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Sea Water / Salt Water Piping Application

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RobsVette

Mechanical
Apr 15, 2009
94
Good Afternoon,

Im relatively new to heat exchanger piping and vessels that stand up well to salt water service and was hoping someone could provide some guidance on the difference between super duplex stainless steel (2507) vs titanium for piping and heat exchanger tubes.

The pressures are low, less than 50 psi.

The temperatures are also relatively low, less than 100F.

The titanium option is much more expensive and just wondering if the super duplex SS will be a problem.

In my experience from the past with 316 and lower grade SS, the welding seems to be the weak point with SS. I'm wondering if the super duplex stainless steel would resolve that weakness.

Any insight or direction on the application difference would be appreciated.

Thanks in advance for any assistance.

Rob
 
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y sort of answer you also need o define the chloride content / "saltiness" of your fluid.

And also the level of oxygenation.

Aluminium Bronze is also used a lot in seawater applications.

Try a search here and int he piping forum. Using the button on the top left limits the search to that forum.

Things like this pop up.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
Sea water is ~30,000 ppm chloride, nasty stuff

What type of heat exchanger is this/construction method? 2507 is subject to crevice corrosion at temperatures seen in some oceans ~90°F. This will be a problem for gasketed plate heat exchangers or shell and tube with swaged tube sheets.

Copper-nickel is standard for shell and tube construction while titanium is standard for plate and frame constructions when talking water-water or water-oil exchangers at those temps and pressures.
 
For 'seawater' in general there are many alloys in use.
Cu/Ni (91/10 and 70/30)
specific brass and bronze alloys
superaustinetic SS (such as AL6-XN)
superferritic SS (SeaCure)
superduplex SS (2507/2707)
Ti (CP commercially pure, usually gr2).
The issues are different for piping compared to HX tubes.
All of these materials are found in HX tubes.
The all have different issues when it comes to corrosion resistance (both sides), erosion resistance, strength, and vibration resistance.
Of course they have different heat transfer properties also, but this is not usually the biggest issue.
If you have had issues with welded SS then it is the fault of you having a weak specification.
I have supplied hundreds of millions of feet of welded SS tube for a wide range of services.
When it comes to pipe you have to be very careful because of the size and weight involved the cost impacts are very different.
For example you do not see superferritics (not possible metallurgially) used in piping.
For many of these alloy groups the issue of piping comes down to 'does anyone make it'.
And with piping systems you need fittings and valves also, which may further limit alloy availability.
And just a word of caution about superduplex.
You MUST require corrosion testing and testing for intermetallics on every lot.
The heat treatment of these alloys is very demanding and easy to mess up.

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P.E. Metallurgy, consulting work welcomed
 
Thank you very much for the quick responses. This is in reference to a commercial office building chiller plant replacement project here in NYC. We will see leaving river water temperatures of about 90F.

There are many such buildings that have these types of cooling systems here in NYC and the formula that seems to work the best is plastic piping, valves that are rubber seated and have nylon coated discs, pumps that are epoxy-coated cast iron with bronze impellers, and chillers with titanium tubes.

In past experience here in NYC, Cu/Ni has been tried in many places but never to successful use with chiller tubes.

Recently we were part of a discussion where it was suggested to use a super duplex filter assembly which would have a 2507 shell and 904L ss internals. This is for a project where the chillers will have titanium condenser tubes.

I was concerned with the use of Duplex SS. The vendor has an option for Titanium shell and Monel internals, which costs twice the price. My question would be, does the Super Duplex hold up to salt water exposure the way the titanium does?



 
Copper alloys are sensitive to pitting from sulfides. If you are using river water this could certainly be a problem.

Have you considered gasketed plate and frame heat exchangers with titanium plates? These exchangers use steel shells and eliminate the tube sheet which saves a lot of mass of the expensive materials. Repairs can be performed in house by operators, no specialists required.
 
Plate-and-frame Ti HX is a good option if you have very clear water.
If you go with superduplex for a filter it should all be superduplex.
904L is not a suitable seawater alloy.
The 2x price differential for Ti/Monel is what the market will bear, not the real cost of materials.
Cu alloys hate polluted water, sulfur compounds eat them up and it gets much worse when the water is static.
And SS alloys all have various degrees of biofouling risk with untreated waters.
Going all plastic is a much safer route as long as pressure isn't an issue.
And that you remember that the plastics will age and become brittle, they have a finite life.


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P.E. Metallurgy, consulting work welcomed
 
Thats actually what they have now and because the water is very dirty, they clog up very quickly. From a corrosion standpoint the PFHX works very well. From a heat transfer standby because of the dirty water around NYC it doesnt work very well.
 
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