Piping Materials for Salt Water Environment

[TrippL]

I am replacing a 4" domestic water line and a 10" salt water fire line under a pier. Thinking of using copper or 316 stainless for the domestic and ductile iron for the fire line. Does anyone see any problems with this?

 

[gr2vessels]

I don't think anybody would have problems with your selection. It is your private business and the question is quite intimidating. However, if you don't hurt me for asking, is your question related to the durability of the pipe or just a generic question? I assume the salt water is sea water with some 20000 ppm of chlorides at 10-20 deg C temperature. I assume the sea water is not stagnant and is continuously flowing with some velocity. I also assume the sea water contamination with internal rust is not affecting any equipment downstream, the biological growt is not a problem, etc. Am I wrong with my assumptions?
Cheers,
gr2vessels

 

[TrippL]

The 4" domestic pipe and 10" fire water pipe will be stagnant unless in use. When in use the domestic water will have a flowrate of 20-50 gpm and the fire line will be flowing 2000 gpm. The domestic line will be in use most of the time and will have potable water. The fire line will be tested probably monthly and will have brackish river water flowing through it. Both pipes will be exposed to brackish river water periodically from the tide.

 

[gr2vessels]

I suggest use FRP piping, with some good corrosion barrier inside outside, like 3 mm PVDF lining. Again, you keep secret the process conditions and you expecting miracles from the sky. I still have to make assumptions, instead you providing the information. If the brackish water is treated, then you don't have marine growth problems, otherwise you will see fouling. I assume in absence of your secrets that the sea water temperature is ambient, perhaps below 30 deg C. If that's the case, the FRP pipe is fine. If not, depending on the temperature, you could see some minor problems with the thermal expansion and the pipe flexibility. I also assume the pressure will be around 10 bar, not more than say 15 bar. If higher, then special FRP will be needed. The carbon steel or iron piping will rust regardless of you efforts to prevent corrosion, require heavy supporting structure and is generally not suitable for stagnant sea water/brackish water use.
Cheers,
gr2vessels

 

[EdStainless]

Is there any chance that the outside of the potable water line will ever see any seawater? If so then stay away from SS.
FRP for both would be a good option.

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Plymouth Tube

 

[rconner]

I suspect one of the areas/entities with some of the most long-standing and extensive experience (I think a lot of pipe over MANY decades) with specifically salt water or sea water fire protection piping systems may well be the Water Supplies Department of Hong Kong. I noticed in a feature article of a notable Engineering periodical not long ago that some quite large new construction in this area chose the materials of cement mortar lined ductile iron and also cement mortar lined steel pipe for very important new parts of their systems. It appears they have probably gravitated mostly to cement mortar lined ductile iron piping for common, smaller mains (see e.g. page 42 of the document now at

http://www.wsd.gov.hk/filemanager/en/content_150/HBonPIB.pdf

). While I’m not sure if they have operational similarities to yours, I believe a whole lot of such pipes are also used e.g. in various pressure piping applications at DOD docking and berthing facilities etc.

It probably also makes sense for pipe-on-supports applications that may be exposed e.g. to aggressive sea/salt spray and wet and dry cycles etc. to be provided with a good quality primer and finish coating system, to minimize exterior corrosion and improve aesthetics, if those are concerns. It is best in these cases to order the ductile iron piping specially made from the manufacturer with the primer, as opposed to a standard asphaltic coating system per AWWA standards, in that the latter greatly complicates surface preparation and finish coating of those pipelines (if the asphaltic is not the finish coating desired). It probably also makes sense to provide some padding between the pipe and supporting/bracing systems, to reasonably protect same. It may be best to contact the manufacturers and/or consultants familiar with surface preparation and application of finish coating and on ductile iron pipe for specific recommendations on coating systems.

Ductile iron piping is also available in the 4” size.