We have a 660 Mw power boiler at our power plant with a deaerator vessel and DA storage "tank". The DA vessel has a National Board number and a MAWP of 200 psig at 400 degrees F (so stated on the manufacturer's name plate). The DA storage tank (located below the DA "vessel") has NO National Board number but is made by the same company as a unit.
Is the DA vessel a Code vessel? It seems to me that the DA vessel and the storage tank below it are both vented to atmosphere (?) but there is a safety relief valve on the upper DA vessel. How could the DA vessel be containing pressure (200 psig?) if it is connected to the storage tank which is vented? If the DA vessel is not pressurized why is it National Board registered and why does it have a MAWP of 400 psig?
The questions come up because I have been told that if a "vessel" is National Board registered it must have a "R" stamp company do any weld repairs. Our DA vessel and DA storage tank need to have some cracks welded and the contractor says that we DO NOT need to have a "R" stamp organization do the work. I have read the Pensylvania Boiler regulations but I can't see any mention of DA vessels (if they really are pressure vessels that is a different story).
I am so confused.