SS and HCl

[ivanhoe374]

How would 316SS go within a water treatment plant wereby i am periodically dosing 400mg/L of HCL (0.04%) into a stainless steel line. At ambient temperature and a pressure of 1-3 bar.

I know that HCL has a severe effect on stainless at high concentrations but i am not sure at such a low concentration?

 

[Metalguy]

What is the "normal" amount of chlorides? Is flow constant?

Many other questions.

"When the Eagles Fall Silent
The Parrots Start Jabbering".
Winston Churchill

 

[ivanhoe374]

The application is a chemically enhanced backwash on an ultrafiltration plant (using Norit Xiga membranes)

The flow rate during the CEB is 60m3/hr, dosing 30% HCL at 1330mg/L , this occurs for a short period before 10 minute soak. The chemicals are then rinsed out using ultrafiltered water.

 

[gr2vessels]

ivanhoe,
The chloride concentration you mentioned is approximately equal to 400 ppm at ambient temperature. I also assume that the water contains the other bad ingredient, the oxygen. The stress corrosion cracking for this concentration would not be an issue until the temperature of water (during the exposure to HCL) remains ambient, say below 100-120 F. Also, the rinse after the chemical treatment reduces considerably the risc of corrosion with catastrofic consequences. You should start being concerned at around 1000 ppm (1000 mg/L)at temperatures at / above 140F. Also, the presence of highly stressed components (sharp bends with stagnant pool of high chloride concentration, etc)could cause concern. Your description of the equipment and process seems to guarantee many long happy years 'till retirement.
However, on the safe side, check the remaining water after rinse, for chloride concentration and make sure to wipe or mop-up the remaining water (don't dry with air or worst, with hot air).

Cheers,

gr2vessels

 

[ivanhoe374]

Thanks gr2vessels.

There will never be "drying" stage. The membranes will always remain wet. Basically, after the chemically enhanced backwash is complete, the system automatically goes into a filtration cycle wereby treated water will continously flow where the HCL solution was. There is also comprehensive deaeration steps designed into the membrane plant so there shouldnt be too much oxygen about.

The temperature will not get above 45C.

Do you have any references to the chloride concentrations that you mentioned?

 

[moltenmetal]

Obviously care has to be taken at the injection point, where localized acid/chloride concentrations (and heat of dilution) will make corrosion conditions much more severe than in the average condition.

 

[tomwalz]

In our experience with high conc. HCl the problem is at the welds. Might want to make sure how those are done.

Tom

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[EdStainless]

Without doing the calculations I'll just guess, the pH must be something like 1.5.
At these chloride levels and temperature I am not too concerned with CSCC.
You probably will see some general roughening of the surface over time with some metal loss in welds. At your temperatures this will be gradual and take a long time.

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[bimr]

ivanhoe374,

You can probably find reference information here:

http://www.nickelinstitute.org/index.cfm/ci_id/86.htm

I doubt that you will be backwashing and cleaning your UF membranes more than once every week or two. The short exposure will tend to minimize the possiblity of corrosion.

 

[EdStainless]

The reason that you will see metal loss at welds eventually is that the acid will attack the delta ferrite in the welds. This is a very uniform attack, and at low temp it will take a long time. You might want to have an inspection port or something so that every couple of years you can check on it.

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[ivanhoe374]

The chemically enhanced backwash, using the HCL happens every 24hrs under normal conditions, and every 8 hours when the raw water quality starts to deteriotate.

With regular CEB's occuring there is no requirement for an extended CIP clean.

"Moltenmetal" raised an interesting point, that being the dosing point, where the concentration will be higher....the injection fitting itself will be plastic however this fitting will be screwed into the stainless pipe.....am i likely to get problems around this fitting?

 

[moltenmetal]

Yes...probably.

Consider doing the injection in a piece of PTFE-lined pipe, preferably with a static mixer. Or risk localized attack in your "mixing spool". It will see higher local concentrations but may not be much of a problem due to the short duration of exposure followed by lots of flushing- but you will experience problems anywhere it will be here.

 

[EdStainless]

You need a quill into the center line of the flow and some fins to cause mixing. Don't use as-welded stainless in this area. If you do want to weld then anneal.

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