Steel pipe in water for 2 years 1

[CostasV]

Hi all,

Recectly, it came to our responsibility a natural gas network that has underground concrete chambers (3m X 3m X 3m) as valve stations. In these chambers, there are the gas pipes (main pipe 3m long, 10" diameter, secondary pipe 6m long, 2" diameter) and the valves (of the same diameter). In one of these chambers, we found it full of water. We remove the water and we notice that the paint of the pipe was "wetted" and could be easily removed even with the bare hand, leaving a brown color surface. Any thoughts of what is to be done regarding the pipes? I am thinking of cleaning (removing the old paint) the pipe, drying the chamber, and repainting it. Is it worthy to measure the thickness (by ultra sound)?


Other Data
1. Water inside the chamber must have entered either from the top opening, or from the entrance of the 10" pipe accross the concrete chamber (which is about 2m below the ground level). It is rain water and we estimate that it should be there for about 24 to 30 months.
2. The material of the pipes is API 5L Grade B.
3. According to our specifications, the painting should be in 3 layers, first layer 50 micron thick, second layer 30 micron thick and third layer 160 micron thick. Durability of painting system should be per MIL-P-23236 I/4556 D (Sorry, I don't know what is this).
4. The pipes that are inside the concrete chamber are NOT cathodically protected

I appreciate any recomendations

Costas

 

[Metalguy]

After you clean the piping, look for any pitting damage. It's possible the coating protected it and the steel is fine.

You could do some spot checks with a UT thickness tester just to be sure.

 

[flames]

Since your paint is damaged, you must clean the pipe to a clean surface. Your inspection should be fairly easy as you are expecting external corrosion so you can visually inspect the line. Any noticable pits can be measured using a pit gauge. You can use UT to confirm the wall thickness, but like I mentioned if you can see the pits, the pit gauge is the way to go.

The water could be entering around the pipe as we have had this happen. You can try to re-seal the opening, but you will have to periodically come back to check to see if the vault has any water in it as well.

 

[kenvlach]

Hi,
The current paint specifications and qualified product list can be downloaded at the Department of Defense’s ASSIST site (

http://assist2.daps.dla.mil/quicksearch/).

MIL-PRF-23236B PAINT COATING SYSTEMS, FUEL AND SALT WATER BALLAST TANKS and its Qualified Products List, QPL-23236-73. “This specification covers paint coating systems for application on surfaces subjected to fuel and salt water in shipboard fuel and salt water ballast tanks. These coatings are not applicable to fresh or potable water tanks.” Type I -- General Use.

The 2nd specification and QPL are MIL-PRF-4556F COATING KIT, EPOXY, FOR INTERIOR OF STEEL FUEL TANKS and QPL-4556-27. “This specification covers a two-component epoxy coating system for protecting interior surfaces of steel tanks used for the transportation and storage of fuels.”

As to why the paint failed: your description indicates a general permeation of the paint. This suggests improper curing (improper timing between coats or solvent fumes in an enclosed space?) or else solvent (gas or water) attack or perhaps old age (age of the paint coating?). MIL-PRF-23236B, paragraph 4.5.2 Immersion Resistance indicates 24 hours between coats and 1 week after final coating before immersion service.
Also, the paint may not be of high enough quality for long-term service without periodic inspection and touch-up: MIL-PRF-23236B, paragraph 3.10 Service Performance. “The coating systems shall continue to provide protection against corrosion with a total of 10 percent maximum touch-up for a minimum period of 3 years.”

I agree with the recommendations of Metalguy and flames. Scrape and blast clean to bare metal, then inspect. Owing to the possibly humid conditions, I suggest using a phosphoric acid-based cleaner or wash primer to handle any flash rusting prior to painting:

http://www.finishing.com/198/33.html