Plan 74 is a barrier gas plan for dual gas seals. All pressurized dual seal arrangements have a barrier fluid at a pressure greater than the process pressure being sealed.  Dual gas seals differ from other pressurized multiple seal arrangements in that they do not have circulation of a fluid between the seals, but rely upon an external source of inert gas to pressurize the seal cavity provided by the seal cartridge.  Flow into the seal cavity is the result of leakage of the inert gas past both the outer seal faces and to a lesser extent the inner seal faces. Typically the external source is a pressurized nitrogen line within the plant.  In some very special cases the nitrogen source can be a nitrogen bottle or a bank of nitrogen bottles.  Bottled nitrogen can be an expensive, unreliable system and the maintenance is high to ensure that the bottles have sufficient pressurized gas at all times.

A problem that can develop with Plan 74 is that the barrier gas leaks into the pump.  It is possible for gas to accumulate in the standby pump and for that pump to become vapor locked.  This is especially true on small hot pumps.  Therefore, venting the standby pump before starting  is recommended.