion under insulation (offshore)

[SAeng]

I have come across a problem with Corrosion under insulation(CUI). The problem is that moister (mainly form rainwater) is accumulating under the insulation of pipework. It is unknown if there is a coating on the carbon steel pipework.we are considering removing all insulation that is not absolutly required to prevent the any further attack. however there are close to 700 lines to me manually inspected. Has anyone had similar experience with CUI on offshore facilities.

some questions:
1. do process hot lines normally dry up the moisture preventing the problem?
2. What about areas under shelter?
3. If an alternative personnel protective device(wire mesh) is used will teh problem still persist (around clamp area).

I am looking at a way of removing or even combating CUI without replacing pipework and if possible while the plant is running.

Can someone shed some light into this problem.

 

[rinky]

Process hotlines do not dry up the moisture, during operation the moisture may migrate away from the pipe surface but will be retained within the insulation and the atmosphere will be very humid.

what are the temperatures of these line?

To combat this problem without disassembly, a specialized liquid rust inhibitor is introduced into the areas of concern.

Any liquid water should be drained, the drain hole plugged, install the inhibitors.

Terry







What is the approximate exterior temperature of these pipes?

What kind of insulation is used?

Can you drill a hole at the low spots to allow the liquid/moisture to drain off then seal the hole.

 

[corrosionman]

Is the insulation then externally clad with stainless steel sheet ?? I have seen so much cladding where the overlaps are wrong way up -- -- Instead of shedding rain they allow it into the insulation. You then got mild steel pipe + soggy wet + stainless. If then the Stainless ans mild steel are in contract this is the best arrangement possible for dissimilar metal corrosion which can increase tenfold the damage rate for the MS. Cheers CM

 

[ferchuvicente]

Saen: I would like to know, how i can to performance a NDT (non destructive testing)trying to look for CUI in an cryognic equipment in service. Is good practice Infrared Thermography?

regards

FERNANDO

 

[EdStainless]

There has been a lot of CUI work done by MTI. I believe that non-members can purchase the reports.

The suggestions are simple and harsh.
1. the insulation must have no leachable componants.
2. the insulation must not absorb water
3. you must keep the insulation from ever getting wet

Most coastal chemical plants require that all piping that will be insulated must be painted first.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[James01]

We have been running a CUI campaign across our installations for the last 8 years. We are currently in a position of 100% removal of lagging and visual inspection of the pipe surface. We have arrived at this after trialling various NDT techniques and targeted removal campaigns. Our current strategy is as follows:

1. Identify all insualted pipework
2. Rank pipework according to consequence of failure
3. Challenge need for insualtion with Process Engineers, we have found that at least 50% of the insualtion is not required.
4. All insualtion for pesonnel protection has been removed and only replaced at locations adjacent to walkways.
5. We run a multi-disciplined team consisting of Rope Access NDT & Fabric Maintenance personnel.

We have found that CUI occurs in the strangest of places and if you are serious about its detection the only way forward is to remove and visually inspect.

This also depends on the age of your plant, if its less than 10 years old you probably only have to look at small diameter thin wall pipework. Going up through the diameters and thicknesses with increasing age.

 

[SAeng]

Thanks Guys,
You have been very helpful.
We were going to go through the same process that James has described. But some have debated before we have even started that all insulation is there for a reason can should not be removed. I think the the only way is to identify all insulation that can be removed and go out there and inspect them.

Looks like a big long job!!

Thanks your your input.

 

[robsalv]

CUI can occur where ever the temperature is between -4C to ~126C. You don't need a lot of moisture to set up CUI. If the line cycles then moisture can be drawn in!

In our plant we use targetted partial insulation removal on equipment and flash radiography on our piping to look for CUI damage. Any doubtful profiles on the piping flash get's full strength RT to check for remaining wall thickness. Then we consider stripping.

Corrosion is commonly dealt by cleaning and painting. A coated line/equipment helps avoid CUI, based on barrier protection, but a zinc primer system could actually promote corrosion!! I've not seen this myself, but apparently, above 70C under moist insulation, the zinc becomes cathodic to CS.

Apparently best practice is to coat new lines with thermally sprayed aluminium and CUI is never an issue again.

IMHO, an insulated uncoated CS line/equipment in the CUI temperature range is a CUI issue that should be thought of as a matter of when, not if.

The rules in Edstainless' post are absolutely right.

Cheers

Rob