Limitations of replacing C.S tube with S.S tubes in a C.S shell and tube exchanger

[inammanj123]

Hi,

My client want has a to replace existing carbon steel tubes in a shell and tube heat exchanger with stainless steel tubes.
Can this be done?
what will be the limitations?
Does TEMA and API 660 addresses this dissimilar material situation and what what guidelines are there in these standards?

 

[SnTMan]

inammanj123, is this re-tube or new construction? For a floating tubesheet or U-tube unit, mechanically, there is not likely to be any real difference, unless the tube material yield is higher than the tubesheet, for example duplex stainless tubes and CS tubesheet. In this case expanded tube-TS joint can be difficult or impossible to seal.

If a fixed tubesheet unit, the stresses due to differential thermal expansion will need checked as per Part UHX or similar.

The real issue is likely to be performance-related as the thermal resistances of the two materials are different.
The thermal & hydraulic performance will need to be verified per HTRI or similar.

I don't think TEMA or API 660 has any particular limitations on SS tubes in a CS exchanger, it is a fairly common construction. Some client specs may mandate tube and baffle materials be similar materials, this may or may not apply to your situation.

Regards,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[EdStainless]

Just remember to check the thermal and pressure calculations.
It is common to do this, and often the support plates are switched to SS also (but not always).
In many cases the SS tubes are thinner because there is no corrosion allowance, so in the end the thermal performance ends up being very similar.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[weldstan]

As Ed stated, it is quite common to change from CS to SS tubes.

 

[inammanj123]

Thank for feedback,
basically it is AES type(having floating head) and client is saying that corrosion has been seen on tube due to tube side fluid, so they want to change the tubes to S.S 316.

I have following conclusions:

1-In the light A.4.4.7 OF API 660, which is talking about dissimilar tube and tube sheet material, I can go with carbons steel tubesheets with SS 316 cladding on tubeside. The rest of tubeside items, since not being welded to S.S tubes, can remain carbonsteel.

2-second option is to change the whole tube side material to S.S 316. The reason they want to change tubes is due the corrosive nature of tube side liquid, so why just change the tubes only? for increased life of equipment, whole tube side should to change to S.S.

Regards,
Manj

 

[SnTMan]

inammanj123, the choice between options is primarily one of cost. It can make sense to change tube material only, as the tubesheets and head cover are relatively thick and presumably suffer less from corrosion than the tubes. It the joints are welded, clad tubesheets make sense, but the head cover could remain CS, or internal clad / WOL.

All stainless tubeside is perhaps the most bullet proof, but of course highest cost.

Regards,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[inammanj123]

I just found out that tube to tube sheet is only expanded, so it means that in this case there is not code restrcition to use cladded plate over tube sheet. Right?

client want to replace whole tube side to S.S 316, but Client ask to make sure there there is no galvanic corrosion. One way to deal with galvanic corrosion is to use insulating gasket kit. Right? This insulation kit will be used on bolts of floating head side and also on shell/tubeside girth flange. right?

 

[inammanj123]

I think since joint between tube and tubesheet is only expanded with groove, there is no need to change tube side material, besides tubes. This will save a lot of money and engineering hours. What do guys suggest?, but if client thinks that based on corrosion rate he will go with option of changing whole tube side material to S.S, than it is another story, but my point is that from mechanical point of view since there is no weld between tube and tube sheet, only change tube material S.S.

 

[SnTMan]

inammanj123, if an expanded only joint, there is no mechanical reason to clad the tubesheet, correct.

As to galvanic corrosion, I suppose the potential exists, but again, this is a common construction, if it were a widespread problem I'd think it would be known.

You client should know the service, and determine the potential extent of any effects of galvanic corrosion.

Whatever the case I would definitely avoid any kind of insulating elements in the floating head bolting. That's just asking for trouble.

Regards,

MIke



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[inammanj123]

thanks,

Since there is no requirement to change any thing beside tubes, i will give proposal to client, to replace only tube material, and nothing else. This will save a lot money and engineering hours.

One questing is they will replace the tube bundle(tubes(S.S)+tube sheet(same as existing)) or just re tubing will do the job?

 

[cap1a79]

1. Tube
- Generally, basic purpose of tube is "heat transfer" while it is mass transfer for pipe.
- This means that it is not practical to give sufficient corrosion allowance for tube mostly, covering whole of life time (If the thickness of tube is thick added by corrosion allowance, heat transfer between shell & tube would not be efficiently made.) In the meantime, CS tubesheet & channel can have corrosion allowance. (Refer to No. 2 & 3)
- That's why design life for CS tube is somewhat limited for most of licensor's design (ex., 5 to 10 years) and recommended to replace it periodically.
- Due to above reasons, it is quite common to apply CS tube with periodic replacement or upgrade to SS (or CRA) which can cover whole of design life.

2. Tubesheet
- Regarding galvanic corrosion issue, refer to below reference from API 571 clause 4.3.1.6)

(Quoted from API RP 571 4.3.1 Galvanic corrosion)

4.3.1.6 Prevention / Mitigation
a) The best method for prevention/mitigation is through good design.
b) Differing alloys should not be in intimate contact in conductive environments unless the anode/cathode surface area ratio is favorable.


(Unquoted)
- Galvanic corrosion can be an issue in case the design is related to "large cathode versus small anode". On the other hand, for the case between CS tube sheet(as large anode) and SS tube(as small cathode), galvanic corrosion will not be an critical issue with no need to change the material as the area effect as described has been already favorable to minimize it.

- Meanwhile, as tubesheet contacts with both fluids (from shell & tube sides), the material & corrosion allowance should be suitable for both sides during life time.
- If the thickness is too thick, then the material upgrade can be considered to optimize it.

3. Channel
- Generally, the same material and corrosion allowance as incoming pipe can be considered.(As long as the incoming pipe is suitably selected.)
- However, as sometimes there is erosional issue (ex., solid particle, turbulent flow), it is also common to upgrade the material to SS or CRA for channel side. For the material upgrade, it is recommended to discuss with client. (CS with regular inspection can be one of the options considered, if there is frequent opportunity to open the cover.)


Lee SiHyoung,
WorleyParsons Oman Engineering,

 

[SnTMan]

inammanj123, re-tubing is often done, it can be an option. It will depend (among other things) on the size, condition and configuration of the bundle. Condition is obvious, some constructions may not be easily re-tubed, small bundles may not be worth the cost.

Schedule can be a consideration as well, a new bundle can be fabricated while the existing stays in service.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

TEMA RCB-1.51 CARBON STEEL PARTS
Tubes, bolting and floating head backing devices are not required to have corrosion allowance

Regards
r6155