Mixing high pressure and low pressure condensate increases the back pressure on the lowest pressure header. This impacts condensate evacuation.
It is observed across plants that condensate from all the equipments is connected to a single condensate return line. After flash steam separation from condensate, the condensate and flash are returned to the feedwater tank via separate lines.
However often in such a case, condensate of equipments using high pressure steam and low pressure steam get connected to the same return line. Condensate being at the same pressure as steam, mixing of the high pressure condensate reduces the net differential pressure available across steam trap of the equipment using steam at low pressure.
This restricts condensate evacuation from the low pressure equipment. With the result it will water log and will suffer a drop in performance.
Thus in case of a closed system, mixing high pressure and low pressure condensate increases the back pressure on the lowest pressure header, reducing effective condensate evacuation.
Consider a closed system where two equipments requiring the same steam flow rate but at different pressures (0.5 bar and 3 bar respectively.) The condensate is returned via a common line and the pressure maintained in the flash vessel is 0.5 bar. Thus now the net differential pressure across the steam trap on the first equipment is zero. Therefore the trap will not discharge leading to condensate logging.