API 560 Code Issue - Use of carbon steel with high temperatures above 425¦C

[rvivac]

I have a carbon steel (CS) use at high temperatures issue here . We are just finishing the construction of a Reformer Furnace for a Hydrogen Generating Unit. The designer / supplier used ASTM A36 on an expansion joint on a breeching duct that is subjected to a 515°C stream.

It is fact that the item 12.5.4 of the API 560 says that on temperatures above 425°C, stainless steel (SS) or alloy steels (AS) shall be used, instead of carbon steel due to graphitization and loss of ductility that occurs on this type of material at temperatures above 425°C.

On the other side, the supplier tried to convince us that, since the part is not subjected to stress, the API STD 530 (2008) allows the use of grade B CS’s up to 540°C, since it is the limiting design metal temperature. Note that the API STD 530 scope is for Heater-tube Thickness. They tried to convince us that since the A36 have similar properties of the API 283 Grade C, it can be used like an Grade B in this situation. I did not accept that.

I have seen that this kind of design misunderstanding is common, at least, on our refineries and the maintenance people have to inspect and monitor this kind of joints forever, adding cost and cautions to the process. I would kindly like to know how you deal with these situations and if it is a common design misunderstanding.

 

[rvivac]

This question had an evolution.

Since the item 12.5.4 of the API STD 560 says "If metal temperatures exceed 425 °C (800 °F), stainless or alloy steels shall be used." and the stream is at 515°C.

I have done some calculation and the wall temperature of the ASTM A36 due to radiation and forced convection inside and outside the system is reduced to around 365°C so that reading the standard requirement is reached by 60°C bellow the highest allowed temperature.

A good reference for heat and mass trasnfer Frank Incropera:

http://www.amazon.com/Fundamentals-Heat-Transfer-Frank-Incropera/dp/0471457280.

But since norms like the API STD 560, which provide construction guidelines to "design, materials, fabrication, inspection, testing, preparation for shipment, and erection of fired heaters", is there any norm which provide guidelines for on streams thermal calculations other than ASTM C680 – 10?

 

[Ripz]

The 800F limit is usually set for Carbon Steel, since long term operation above that causes Spherodization (Softening) of the CS.

But since there's no stresses on the joint, then you shouldn't worry about rupture too much.

 

[rvivac]

Thanks Ripz.

I opened another thread about this issue, since no forced convection was considered in the duct:

http://www.eng-tips.com/viewthread.cfm?qid=360364.