Plan 54 uses an external source to provide a clean pressurized barrier fluid to a dual pressurized (double) seal.  The system supplying the barrier fluid can be

• a process pump providing clean cool lubricant under pressure
• a simple lubrication system with minimal components
• an elaborate system with many ancillary components.

Stickily speaking, there is no Plan 54 “system” and the API 682 illustrations show only an inlet and outlet from the Arrangement 3 seals. Plan 54 only means that the dual seals are pressurized from an external source and that the barrier fluid pressure is greater than the seal chamber pressure.

Although piping Plan 54 is very simple, the actual external lubrication system for Plan 54 can be very costly  depending upon the number and type of redundant and safeguard systems utilized.  Because of this high cost, it is not unusual (although not recommended) to have one lubrication system supply several sets of dual seals.   In this case,  precautions should be taken so that a failure of one seal will not drain the entire system.  Precautions should also be taken to prevent contamination of the system barrier fluid should one seal fail.