Full definition
Liquid circulated at controlled pressure between the two seals of a double mechanical seal arrangement. Functions: lubricates and cools the inner seal faces, prevents process fluid from reaching the atmosphere. Common fluids: water-glycol (API Plan 52/53), mineral oil ISO VG 32 (Plan 53A), nitrogen (Plan 74). Barrier pressure must be 1.7-2 bar above process pressure to maintain separation.