Baffle Material for Stainless Steel Tube Bundle 1

[Narbij]

Hello Metallurgists,

I have a Shell and Tube heat exchanger with U-tube bundle for which I'm replacing the bundle.
Shellside service is ethane. Tubeside service is condensing steam.
All material for original exchanger is Carbon Steel.
For the replacement bundle, I'm replacing it with a 316 Stainless Steel tube bundle to address a erosion/corrosion issue caused by some upstream equipment - (yes, I know I should be addressing the upstream process upset issue instead but at this point, I have no choice)

Question is: should I make my baffles 316 SS as well? or are they okay as Carbon Steel?

My reasoning: I'm worried that I will ruin the 316 SS tubes with the Carbon Steel baffles due to iron contamination. Is my worry warranted?

Thanks in advance.

 

[SnTMan]

Narbij, is your erosion / corrosion issue on the tubeside or shellside? That should inform your choice.

I see these things both ways, all the time. If the shell fluid is not especially corrosive CS baffles would likely be OK, if not, perhaps not.

OTOH, probably not a large incremental cost to go with SS.

Regards,

Mike

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

 

[Narbij]

SnTMan: Thanks for responding.

Erosion/corrosion issue is on the shellside. The existing CS baffles are fine though...however the existing CS tubes are suffering impingement damage from amine carryover, hence the change to SS. For a relatively small HX, you're probably right about the incremental cost. However, in this case, I could save some significant $$$ by switching to CS.

Thanks.

 

[EdStainless]

If the erosion is shellside then go to SS baffles.
With more erosion resistant tubing you could easily find yourself eroding out the holes, and then having tubes fail by vibration.

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P.E. Metallurgy, Plymouth Tube

 

[Narbij]

EdStainless: That's an interesting thought...I had not considered that. Indeed this particular HX is quite prone to vibration as well...
I'll take another look at the photographs from the inspection reports.

Thank you kindly for chiming in with that.

 

[EdStainless]

If you have serious cross flow issues causing vibration then you might want to use a tighter baffle spacing in your new bundle. Don't be afraid to solve problems.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

I would hope you know that switching to stainless steel will be a hit on heat exchanger thermal performance. Other than that, if the impingement is on the carbon steel tubes caused by an upstream flow issue, you may still have other problems.

 

If there are no leaks, then the process fluid and steam are clean then this HX environment is pretty benign.
However, trying to solve any and all upstream process problems by ramping up metallurgy can lead to open-ended expenditure, as well as lower efficiency.

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

 

[EdStainless]

The thermal performance could stay the same since the SS version will not have an added corrosion allowance. With walls that 30% thinner the heat transfer will be nearly identical to the CS version.
Make sure that the SS tubes are specified to the designed wall, without added corrosion allowance.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

The thermal performance could stay the same since the SS version will not have an added corrosion allowance. With walls that 30% thinner the heat transfer will be nearly identical to the CS version.

Typical corrosion allowance is small in comparison to the loss in heat transfer using austenitic stainless steel tube materials. I doubt it will remain the same. Most of the heat exchanger replacement bundles I have dealt with at power plants over many years take a hit on thermal performance unless more surface area was added to compensate for the loss in thermal conductivity using austenitic stainless steel tubing versus non-ferrous (copper-nickel),martensitic stainless steel, duplex or CS.

 

[SnTMan]

I had sort of assumed performance had been confirmed. Perhaps not.

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

 

[EdStainless]

Metengr, Yes and no, I understand what you are saying.
I have seen many applications moving from Cu-Ni to duplex or ferritic SS actually improve overall heat transfer. Part is removal of CA, part is using the higher strength of the material allowing the use of much thinner walls, and part is the option of higher flow velocities because of the higher erosion resistance.
In austenitics the walls will not be that much thinner than CS, but flow velocity can be much higher.
I have frequently seen people change tube diameter when moving from CS to SS in order to optimize the heat transfer. Just switching the alloy and leaving the same OD and wall for tubes will be a recipe for disappointment.

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P.E. Metallurgy, Plymouth Tube

 

It needs a specialist HX designer to assess all of the impacts on performance.

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

 

[metengr]

Absolutely agree, ironic metallurgist.

 

[pradipgoswami]

Narbij,


Question is: should I make my baffles 316 SS as well? or are they okay as Carbon Steel?
A- The baffles should be of stainless steell also.Iron contamination, during operation as well as galvanic corrosion(during shutdown)is possible from Carbon Steel baffles.Low ph could be a significant contributing fator towards above,

Baffles are the suppoting assembly for tube bundle. However cheap design could endanger the tube bundle significantly. Definitely upgrading to costlier material means, yiou're redesigning for longer service/operational life.

Thanks



Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
ca.linkedin.com/pub/pradip-goswami/5/985/299

 

[Chumpes]

hi
normal design is usualy to have the same metallurgy for tubes and baffles
regards

 

[Narbij]

Thank you all for your inputs.

1) EdSTainless: I did some more digging onsite, and in speaking with the Chief Inspector, found out that this was definitely a warranted concern - he had three examples where precisely this occurred. Thank you immensely for cluing me in here.
2) THERMAL PERFORMANCE - SnTMan and metengr: Indeed the hit on thermal performance from switching CS to SS was considered and optimized to drop thermal performance by a mere 1.6%. In this case, it was not an issue because of the service of this exchanger as well as the overdesign in the original design.
3) IRON CONTAMINATION: Agreed, there can be cases where this is a concern - specifically, in my understanding, in a wetted environment with the presence of oxygen, iron contamination from baffles on tubes can lead to stainless no longer being "stainless" - I have specified HX's in other services with full SS bundle cage assembly to avoid this. In this case, I did not deem it a concern.
4) GALVANIC CORROSION: again, requires a wetted environment. The galvanic cell would cause the CS to preferentially corrode and, galvanic corrosion being local, may cause the holes to widen and the same concern would as EdStainless raised would be applicable.

I chose to implement a full SS bundle including baffles.

Thanks again for chiming in and educating me. As always, appreciate this community very much.

JA

 

[SnTMan]

Narbij, appreciate the update.

Good Luck

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