Turbine critical speed

You’ve got it backwards. When the vacuum level is higher than the critical value, more steam can enter, which accelerates the turbine’s transition to the supercritical state. In fact, a high exhaust pressure in the turbine hinders this transition (the higher the exhaust pressure, the lower the vacuum level), which affects the amount of steam that can enter the turbine. When the amount of steam entering decreases, the turbine cannot accelerate as quickly as needed; there isn’t enough steam to meet the requirements for rapid acceleration, resulting in a decrease in the turbine’s acceleration rate or even an inability to increase its speed. In severe cases, the turbine cannot reach the supercritical state quickly, leading to increased vibration in the turbine’s or compressor’s (or generator’s) bearing shells, and this can trigger an automatic shutdown of the equipment.