Galvanized steel or CuNi pipe for sea water system on boat?

[tonylazyz]

I know the answer to this question but I need some ammunition to argue my point with my boss. We have a small (58-meter) ocean-going ship, 6 years old, and the original galvanized steel pipe used in the sea water (fire and bilge) circuit is starting to develop pinhole leaks at the welds. The shipyard used CuNi (70/30) for the main seawater crossover pipe and for the pipes feeding cooling water to all engines, but opted to save money by using galva on the bilge/fire system.

We need to change the pipes. I want to use CuNi, but my boss is pushing to replace the galva w/ galva and "paint the welds" I don't know how we'll successfully paint the inside welds of a 65mm ID pipe when the weld is 2 meters from the end of the pipe. It is my understanding that galva pipe has to be re-galvanized after welding.

I see the useful life to the galva as 5-6 years (what we got from the original pipework) and the life of the CuNi as 10-15.

Cost is not really an issue as long as we can budget ahead of time for it. Also, the cost of the labor to remove the original pipes is huge, so may as well go CuNi now rather than do it all again in a few years.

Does anyone have any strong feelings one way or the other?

Thanks

 

[CoryPad]

CuNi for the reasons you have stated.

 

[stanweld]

70-30 Copper Nickel and the life expectancy should be at least double your predictions.

 

[EdStainless]

Stay with CuNi for this. These applications are the natural for it.
On military ships the CuNi fire water systems last the life of the ship.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[MPEngineer]

Another one you could bring up is when you weld galvanized pipe, it becomes a health hazard. Any time your work on galvanized pipe, cutting, milling , drilling, etc., the coating becomes airborne and can get into your lungs.

 

[TomDOT]

CuNi, especially since the install/replacement labor costs are high.

Also, do NOT connect CuNi with galvanized pipe unless you electrically isolate them. Otherwise, the CuNi speeds the corrosion of your galvanized.

 

[rmw]

Tell your boss you are going to stick with steel, but it will be 6-Mo pipe. Then maybe Cu/Ni won't sound so bad to him.

rmw

PS: I once saw this on a client's wall:

Rule #1 - The boss is always right.

Rule #2 - When the boss is wrong, refer to rule #1.

 

[strider6]

Galvanized steel in a seawater service is not a good material selection, unless you plan for frequent maintenance. However the corrosion rate depends also on the exposure time to seawater, temperature.
Welding galvanized steel and thinking to touch up the weld inside the pipe seems an impossible thing to do. If you leave the weld without galvanizing the time for maintenance will be even less considering that you'll have carbon steel exposed to seawater.
I'll definetely go for at least the cupro nickel 90/10 or higer stainless steel like duplex or 6 mo. Cu/Ni have some limitationt on the max seawater velocity but i don't know if this can be an issue in your case. However you should carefully consider the likelihood of galvanic corrosion due to contact with other metallic materials.

http://www.copper.org/applications/cuni/txt_sea_water_system_design.html

http://www.copper.org/applications/marine/seawater/seawater_corrosion.html

regards
Vict.

Corrosion Control

 

[BrianBT]

No, I am with your Superintendent.

Cunifer is great for sea water systems but for a survivability ( impact for example) I would go with steel. If you want it to last you can use schedule 80 or use one of the many internal coatings that are now available and not at great cost. Done right you will at least match the service life of Cunifer

Steel pipes are readily repairable by ships staff in a more permentant way, the repair to cunifer pipe is not always practical with the material becoming soft after repair. it also work hardens in service making liable to cracking. Bottom line is that any problem and the only reasonable solution is complete replacement.