An improved package yarn dyeing system adapted for operation above the boiling point including a dye chemical add tank receiving a predetermined amount of diluting bath liquid before it is siphoned into an expansion tank and fed from there to the pump inlet feeding a kier through a heat exchanger. Further bath dilution and line washing as well are achieved by an add tank bypass line through which bath liquid is circulated. The add tank fill line emanates from the pump outlet and the kier dome is tapped by an additional expansion tank feed line to enhance and speed up dilution uniformity. The part of the equipment for circulating the dye back and forth through the packages may be isolated from the rest of the system and is protected against pressure damage by an expansion chamber containing a small volume of entrapped air.