By -
Date 09-26-2001 05:43
We are experiencing a corrosion problem. The situation is: piping running through seawater ballast tank (seawater exposure on the outside surfaces of the almost always internally dry pipe); pipe is 90/10 CuNi connected with an insulation kit at a flanged connection to a carbon steel pipe (A105) that penetrates the bulkhead;corrosion includes through wall holes in CS pipe near the CS pipe to flange weld (the CuNi itself looks fine); has been in service less than 4 years.
Our quandry relates to fixing this specific problem. For prevention, we have decided to inspect and replace the insulation kits at other similar joints more frequently. However, to fix this thru-wall leaker, we have some disagreement. We need to avoid damaging the bulkhead, so removing the penetrating pipe is out.
I have suggested that we remove the CS flange, reface the CS pipe, and weld a CuNi flange to the CS. From what I can determine, carbon steel and CuNi are near enough on the galvanic table to indicate that there should be no big galvanic corrosion problems with this joint. Some of my cohorts disagree on this, however, and think that we should instead fill the thru-wall hole with a powdered metal putty product and replace the insulation kit. This would certainly be cheaper, but I think less effective in the long run.
Does anyone out there have any thoughts on this? Experience with CuNi/CS dissimilar metal joints in service and associated corrosion performance? Any thoughts or recommendations on the welding of same? How about experience with these metal putties?
Thanks very much
Mankenberg
As I am not in the shipping industry, I am not sure what you mean by an insulation kit. Maybe you could explain this concept a bit further. At any rate, I thought that I would give my ideas.
Carbon Steel will at the best times corrode in the presence of sea water. You therefore need to protect the CS from the sea water. There are special tapes that may be good enough for this purpose, or you may consider something like epoxy powder coating or other purpose made coating. If almost all you corrosion seems to be concentrated in the vicinity of the CuNi / CS joint, then it does rather apear that the corrosion is being aggrevated by some sort of a galvanic effect. To eliminate this, ensure that you have a non-conducting gasket in the joint and that the bolting does not come into contact with both sides of the joint. (This can be achieved by placing special plastic/rubber inserts into the bolt holes.)
Do you need this pipe to transmit any substantial forces? If not, try inserting a plastic pipe through the steel section. HDPE piping should be just fine in a sea water application. As plastic has much lower friction losses than steel, the smaller diameter should not be a major problem. You may even consider replacing the whole line with an HDPE pipe.
Regards
Niekie Jooste
By -
Date 09-28-2001 00:35
You describe an insulation kit well. It is a nonconductive gasket with plastic sleeves for the bolts and washers to prevent metal to metal contact at the dissimilar metal flanged connection.
Your plastic piping idea is interesting, but for economic reasons I'm sure would not gain serious consideration.
We will likely simply replace the CS flange and damaged CS pipe, coat as per spec, replace the insulation kit, and perform maintenance inspection on these joints more frequently.
I am still very interested to hear from anybody out there who has some experience with this sort of situation, however.
Best Regards, Mankenberg
By -
Date 09-30-2001 18:51
Thanks for helping me get the terminology understood regarding the "insulation kit". Regarding the plastic piping idea, I would like to add that it is precisely the economics that supports it the strongest. It is extremely cheap (Much cheaper than the whole cladding scenario.) to install plastic pipes that could potentially last 25 years.
Just make sure you get people that know what they are doing. The typical plumber types are not your best bet. Get guys that have a good reputation installing piping in the petro-chemical environment.
Some of the plastic piping installation/welding methods are really great. For example, electrofusion welding allows you to perform "tie-ins" into live pipes by using tapping tees. Welds can be performed under wet conditions. (although not ideal) They can be made by people with the minimum of training, within minutes. You just need the correct (Automated) welding machine.
I also referred to the impregnated tapes. Probably THE trade name in these tapes is the "Denso" trade name. If you are still going to use the steel pipe, I would definatelly wrap the whole joint and exposed pipe in the relevant denso tape. The denso people will be able to give you the correct one for the job.
Regards
Niekie Jooste
We have sovled this problem two ways. First we cut the threeded end of the tubing off (right at the weld on the bulkhead), then we cut a piece of CuNi (90/10) pipe to make up the opening (area between the piping and the tubing, then we welded the CuNi pipe to the C.S. pipe using gas metal arc ERNiCu-7 wire and 100% argon, then TIGed the CuNi to itself. The second thing we did was to stop using the tubes and welded the CuNi directly to the bulkhead or to a pipe nipple insert with the ERNiCu-7 wire and have had no problems.
Ronald Bergeron
Bollinger Shipyardss, Inc.
By -
Date 10-04-2001 05:02
Thanks very much for the advice everyone. I appreciate the response. We will give strong consideration to all of the advice that we have received for purposes of repairing our in-service problem and for avoiding it in our new construction projects. Once again, thanks to all.
Mankenberg
