If there is any chance of loosing seal gas supply from an external source, the seal gas system should include automatic cut-in of a back-up source of gas. Tandem seals should typically use discharge gas as a back-up. In this case, it might be necessary to design for potential differences in liquid removal requirements between the different types of seal gases. Double seals using nitrogen can be backed up with nitrogen bottles, although this supply will be limited. If bottles are used, pressure monitoring/alarming of the bottles should be included in the design in order to assure their readiness over many years. In addition, an alarm and a compressor shutdown are also recommended in the event that seal gas pressure approaches the actual sealing pressure. The shutdown set point should be selected to allow safe coast down before the primary seal differential pressure reverses enough to cause damaging face contact.