Could somebody please provide guidance on sizing the emergency relief vent for an atmospheric tank (~35mbarg) where there is some non-condensable gas evolution from an aqueous solution when it is heated? Its not a reaction as such, just gas evolution from solution due to an increase in temperature. The relief device is a conservation vent set at 10mbarg. I don't have any gas evolution rates, just partial pressure Vs temperature data - do I need more than this? I have used the API2000 equations to check the size ignoring the gas, but I don't think this is enough.
Thanks.

Gas evolution from heating is the same mechanism as boiling a liquid due to excessive heat input, i.e. fire. So you can go through the detailed caculations for heat input to the vessel (taking into account heat transfer through the vessel into the liquid) OR you can just size the vent based on the API 2000 fire scenario; probably will be very conservative but it will save you a gazillion engineering manhours.

API 2000 normal vent sizing takes into account pump-in/pump-out rates added to thermal in and out breathing. They also provide a calculation for emergency venting due to fire.

Linus:

This will serve to reinforce what Phil has suggested. Conservation Vents and Emergency reliefs for API tanks are relatively very cheap and inexpensive. I always recommend a conservative design using API 2000 - which is conservative itself.

What I would do if it were my tank would be to install a Conservation Vent (which contains both pressure relief and vacuum relief) sized for the operating, normal relief (such as in-breathing, outbreathing, gas evolution, etc.) and right beside it an Emergency Vent. The emergency vent is nothing more that a weighed, blind flange that is either chained or hinged to the tank nozzle on which it sits. The emergency vent is meant for just that - an emergency (such as the fire case, a ruptured internal heating coil, or a blow-through case). That way, you would be covered redundently (if that is the correct word...).

Be sure to document all your basis, assumptions, and calculations in your plant files.

I hope this helps.