in CCPP power plant, in triple pressure boiler, we control LP drum pressure, some times IP drum pressure, but not HP drum pressure.
whats is reason behind this?
The HP drum pressure is limited, rather than controlled, by the HP bypass pressure control and ultimately by the drum safety valves. Any minimum pressure control is via the steam turbine inlet pressure control, which usually just lets the steam turbine use all the steam available.
The pressure control for the IP and LP is to make sure they maintain a minimum pressure. The source pressure for this is usually the HP drum for the IP and the IP drum for the LP.
