What is Critical Speed of Turbine ? What are steps to avoid it during Rolling of turbine ?

Turbine shaft material has its own natural frequency, when 

turbine rotates on such a speed that frequency of shaft 

become close to its natural frequency, machine causes noise 

& high vibrations because of resonance due to matching of 

frequency. Running of Steam "TURBINE" on this speed is 

avoided & this is called Critical speed. A turbine may have 

more then one critical speed, which may depend upon number 

of couplings.