I am not aware of any codes that address this question.
There are several concerns involved in welding around piping and equipment that contain flammable fluids. Leaks are the first and foremost. You need to assure that there are not leaks or flammable vapors in the area where the welding is going in on. You want to be sure no vapors can be blown into the area by the wind.You also need to be sure no one will be releasing any vapors during the welding process. You will need someone assigned as a fire watch with the appropriate fire extinguishing equipment at hand to suppress an fire that might get started. Finally you need to inform everyone in the area about the welding activity and only have the absolutely necessary people in the area at the time.