EPA attempts to define and quantify how far apart separate vessels have to be to be considered different processes. There is no hard-and-fast rule for how great this distance should be before you can consider the vessels as part of one process.
Two vessels at opposite ends of a large warehouse room might be considered one process if the entire warehouse or room could be engulfed in a fire.
Two vessels separated by the same distance outdoors might be far enough apart that a fire affecting one would be unlikely to spread to the other.
You may want to consult with your local fire department. You should then use your best professional judgment. Ask yourself how much of the regulated substance could be released if the worst happens (you have a major fire, an explosion, or a natural disaster).
There is no hard-and-fast rule for how great this distance should be before you no longer need to consider the vessels as part of one process. Two vessels at opposite ends of a large warehouse room might be considered one process if the entire warehouse or room could be engulfed in a fire. Two vessels separated by the same distance outdoors might be far enough apart that a fire affecting one would be unlikely to spread to the other. You may want to consult with your local fire department. You should then use your best professional judgment. Ask yourself how much of the regulated substance could be released if the worst were to happen (a major fire, an explosion, or a natural disaster).
Source: www.epa.gov/rmp/process-identification-and-distance-between-vessels