I have existing boiler (4MMBtu/hr), and the condensate return from the Process side is going directly to Boiler Feed Water Tank. Plant is not in use since last couple of years and client is planning to start the plant. It is a small boiler so it doesnt have deaerator.

The concern is from the process side (from steam traps) condensate header is going directly to Boiler Feed Water Tank, so Tank is also acting as a Flash vessel.  

The Boiler Feed Water Tank(Resilient Seated Gate Valves) is Atmospheric and it has vent on it. I am not sure if the Boiler Feed Water Tank has to be pressure vessel.

The condensate is probably cold enough to be storred in atmospheric tank. There is no flashing (or minimal), because the condensate returns always in a colder water.
If it is fitted with a nameplate and there is mention of ASME VIII or similar, then is a pressure vessel. Otherwise is not.

As said above, the tank must be stamped as a pressure vessel, to be used as such. On that system, it is most likely not a pressure vessel.

Since the system has not been used for a couple of years, my only question would be-How was the boiler "laid-up"? Hopefully it was in dry lay-up packed with desiccant for the last couple of years, otherwise it could be very corroded on the inside.

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