Stainless Steel in Oleo Processing Plant

[Mizrae]

Hi everyone, I am doing a corrosion study for an oleo plant handling mostly fatty acids and fatty alcohols (C12/C14 & C16/18). The materials used are mostly CS (SA 516 Gr.70) and SS (mostly SA-240/316L, SA-240/304, SA-240/TP304L & SA-240/TP316L). Can I assume that the fluids would be free of H2S and chlorides, hence I do not have to take into account SSC or SCC? I'm not 100% comfortable with this myself.

Also, I believe fatty acids are not strong acids such that they would attack the SS passive films.

Appreciate any comment and advise. Thanks

 

[TVP]

I have a difficult time imagining any fluids containing H₂S. Cl-, on the other hand, depend on whether water is used in the process stream, and what that water quality would be.

 

[metengr]

Don't assume anything. You should have an analysis of entering process stream fluids to better understand potential corrosion issues.

 

[EdStainless]

The last time that I checked there was Cl in many of these chemicals.
You had better check.
The point about checking cooling (or heating) water quality is also very valid.
Do they ever clean these systems (internal or external) with Cl containing fluids? I have seen hot water lines crack from the outside contamination.

What is the lowest pH that you will see at any point in the process? That will be the key.

My hunch is that you would be better off with a lean duplex stainless (2101, 2201, or similar).
You get higher strength (thinner walls), corrosion resistance that is a bit better than 316 (pitting and crevice), and you get usable SCC resistance.

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Plymouth Tube

 

[moltenmetal]

You cannot assume these materials will be free from chloride. Some is there naturally, some as a result of processing. If the source fats were recycled, such as is often the case with biodiesel manufacture, you can encounter a LOT of chloride- think of those guys salting french fries right next to the deep fryer!

There can be water associated with these oils- water dissolved in the oil or coming along with the oil. If it comes out of solution, everything will be concentrated in it- chloride, acid or base you might add etc. The assumption of homogeneous solution of ionic stuff into oleo-based materials has been responsible for a lot of poor materials choices over the years.

As to whether or not that chloride is an issue with respect to Cl SCC for the austenitic stainless materials depends on temperature and concentration, so obviously you need some data- and hopefully not just a handful of measurements, as it varies with feedstock.

The other issue might be contamination (colour pick-up) from the carbon steel parts, rather than a concern about corrosion per se.

The long-chain organic acids don't appear to be much of a problem themselves unless they're really hot. The shorter chain ones like formic and acetic can be very, very nasty indeed.

 

[TomDOT]

Acetic actually does a better job of trashing many coatings/linings than hydrochloric or sulfuric.

 

[Mizrae]

Thanks everyone!

 

[Mizrae]

Unfortunately not all clients provide detailed analysis of their process and utility streams. I often have to work with knowing just the normal fluids inside the equipment and piping, op temp, op pressure and design info. So yes for this project in the end I requested for water analysis data and as expected there was 30-475 ppm chlorides in the water streams.