Denickelification of 70/30 Cu/Ni tubes

[Anwar alasmi]

Hello
Sea cooling water system provides the main cooling medium for heat removal in to our plant through heat exchangers. The cooler details are as follows.
Fixed tube ASME code, section VIII, Division 1, 1998 Edition and 1999 addenda
Tube material- ASTM B111 C71640
Tube side: seawater / shell side: Natural Gas
Design Temp tube side : 85/-64 °C & shell side : 139/-64° C
Max working Temp tube side : 40 ° C & shell side : 129° C
Actual working Temp tube side : 25 - 35 ° C & shell side : ≥ 130 < 140 ° C
Issue to be discuss
Every four years and in some cases less than that. We have to replace the top raw of the tube due the size of the defect more than 40%. we have observed discoloration at outer layers of the tubes opposite to the gas inlet nozzle and internally evidence of pits with sing of pink/red discoloration.
The internal corrosion observed is characteristic of Hot spot corrosion ( denickelification). However, this failure occur frequently and we know it, also it has been addressed in many paper such NACE paper No. 599.
My concern as following:
What is the tube outer skin Temp assuming the shell side operate above 130° C and tube side 35 ° C , tube wall 1.626mm ?

Is the tube material suitable for such application knowing that the offset Temp of the process goes beyond the threshold temp of the tube? Despite, its good corrosion resistance to sea water.

Some of the literature address the denickelification occur above 90 ° C. and some of them above 130 ° C. which temperature we should consider during material selection?

Is any fellow face such failure, if so, how it has been mitigated?

Is the water flow contribute in such failure?

thanks in advance

 

[EdStainless]

In this portion of the exchanger you have very high flow of hot gas, and maybe too little water flow.
The tubes are probably getting within 5C of the gas temp.
And this is way too hot for CuNi.
Either look at the design, and try to find a way to get less gas/more water in those areas, or look at other materials. Today these exchangers would be built in Ti, no questions.
We have supplied superferritic stainless for some applications like this, but if the local heating is too much (reaching boiling) then there is a risk of pitting even with these alloys.

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P.E. Metallurgy, Plymouth Tube

 

[weldstan]

Agree with Ed. These tubes in the USA would be made from Ti or superferritic stainless alloys like SeaCure.

 

[georgeverghese]

Tube OD skin temperature will a function of the tube ID heat transfer coeff, the OD shellside htc and the mean bulk ID and OD fluid temps at the local zone of interest - a process engineer can compute this for you if these values are known.

 

[pradipgoswami]

Hi Anwar,
You may read the following papers for additional information on "denickelification". Hot spot issue is the major cause of damage.The first article(under localized corrosion) describe on simple steps to mitigate this damage resulting denickelification.The interpretation is :While dealloying can occur in copper alloys, it is not a common occurrence in copper-nickel alloys.If process improvements to lower tube temperature is not feasible then switching to another alloy could be the potential solution.
If change of material is thought of,as along term solution, a switch to "Duplex" or 254 SMO alloys may be wise choices. This is so as both of these alloys have proven history in sea water environments and fabrication resources are plenty.

Thanks.

http://www.stainless-steel-world.net/pdf/12014.pdf

http://cradpdf.drdc-rddc.gc.ca/PDFS/zbb57/p507340.pdf

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