API Plan 52 is an unpressurized seal support system used for dual unpressurized seals, where the outboard seal acts as a containment barrier. It utilizes an external reservoir to circulate a buffer fluid between the inboard and outboard seal chamber via natural convection or forced circulation. The buffer fluid acts as a cooling medium, helping to maintain seal performance while preventing the process fluid from reaching the atmosphere in normal operation. However, if the inboard seal fails, process leakage may mix with the buffer fluid, which is vented or drained safely.

This plan is ideal for non-hazardous, non-toxic, and non-volatile fluids where minor leakage into the buffer system is acceptable and where a pressurized barrier system (Plan 53A/B/C) is not required.