Why is it important to avoid reaching critical speed in a steam turbine?

• A. Turbine will vibrate violently
• B. Turbine will accelerate uncontrollably
• C. Turbine will stall
• D. Turbine will overheat

Answer: (A)

When the rotor’s natural frequency matches the shaft’s rotational speed, resonance occurs. At that critical speed, even small imbalances can produce very large vibration amplitudes. This violent vibration drives high dynamic loads into the bearings, shaft, and blades, leading to fatigue, wear, contact, and potential catastrophic failure if components rub or break. So, avoiding crossing or sitting at this speed helps keep the machine stable, reduces the risk of damage, and preserves safe, reliable operation. Reaching critical speed isn’t mainly about accelerating uncontrollably, stalling, or overheating—the danger lies in the excessive vibration from resonance.