HI,
I have been started analysing High temp lines connected to Fired heater. Since the Heated is a Major critical equipment, kindly advise me what are all the areas need to give the more attention apart from the nozzle loads should satisfy the API 560 allowables.

Rams.

In fired heater analysis, your stress model should capture the type of heater(box,cylindrical). Make sure how the tubes are supported (bottom or top). Accordingly the y- supports on the piping need to be placed to minimise differential growth. It is advised to look at the heater drawings before they are issued for fabrication.Work with the Fired heater engineer from the quotation stage. You may have to specify stops at the heater nozzle face to control movements. Also note that API 560 has an allowable MOVEMENT range also in addition to allowable loads. Make sure the connected piping does not impose movements on the heater tubes.
While taking supports from the heater structure ensure they are stiff enough.

Regards

All good points by Neelam. To summarize - you need to understand that a fired heater is really just an extension of your piping system from an analysis standpoint. It's just that that section of the piping system is encased in a firebox.

Typically, the nozzle penetrations through the box are not fixed points. I've had bottom outlets heaters with the lines supported at the top of the box, resulting is 6" of downward movement at the flange face. The external piping system that you design needs to account for whatever the heater piping is going to do.

Also, the heater shell is going to have a skin temperature that will need to be accounted for - I've typically seen the refractory thickness set so that the skin temperature is about 200°F.