Urea Tank

[TugboatEng]

I have some negative feelings on this project and am interested in other opinions. We have commissioned the construction of a tank that will hold 32% urea solution at ambient conditions. The tank is constructed from 3/8" 304 stainless. It is being constructed near carbon steel fabrication. The tank has skip welded baffles and backer bars on the tank top. The tank will be cleaned with a consumer grade oxalic acid based product.

I know this product is doomed to fail if there are chlorides present. Does it have any chance at all assuming near zero chlorides?

 

[3DDave]

Is this is near the salty sea?

Is it bigger than a bread box? (Actual question: isn't there a rotomolded tank/tanks that could be used?)

 

[TugboatEng]

Yes, it is very near sea. It is inside of a boat that floats in the sea.

The tank is fabricated to conform to vessel. It has a T cross section. Capacity is 500 gallons.

I am the details guy. I get to see the plans at the end of the design phase. I make the critiques and then catch a bunch of grief for delaying the project or causing change orders.

 

[3DDave]

Should be a review of the requirements ahead of the design phase to chose materials at that time.

What if a couple of rectangular rotomolded tanks were plumbed in and you only got 480 gallons and no corrosion worries? Someone upstairs going to have a stroke over sunk costs?

In other industries there would be a systems engineer to take the grief, though most of those I've had the misfortune to work with are slimier than slugs when it comes to taking responsibility for not doing their part of the job.

 

[TugboatEng]

There will be 8 of these tanks eventually so it does make sense to invest in a better process. Time is an issue, though.

 

[3DDave]

Just an option. I worked on a really small and highly configured tank for a military vehicle hydraulic tank. The original was 3-4 deep drawn stainless pieces with a bunch of bosses welded on and, being opaque, required a dipstick to see the level. TACOM became very interested after some General got the chance to operate the system, but the system had been over-filled and when the fluid heated, it barfed on him. The translucency of the rotomolded tank allowed clear (pun!) visibility of the fluid level (though your tank might not be positioned to make use of that.)

The molds aren't too expensive; they don't require being pressure tight. Getting into the schedule is likely the biggest task and then there are the tweaks to the turning cycle to get a uniform wall thickness.

If the configuration is simply a T-section prismatic shape then maybe making two tanks, one for the top of the T and another for the vertical part will significantly reduce cost and make installation much simpler at the expense of a hose to connect them.

 

[TugboatEng]

That's an interesting thought about two tanks with a small hose. These only deplete at a few gallons per hour max.

One challenge is that the urea is so concentrated that any leakage shows up immediately as a ball of white crystals. This is alarming to most operators. I can't convince the operators that this white ball isn't toxic, isn't corrosive, and can be rinsed away with some water. Instead, they want a helium right system because they're afraid of the scary white substance.

 

[3DDave]

It's a main constituent of urine and is less problematic:

Human urine is composed primarily of water (95%). The rest is urea (2%), creatinine (0.1%), uric acid (0.03%), chloride, sodium, potassium, sulphate, ammonium, phosphate and other ions and molecules in lesser amounts

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6934465/

The uric acid could be a problem - it eats through aluminum with enough time and effort (dog vs. AC outside condenser fins, for example)

Plain urea breaks down to ammonia in soil as a fertilizer, is part of animal feed for protein building, and skin creams, among others.

Anyway, best not to leak at all.

I recall another reason for designing the little tank - it was a pain to clean out the tank from all the little bits from welding and the post-weld tapping of the bosses for the hydraulic fittings. Since the fittings were molded in, they didn't generate swarf and the way the process works, there aren't any places to trap cleaning fluid, which really wasn't needed due to the smooth, continuous, internal surface. I suppose they had to drill out where the plastic bridged over the fittings, but that would be long threads of plastic that would just dump out.

 

[TugboatEng]

Our spec is for Swagelok fittings, they don't leak much. However there are always hose and JIC is the favorite hose end. These leak sometimes. That's ok, if one does we can add a Second washer.

 

[EdStainless]

Is the outside of these painted?
I am more concerned about external corrosion in the short term.
In the long term there will be some salt inside the tanks.
And pitting of the inside is eventually going to be the issue.

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P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

The tanks are installed in a protected location inside the boat. As for internal chloride accumulation, I am counting on the consumption rate to keep chloride levels low. These tanks will be drained to 25-35% and then refilled about once every 3 weeks.

 

[MJCronin]

Just a thought ...Have you gotten a price for tank fabrication, but from C-276 Stainless Alloy ?

(It seems to me that you are in a condition of both internal AND external corrosion)

C276 has reasonable corrosion resistance to chlorides ... for a reasonable price

I also a 3/8 think 500 gallon UREA tank seems to have a "too robust" wall thickness

I would be very tempted to get a "budgetary grade quote" for your tank geometry

... but in both 0.25" and 0.375 C276 Construction

Best Regards, Tug !!

MJCronin
Sr. Process Engineer

 

[EdStainless]

For an in between price point you could look at AL-6XN.
The raw material price of C276 is steep.
My only concern about 316L is the welds.
The residual ferrite in the welds will eventually be attacked by the Urea.

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P.E. Metallurgy, consulting work welcomed