stagnate boiler feed water with 321 stainless steel

hi
is your BFW well treated (inhibitor, oxygen scavenger)
what is the O2 content ?
I would say that well treated BFW, even stagnant, is not corrosive to SS 321.

regards

If the stagnant boiler feedwater contains oxygen it will result in corrosion pits to the 321 stainless steel tubing. There should be no feedwater touching these tubes, if possible.

My preference is definitely to keep austenitic heater tubes under dry, O2 free nitrogen purge if at all possible.

Be careful, depending on what the furnace has run, polythionic acid stress corrosion cracking could be a concern, even on 321 tubes (impossible to know that you haven't had a hot spot that's hot enough to sensitize the tubes). Not just on the ID either, if there's sulfur in your fuel gas, you can get OD initiating PASCC. If you're dead set on filling them with water and there's any sort of sulfur in whatever is being heated in the tubes, you'll probably want to use a soda ash solution rather than straight boiler feed water.

Nathan Brink