By Vincent A. Carucci
The simple answer to the question posed in the title is “As thick as it needs to be.”
A relative of this question is “Do I need to add a reinforcing pad to a storage tank nozzle that doesn’t have one now? Here again, the answer is simple - “Only if it needs one.”
While those answers sound rather “flippant,” they are both absolutely correct.
Since API-653 was first issued, companies had to take inspection and maintenance of their existing aboveground atmospheric storage tanks more seriously than perhaps they had been doing before. With increased inspection activity comes the inevitable need to evaluate what the inspectors find. It is not uncommon to find cases where existing nozzle neck thicknesses do not meet current API-650 requirements - not because they corroded, they were thinner from the beginning. It is also not uncommon to see a nozzle over NPS 2 (50 mm) that does not have the reinforcing pad that API-650 would currently require. Such situations are more likely to come up with relatively “old” tanks for a variety of reasons. What do you do in these cases?
In their desire to meet current industry practices, be conservative, and “safe,” some folks might decide to add reinforcing pads where there are none now, and to replace the “thin” nozzles with new ones that meet the current API-650 thickness requirements. But, do you have to? Not necessarily.
API-653 does not require us to upgrade existing storage tanks to meet current API-650 requirements. API-653 “only” requires that an evaluation be made to determine if the existing installation is acceptable. In fact for “old” tanks (i.e., those erected before 1980), it is generally preferable to avoid welding to them if at all possible in order to minimize the brittle fracture risk. Don’t misunderstand me - welded repairs and alterations can certainly be safely made to old tanks if appropriate procedures are followed to minimize the brittle fracture risk. It’s just preferable to avoid doing it if possible.
So, how do you deal with the cases of “thin” nozzles or no reinforcing pads? I’ve generally used the following approach:
Again, keep in mind that API-653 does not require that we upgrade
tank nozzles to current API-650 requirements with respect to
thickness and reinforcing pads. We just have to decide if they are
acceptable the way they are or not. Of course, if you decide that
changes are needed (e.g., adding a new nozzle, adding a
reinforcing pad, etc.), then they must meet current API-650