Material Selection for Double Pipe Heat Exchanger

[Abigail_281106]

Hello Everyone,


I got to design a double pipe heat exchanger, where the natural gas inside the inner pipe will be heated up by steam, which flows at outer pipe.
The operation data of natural gas and steam are as follow:

1. Natural Gas (inner pipe)
- maximum operating temperatur: 118C (and normal operating temperatur is only 78C)
- maximum operating pressure: 59,2 bar

2. Steam (outer pipe)
- maximum operating temperatur: 260C (and normal operating temperatur 220C)
- maximum operating pressure: 7 bar

And i got two question:
1. To determine the design temperatur for inner pipe, should i use the maximum operating temperatur from steam (outer pipe), which is 260C.
2. What is the suitable material for both, do i have to design it with the same material, for example both pipes made of stainless steel? or can i design it with different materials for example inner pipe with stainless steel but outer pipe with carbon steel? Is there any book or reference to select the materials for double pipe heat exchanger?

I thank you for every answer.
Happy weekend.

Abigail

 

[TiCl4]

1. Yes, 260 C, if that is indeed the highest temperature the Hx will see. Check to make sure that the 260 C and 7 bar(g?) are protected upstream by a PSV on the steam header or other limiting functionalities.

2. I do not think a pipe-in-pipe heat exchanger falls under ASME Section VIII, but is rather normally specified as a jacketed pipe and built as such (ASME 31.3 would be a relevant code, as would be plant piping specs for steam and natural gas). Carbon steel's coefficient of thermal expansion is ~66% of stainless steel (grade dependent), so thermal expansion will be a bigger factor if you are mixing metals.

 

[Abigail_281106]

Hello @TiCl4 !

thanks a lot for your answer and suggestions.
1. Allright i'll make the design temperature for both pipes 260C. Yes i proposed already a PSV on the stream header in case of the inner pipe break.
2. So it would be better to use the same materials for both pipes, in order to prevent thermal expansion problem. Thanks a lot.

I did calculation for double pipe heat exchanger with HTRI Simulation, so it consists of two inner pipe and two outer pipes (shells) in series, with total length of 3 m.
Is there any concern from the simulation results.
Does it look already good?


Thank you.

Abigail

 

[0707]

I think A106 gr B would fit your aplication.

luis

 

[EdStainless]

Under normal conditions the various walls will be different temps and so they will expand differently.
Using steel gets you lower expansion making it easier to manage.
Sometimes you see these as 'hairpins' making differential expansion easier to accommodate. Or keep each section short.
These are very common in food processing where you want to heat or cool something gently. The pasteurize milk in these, they serpentine back and forth and are up to 140' in total length (but only about 26' of floor space).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Abigail_281106]

Hello @Luis 0707 thank you for your suggestion. So carbon steel is suitable for both natural gas and steam pipe?
Ok. Noted.

 

[Abigail_281106]

Hello @EdStainless !

Thank you so much for the technical suggestions ad information.
In which case is a stainless stell required and in which case carbon steel is adequate?

If i designd both pipes including inlet and outlet flanges with stainless steel, but the outer pipe (steam stream) will be connected flange to flange with the existing carbon steel steam piping, would it be no problem right?

Thank you.

Abigail

 

[TiCl4]

Abigail, does your plant/company not have any pipe codes to use as reference? Carbon steel is used frequently for both applications for piping, but internal corrosion can be a bigger issue here than for a simple steam pipe.

I’m not sure I’ve seen stainless on NG piping. Not that it isn’t fine, but I don’t know about application.

Also, you may want to consider an impingement plate on the shell side steam inlet - the steam can be quite erosive if it has any entrained water droplets.

Stainless to carbon contact technically does have an electrochemical potential. Not sure if protection would be required though.

 

[0707]

Abigail_281106 consult Tubular Exchanger Manufacturers Association, TEMA® Standards.

luis

 

[Abigail_281106]

Hello TiCL4,

Thanks for your insights !

Yes, that's the problem, we have kind of data bank, and it was said, that i can use carbon steel and stainless steel.
I just do not understand, for example, at one plant, for natural gas 130 bar, 550C, they are using carbon steel.
Meanwhile at other plant, for the same natural gas 60 bar,120C, they are using stainless steel.
So, i was not clear, when do we choose carbon steel or stainless steel?