1. Relief is not required for on-plot lines or for lines to plant rundown tankage where line length does not exceed 400 feet. Relief valves are provided on plant equipment, and it is assumed that all on-plot and rundown lines will be open to equipment that is relieved.
2. Relief is not required in off-plot manifolds confined by valves of a type which can be expected to leak, such as typical solid-wedge gate valves, if there is less than 20 feet of pipe per valve. (If the amount of pipe per valve is between 20 and 200 feet, use the “Doubtful Case” approach given below).
3. Relief is not required in areas where no source of external heat energy is available, including most underground lines.
4. Relief would not be needed if a line is always hot and if it can be blocked only when hot. The chances are slight that it can be heated further by solar radiation. For this reason, relief valves need not be provided in liquid hydrocarbon or water lines which can only be blocked at temperatures above 150°F. A line which is hot when blocked and then cooled will be subjected to an internal vacuum. Since standard wall pipes up to NPS 30 can withstand vacuum of 14.7 psi, and since the possibility of pulling this much vacuum is slight, there is no need for vacuum relief on standard wall pipe lines in the sun. (The assumption that a line can be blocked only when hot is not completely accurate.)
5. Relief is not needed on lines insulated for heat retention where the source of the hot liquid has thermal relief provision.