Material for Heat Exchangers

[brimmer]

Dear Corrosion Experts,

I am seeking advice on materials selection for heat exchangers in petroleum refining environment. Currently we have a fresh water cooling system, the pipine and exchangers are made from low carbon steel. There has been evidence of MIC and fouling is a problem. Sludge build up is also a problem so under deposit corrosion is likely. There are areas of stagnant water at times. We currently treat the water in the cooling towers (wooden) with inhibitor (bleach and bromide compound to disrupt biofilms), and perform a kill with chlorine twice per year. Normall Cl levels in the water are 0.7 ppm, and get to 3 ppm during the Cl treatment. We will replace the exchangers, and I am considering a stainless option. Would the stainless be better for the described conditions in general and in preventing MIC conpared to low carbon steel? Be better in preventing fouling? And would it withstand the Cl concentration in the water (SS304 or 316)? Could under deposit corrosion be worse in the stainless?
I have also considered brass, but this can lead to other problems. Any advice on best materials resistant to corrosion in the above conditions would be greatly appreciated. Thank you.

 

[EdStainless]

The lowest cost option is to upgrade your water treatment practice and stick with CS.
If you go to stainless I wouldn't suggest wither 304 or 316. If you can keep total chloride content below 50ppm then you might consider 439, but this will need better water treatment.

If you are going to have higher Cl levels and ANY risk of bio-fouling then you would need to use a super-ferritic like S44660 (Sea-Cure). There are other alloys that would stand up to this, but in todays market they are much more expensive.

The fouling will be the same on any material. You must fight that with water treatment.

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[0707]

Your water should have more than 50ppm of chlorides for that reason SS304 or SS316 would be subjected to SCC in this environments.

CuNi 90/10 or a duplex SS such as SAF2507 or 2505 would be good alternatives to your application.

 

[EdStainless]

My concern about 304 or 316 is under deposit corrosion. If you are getting a lot of fouling then it would lead to problems.

90/10 is a possibility, but pricey and still sensitive to under deposit corrosion.
A superduplex would work as well, but a lot more expensive than a superferritic, and really difficult to find.

One thing that you need to do is measure temperature rise across some of your heat exchangers and use the approach temp as a guide for when to treat your system. You may need to burst chlorinate every two weeks in the summer, and then not again for 9 months.
I doubt that you are actually holding 0.7ppm active Cl. This may be your addition rate, but it sounds like you still have a lot of growth. Get the sludge under control first.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[rzrbk]

I would not want to try to sell a higher metallurgy than brass (CuNi 90/10)to management for a water corrosion issue, and would agree with edstainless that looking at your water may be the way to go.

In addition to the treatment, looking at ways to get CW velocities up may be an option to reduce fouling. (remove some tubes?)

Another option may be a new CS bundle with a baked on phenolic coating on the tube ID. This should cost equal or less than a new brass tube bundle. Coating companies will tell you they can coat an exising bundle but I would not try it.