Leakage from Plate Type Heat Exchanger 2

[ReliabilityEngr2022]

Hi,

I am looking for some help in figuring out root cause of a plate type heat exchanger leakage at our site.

It's an evaporator which receives sodium hydroxide at 38% concentration and increases the concentration to 46.5% by heating the contents by saturated (condensing) steam at 135 degree C. Material of the plates is Nickel 200. Exchanger has been in service for last 3 years. Thickness of plates is 0.8 mm. According to OEM, the exchanger plates should last for 14 to 15 years without any leakage. However, they have cautioned against contamination of sodium hydroxide feed with sodium chlorate or sodium hypochlorite as these can cause accelerated corrosion. Unfortunately, we have not been able to control the chemistry and have known to introduced small quantities (less than 1%) of sodium chlorate / chlorides in our feed.

Leakage occurred at the bottom of the plates near the discharge port as can be seen in the attached pictures.

Please fee free to ask further questions for better understanding of the situation.

Regards,

 

[EdStainless]

Corrosion.
You exceeded the service conditions that Ni200 can handle.

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P.E. Metallurgy, consulting work welcomed

 

Corrosion.

A plate heat exchanger is basically the highest concentration of crevices ever conceived. Very efficient ... while they work.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[ReliabilityEngr2022]

Thanks for your replies.
We have noticed that for caustic under these conditions (46.5% concentration and 135 degree C temperature) Nickel is the best material available. However, the chlorides in the process stream are ending up causing crevice corrosion.
Is there any better metallurgy which we can use here (Titanium may be?) which addresses the issue of crevice corrosion and is also good for caustic under these conditions?

 

According to OEM, the exchanger plates should last for 14 to 15 years without any leakage.

OEM should not be making such promises given your severe service.

Hot caustic is a tough one, and the chlorides obviously don't help.
If Ni200 can't handle it then you might need to consider a different HX type.

First thing to do is a failure investigation to identify the mechanism at least.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

A tube type HX with Ni200 tubesheets and EBrite tubes is the other solution for this application.
It doesn't care about the Cl-.
Or clean your caustic better.
The only two ways to do this.

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P.E. Metallurgy, consulting work welcomed

 

[ReliabilityEngr2022]

Thank you all for your useful suggestions.

Can anyone guide me towards relevant technical literature (research papers or case studies) through which I can understand corrosion mechanism at play when we have chlorides dissolved in caustic solution?

 

[Chumpes]

hello
maybe start with NACE SP 0403...

 

Find a reputable local HX designer.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[MedicineEng]

Please have a look on this document. I hope it helps.

https://nickelinstitute.org/media/4341/nickelpub10019_causticsoda_20pg.pdf