Banking refers to a condition of readiness of a boiler to deliver steam almost immediately on demand.
A cold boiler requires considerable amount of time with controlled heating to reach its full pressure depending on the size and construction. The amount of refractory in a boiler decides as to how fast it can be heated or cooled. At the time of shutting down, if a boiler is boxed up at full p & t and it is cooling naturally by itself, the boiler can return to service usually in <2 h even after a day. When there is a process interruption or ST trip the boiler is boxed up and periodically fired to raise the pressure, so that it can be loaded immediately when the fault is set right. This free floating hot standby condition is called the banking of boiler.