What is critical speed in turbine?

Critical speed of the turbine is the rotor speed at which natural frequency of the assembled rotor (rotor shaft with discs, blades, shrouding strips etc in assembled condition) becomes equal to the operating speed. This is usually a expressed as a range (critical speed range).

What does occur at critical speed?

The critical speed is the theoretical angular velocity which excites the natural frequency of a rotating object, such as a shaft. As the speed of rotation approaches the objects natural frequency, the object begins to resonate which dramatically increases systemic vibration.

What are the causes of critical speed?

Critical speed of shafts All rotating shafts, even in the absence of external load, will deflect during rotation. The unbalanced mass of the rotating object causes deflection that will create resonant vibration at certain speeds, known as the critical speeds.

How do you find critical speed?

The critical speed is the same as the frequency of traverse vibrations. Where m = the mass of the shaft assumed concentrated at single point . The centrifugal force on the shaft = m ω 2(y + e) and the inward pull exerted by the shaft = y (48EI / L 3).

What is second critical speed of turbine?

A turbine may have more then one critical speed, which may depend upon number of couplings. A second critical speed is when the Turbine blade tips approach the speed of sound. This effectively limits the speed of a turbine and explains why power plants tend to have turbines of the same capacity.

What is meant by the term critical speed and why it can be a problem on ships?

Critical speed is the speed at which unwanted vibration happens when a vessel is rotating. The crushing of particles in a traditional mill is generally accomplished by rotating the mill near the critical speed. Running a mixer in excess of the critical speed can cause destructive mechanical vibrations.

What are critical resistance and critical speed explain these with the help of the open circuit characteristics of the generator and voltage build up process?

Critical speed is defined as the speed at which the given shunt field resistance is equal to the critical resistance. It is the speed at which the shunt generator just fails to build up its voltage without any external resistance in the field circuit. It is denoted by Nc.

What are the significance of critical resistance on build up process?

The shunt generator will build up voltage only if field circuit resistance is less than critical field resistance. It is a tangent to the open-circuit characteristics of the generator (at a given speed).

Why the critical speed of propeller shaft is important?

The bending depends upon rotational speed & length of propeller shaft. The speed at which shaft run with additional deflection of the shaft from axis of rotation became infinite, known as critical or whirling speed. Hence, the operating speed of shaft is less than critical speed to avoid failure & violent vibrations.

How does critical speed lead to engine failure?

The critical speed of a shaft occurs when the shaft rotational speed is at or close to resonant conditions. In this condition the torsional vibration of the shaft increases greatly, and will impose very high shear stress on the crankshaft. These levels of stress could even cause crankshaft failure.

What is meant by critical speed of a 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.

Why do wind turbines need to be shut down?

Turbines are generally designed to operate at only one speed and the blade angles and direction are adjusted to maximize the power output at this rotational speed. If the wind speed is to great for the power of the generator to keep it at this speed, it must be shut down to protect the turbine (the cut-out speed).

What happens to a steam turbine when it reaches thermal equilibrium?

Once the thermal equilibrium is attained, the turning gear is disengaged and the turbine is allowed to reach its working speed with the help of steam flow. During shut down of the turbine, it is never brought to standstill condition instantaneously. The speed of the turbine is slowed down to a certain level by reducing steam flow through it.

What happens during the cold start up of a turbine?

During cold start up of a turbine, steam is not let into the turbine with full pressure and flow. Instead, steam is let in at a relatively small flow rate so as to heat up the turbine parts and attain thermal equilibrium. For facilitating uniform heating of turbine rotor, it is rotated at a slow but constant speed using a Turning gear.