Plan 54 uses an external pressurised barrier fluid supply system. This system supplies clean fluid to the mechanical seal chamber where the liquid seal of system 3 is used. The barrier fluid is maintained at a pressure higher than that of the seal chamber. It circulates through an external pump or pressure system. Plan 54 systems are also pressurised double seal systems in which leakage from the inner seal is directed into the pumped fluid. In Plan 54, the cool and clean barrier fluid comes from an external source and should be supplied at a pressure at least 0.14 MPa (1.4 bar / 20 psi) higher than the pressure on the inner seal side. This results in a slight leakage of barrier fluid into the process medium. This plan (with system seal 3) must not be used if the pressure of the barrier fluid is lower than the pressure in the sealed chamber. Otherwise, failure of the inner seal can lead to contamination of the entire barrier fluid system and damage to the seals.
