Graphitization

[kyong]

I'm designing a boiler steam drum/evaporator.
Problem is temperature. Flue gas design temperature is 685 C(1265 F).

Flue gas flows thorough the tubes in the drum.
Tubesheet(or called head) is also exposed to flue gas.
Tubes material is SA192 and tubesheet material is sa516-70N.
Both materials are known as having graphitization at higher than 800~850 F. Of course, because one side of the tube or tubesheet is contacted with water(457 F), the mean metal temperature is lower than 1265 F. But surface of the metal should be almost 1265 F, and mean metal temperature is 861 F.

In this case, are the materials allowed to be used? Or would the lifetime of the boiler unreasonably short due to graphitization?

kyong

 

[JohnBreen]

Hello,

You can count on carbon migration at that temperature. I think that you should use ASTM 335, P22 material as a minimum.

Regards, John.

 

[kyong]

John,

Thank you for your advice.
What is carbon migration?

Steve

 

[JohnBreen]

Hello Steve,

Sometimes it is referred to as graphitization. It is when (due to temperature and time)carbon migrates out of the individual grains and into the boundary layers between the grains. This causes the material to become embrittled.

Regards, John.

 

[davefitz]

I think there are designs which use carbon steel, but the design is configured that the portion in contact withthe gas is always wetted by water, and the flugas surface likely has refractory.

The metal temperature on the fluegas side, even without refractory, cannot be more than 100 F hotter than the fluid in the drum.

If the fluid side of the drum /head material exceeded about 20F above saturation, it would boil, with a heat transfer coefficient of over 1000 btu/hr/ ft2/F, and this is equivalent to a resistance to heat transfer of 1/1000. So the upper bound of the cold side temp is about Tsat + 20 (f).The thick metal represents another resistance to heat transfer , of resistance equal to L/K of about Rm = 0.25 ft/25 (BTU/hr/ft/F) = 1/100 ( assume 3" thk). The hot side exposed to the gas is absorbing heat at a max rate of 15,000 btu/hr /ft2. So the hot side metal temp is:

q/a = 15,000 = (Th-(Tsat+20))*(1/1000 +1/100)

so Th = Tsat + 170F = 457 + 170 = 627 F

The problem is not graphitization ( if the 3" wall is correct) but thermal stress--- a 150 F thru wall temp gradient is excessive- and would be even worse with T22 material ( lower conductivity). A better solution may be to select a type of carbon steel or low alloy that has economical welding requirements and high conductivity but to ensure there is a radiation shield or refractory on the gas side .