Tandem-seal pumps

A disadvantage of tandem seals is that they cannot safely be operated without process fluid because the seal on the process fluid side may run dry. Also, the springs and other internal seal mechanisms on the fluid side run in the process fluid, which may not be good in highly corrosive fluid. The big advantage of tandem seals is that they allow process fluid to be pumped without contamination from lubricating oil, and the chances for major loss of process fluid are small. 

The tandem dual seal is mostly mated to a pressurized barrier tank, or a pumping unit as the support system. Support systems are discussed later in this chapter. 

Semi-hermetic tandem pumps are also used for supercritical gases, in the same way as tandem pumps are used for high system and discharge pressures as described in Section 5.3. In order to separate the liquid-filled rotor compartment from the gas zone, the pump is fitted with two mechanical seals. Accordingly, fluid can no longer be fed from pump 1 to pump 2... 

---