Turbine Trip

Because someone or something has caused/ignored the warning given by the previous Low Level alarm. "Low Low" logic means that something has gone grossly wrong and continued operation in this state places the process in imminent danger/failure. In the case of a LP heater (Low Pressure heat exchanger) the process fluid level has fallen below the design minimum leading to possibly running the boiler dry. Most likely caused by a failed feedwater pump or valve, piping failure or internal failure of the heater itself.

Contact the boiler/turbine manufacturer for more info since control/alarm philosophy varies greatly according to location, client, cost, etc.

sir there is many reason for level low but what it will be effect on a turbine

Please correct me if I'm wrong but you seem to be asking "why trip the turbine if the quality of the steam supply is ready to fail?" Do you want droplets of water impinging on your turbine blades? Or perhaps you want the turbine to stay online as the steam supply diminishes to the point where the generator becomes a motor and drives the turbine?

It's a system, if something in the system is in imminent danger of failing then all components of the system should be safely shut down and the cause of the alarm(s) investigated

PS-the turbine would have tripped anyway when the boiler shutdown on low feedwater.