Material upgrade for Carbon Steel heat exchanger

[a7992]

Hi-
We've had problems with a heat exchanger failing from erosion corrosion near the inlet. It's constructed from carbon steel so it doesn't offer much resistance to this mechanism. We can't change the process conditions or the piping configuration so I thought it would be best to either use tube bundle inserts or replace the tubes with an upgraded alloy (stainless steel).

Is it feasible to upgrade the tubes w/o changing anything else (i.e. tubesheet) or would the differences in thermal expansion be too much? Could we use an expansion joint to get rid of that? Would it be better to replace the entire bundle (including tube sheets)?

Finally, if we used an insert of an upgraded alloy wouldn't there be issues with galvanic corrosion and thermal expansion?

Any advice would help!

 

[unclesyd]

Please describe your problem little better.

Is the corrosion at the inlet of the tubes at the tube ends or a little further in?

Some sizes?

What is the process fluid?

There are several possible remedies like different types of inserts.

 

[MikeMet]

I agree with uncle, we need more information for any specific replies. But some general info might be of help.
> You can install ferrules in the tube ends to prevent corrosion. They can be metallic or ceramic without having to change anything else on the bundle. I have never seen galvanic issues using ferrules.
> If you change tube material by retubing, but you need to perform a thermal evaluation to determine if you need more surface area of if expansion joints are needed. It all depends on your thermal requirements and temperatures in the bundle. If you get lucky you may not have to change anything else on the bundle. Choose a reputable vendor who knows how to run a HTRI
> If you need to change tube material and add an expansion joint, my experience is you are very likely better off building a new bundle. Retrofitting an expansion joint, combined with labor for extraction and insertion of tubing can quickly get to be more than 50% the total cost ( a number some use as the go-no/go for bundle replacement) of new bundle and there is no guarantee the vendor won't run into problems which may extend the time needed to retrofit, which can endanger shutdown timing.

 

[unclesyd]

Here is one of several suppliers of inserts, etc. for repair of bad tubes or tubes going bad. As posted above with a little more information it might be possible to suggest a specific avenue of approach to resolving your problem.

http://www.cti-ind.com/

 

[EdStainless]

Installing SS inserts might work. They don't loosen because they expand more than the CS tubes. Galvanic corrosion will depend on your environment.
Replacing the entire tube bundle will take a detailed engineering analysis.

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Plymouth Tube

 

[rmw]

As a simple fix I also recommend the inserts. You might also consider tube coatings. Plastocor specializes in coatings for just this type of need. Google'm.

To change your tube metallurgy, you will need extensive review of your Hx by someone with some pretty sophisticated Hx sizing software like HTRI, etc. as well as someone that can analyze your flows.

rmw

 

[strider6]

i think it should be necessary to define the type of damage mechanisms; if it's pure erosion then upgrading to SS could not solve the problem.

S

Corrosion Prevention & Corrosion Control

 

[a7992]

Well, it looks more like mechanical damage but I don't think I'd rule out corrosion entirely. We run Naphtha through the exchanger. A colleague brought up using 2 1/4Cr but I have not heard of using this alloy to solve erosion problems...does anyone know if 2 1/4Cr can help with erosion-corrosion? I imagine it would a bit because but I don't think it would be as effective as SS.

 

[EdStainless]

In a lot of applications even just a little Cr will give significant improvement in many environments.
That doesn't mean that it will work for you. You need to do some research.

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Plymouth Tube

 

[a7992]

I have searched and searched and I can't find anything related to 2 1/4Cr and erosion. Actually, I can't find a whole lot of information on 2 1/4Cr in general (not related to high temperature anyway). I also can't find any corrosion data for 2 1/4Cr and naphtha. Any advice on where I can search? I've tried the ASM handbooks and API 571 but am not having much luck. Is 2 1/4Cr called anything else besides alloy steel?

 

[SNORGY]

If the problem is "erosion", then in my mind maybe the material property that needs to change is hardness, not the alloying composition. If the problem is "corrosion", then changing to an alloy makes sense.

If the problem is "erosion", I don't think tube inserts will help much. The velocity will go up; all things being equal, then so will the erosion.

Regards,

SNORGY.

 

[unclesyd]

If we had a little more information it might be possible to determine if the OP's problem is corrosion, abrasion, or corrosion-abrasion. I've personally used tube inserts to successfully mitigate all three forms tube damage.
Specific examples are using Al6XN sleeves in Hastelloy C-276 tubes due to catastrophic corrosion of the HC within the bounds of the tubesheet, Hastelloy C-276 inserts in Carbon Steel tubes in NaOH + metallic catalyst corrosion+erosion?. 316SS inserts in 304 tubes to combat organic acid corrosion to the 304SS tubes.
I've personally haven't done any calculations but have looked at other's and there is no appreciable increase in tube inlet velocity. I have also never seen any differential or galvanic corrosion problems with inserts. In fact we use/used a lot of safe ended tubes to combat both MIC and SCC from the water side.

 

[EdStainless]

And there is always the practical approach. You are loosing tube ends and will have to junk the bundle in 6 months, so stuff SS inserts into it and see what you get.
All of the SS alloys are resistant to flow erosion damage if there is no particulate or corrosion.

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Plymouth Tube

 

[kclim]

Re 2.25 Cr 1 Mo
This is a grade recognised as a creep resisting steel - used at temps exceeding 500ºC, used in refineries and power stations amongst other places.

As far as general corrosion is concerned, the low alloy content of 2.25 Cr does not afford much more protection than a plain carbon steel.

Are the wearing parts exposed to water/steam or naptha? If it is water/steam, it is possible the damage mechanism is FAC (flow accelerated corrosion, specific case of erosion corrosion).
See

http://www.tetra-eng.com/docs/Flow_Accelerated_Corrosion_Assessment.pdf

It's a fairly well documented corrosion mechanism in power gen circles as it has caused fatalities in high pressure piping.

Turns out small additions of Cr (as low as 0.2%) can markedly decrease FAC - 2.25 Cr steel should be immune for all intents and purposes. That's probably where your colleague's suggestion came from.

2.25 Cr is recognised in ASTM standards as 'X'22 grade (where X will change depending on the product form e.g P22 for piping, T22 tube and F22 forging).

 

[davefitz]

erosion corrosion at the inlet of the tubesheet is common , and due to 2 effects.

One is the vena-contracta effect: the fluid streamlines constrict at the entrance such that the effective velocity is 1.6 times the apparent velocity,including the velocity in the boundary layer recirculation eddies.This effect can be mitigated by installing inlet bellmouths or using a tapered / chamferred inlet section, but that option is costly.

The second effect is corrosion due to the type of iron oxidation occurring . If conventional alkaline water treatment with dearation and/or oxygen scavengers is used, then the type of oxide scale that forms is very fragile, and spalls off the surface too easily . I one instead uses the oxygenated water treatment methods, then the type of oxide scale formed (hematite) is a hard scale that does not easily spall off, but it is neccesary to hold the pH in the range 7.5-8.5 and the O2 levels limited to about 250-500 ppm.

Or you can upgrade to 1% chrome alloy tubing, which may provide a 5-10 times improvement in rate of erosion /corrosion damage over plain CS.

 

[a7992]

Thanks everyone for the help! You have put me on the right track!