7.2.2 Pressure control
During start-up and shutdown, when live steam is used, the pressure at the glands is controlled by the pressure reducing valve in the live steam supply line. As load increases, the steam flow to the HP/IP glands progressively reduces and eventually reverses, as leak-off steam becomes available.
Consequently, the rise in pressure at the glands causes the live steam pressure regulating valve to close progressively, thus maintaining a constant supply pressure at the glands and eventually closing completely. The LP glands are now sealed by steam leaking from the HP and IP glands, and the pressure at the glands is controlled by a leak-off valve which dumps steam to an LP heater.
This arrangement ensures that only one regulating valve is in control at any one time and that the changeover from live steam to leak-off steam is fully automatic.
The steam pressure in the sealing line is indicated in the control room and locally; a fault condition is indicated by low pressure alarms.