Material Choices for Heating Brackish Water (6000ppm chloride) 2

[gblackwo]

Hey everyone,

I've stumbled across these forums several times over the years, but this is the first time I've felt the need to make an account. I've found the threads here abundantly helpful lately, so thank you to everyone.

I'm upgrading a design of LP fired burner coils that are ordinarily ASTM A53 carbon steel 3/4" SCH80. The customer wants to run brackish water (6000mg/L) through the burners and suggested upgrading to 316SS. The coils are thermostatically limited to 180F, but this is the water temperature, and I expect that some portions of the coils do exceed this. The operating pressure of the coils range from ~370 psi at the inlet to ~180 psi at the outlet.

From the data I've seen, we would be operating in a range where pitting would easily occur in 316SS. I am wondering if 2205 would suffice for the coils. I am also trying to limit corrosion as much as possible by using CPVC in the system where possible and trying to get the customer to provide clean water for regular automatic system flushes.

Thank you.

 

[EdStainless]

Do you know the pH of the water? And the total chemistry, for example how much will it scale?
If for some reason this water will not scale (which I doubt) 180F at 6,000ppm Cl is a very aggressive service. You need to worry both about pitting and stress corrosion cracking.
So these are externally fired?
I suggest that you consider alloy 800. Though you might be able to do this with a 6% Mo superaustenitic like AL-6XN.
These alloys are 5 times the price of 316L, but 316L will only last for a few hours.

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P.E. Metallurgy, Plymouth Tube

 

[gblackwo]

I haven't seen any other data on the water yet. The external surface of the burner coil is subjected to the propane burner. I am uncertain of how hot the external surface of the coils actually get, only water temperature is monitored/controlled. Burner coils are wrapped with insulation and skinned. It will be difficult to take a reading of the burner chamber.

So here is the real mystery I've come across yesterday: Same customer says they have another install in Arizona with 10,000ppm Cl water, and normal SCH80 carbon steel coils. Equipment is above ground in a non-climate controlled building. Seasonally the temperatures exceed 90F in the building. The equipment only gets used for a few hours a day, and the 10,000ppm Cl water remains stagnant in the system all the time. Reportedly, they only have to replace the coils every 2-4 years. This is why they thought upgrading to stainless would give them more longevity. I am impressed and dumbfounded how these carbon steel coils are lasting this long.

 

[cloa]

Probably the other install is oxygen starved. Stainless Steel will do worse- it will pit

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[gblackwo]

Lack of dissolved oxygen?

 

[EdStainless]

As the water heats it will de-gas.
A steel with some Cr (T12 or T22) will have a better adhering scale that offers some protection.
You either need to stay with steel, or move to a Ni alloy. A stainless is too sensitive to localized pitting or cracking.
Since this does not sound like critical service I would look at Cr alloy steels (these are boiler tube alloys) and see where that takes you.

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P.E. Metallurgy, Plymouth Tube

 

[gblackwo]

All,

Thank you everyone for all the great information.

We will be making one of these systems with 304 coils, with clean water system flushes and sacrificial anodes to help lessen corrosion. The customer understands that we will not be warrantying the coils and we do not know when they will fail. It may be a few months or a few years, but I will try to remember to report back when it happens. As heavily as some of this equipment is used, it may be possible that the coils outlive the machine.

 

[EdStainless]

We saw coils like this in 304, 316, and 310 all fail rapidly (hours of use).
You can do this as a demonstration, but don't expect much other than rapid leaks.
The units that I was working with were gas fired and produced water at 185F.

Even doing these in alloy 800 there were occasional failures.
If I were you I would stick with steel.

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P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

You couldnt do worse than SS in aerated brackish water - suggestion to use 2205 duplex might give you more life than CS.
Else push back on this user - what alternatives do we have to running this fouling, scaling stream through these coils?

 

[georgeverghese]

Believe there is only a small price differential between 22Cr duplex and AL6XN - think you may get more mileage out of AL6XN - higher PREN.

The other problem with this brackish water is possible high Ca / Mg content - these will precipitate out of solution at the tube inside high film temps; resulting in reduced heat transfer, high dp across the heater and possible hot spots when operators fire harder to get back to design heating duty. Antiscale injection will be required no matter what CRA you go for. Under deposit corrosion will also set in. An indirect heating loop will avoid these problems.