What are the disadvantages of duplex ss.

[songkarn]

Dear sir,
I have some project to change tube material of exchanger from cabon steel to duplex ss.
Anyone can tell me? What are the disadvantages of duplex ss.

Best regards,
SK

 

[SnTMan]

songkarn, for this appliction, and assuming compatibility of duplex with the process, a main disadvantage is likely to be the higher yield stress of duplex tube as compared to the tubesheet. This can make it difficult to produce an acceptable tube-tubesheet joint if it is expanded only. Qualification of the joint as per Sec VIII, Div 1, Appendix A for example, is likely required.

Regards,

Mike

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

 

[EdStainless]

You can't use a duplex above 600F, or below -20 to -40 (depending on the alloy).
They can be susceptible to chloride stress corrosion cracking, but they are somewhat resistant.
If you are putting them into CS tubesheets or mating to other CS components the CS may see greatly accelerated local galvanic corrosion effects.
The duplex also has slightly greater thermal expansion than CS. It also has lower Thermal conductivity, but that is more than made up for by the thinner walls.
Remember to not use a corrosion allowance, and calculate the thickness using the properties of the duplex. Depending on the application your thickness will drop by 30-60%. Use the calculated thickness.

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P.E. Metallurgy, Plymouth Tube

 

[MJCronin]

Song,

EdStainless has covered all the major limitations.....

You must tell us more about your existing HX and it's duty...

What are the operating fluids, Max T and P ?

Erosive fluids ? Particles ? Chlorides ? if so, what is the ph and ppm of the chlorides ?

A drawing of the HX would be nice.......

MJCronin
Sr. Process Engineer