14.5 Gland sealing - part 1
Glands on early advanced class feed pumps were of the labyrinth type, with fixed bushes and leak-in/ leak-out connections (Fig 4.60).
Consideration was given to the use of floating ring glands as these would be more efficient than the fixed bush type when new. They were, however, rejected as being vulnerable to loss of water supply, they would not comply with the specified dry running requirement. Under dry running conditions, the fixed bushes operate satisfactorily without an external supply of sealing water, and no significant erosion damage will occur when accepting flashing flow for short periods of time.
A secure supply of gland injection water for the labyrinth seals can be provided in one of two ways. In the first, feedwater is tapped from the discharge of the suction stage pumps and passed through a cooler, where it is cooled by condensate. In the second, the low temperature gland injection water is taken from the condenser extraction pump discharge and fed direct to the feed pump glands. The injection pressure would be controlled at approximately one bar above the gland unloading pressure. To cater for start-up conditions and interruptions in the supply from the condenser extraction pump, a back-up supply is available from a standby injection pump which takes its source of water from the reserve feedwater tanks.