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Analysis of the normal coasting time of turbines and its operational significance: As core power equipment in modern thermal power generation, gas power generation, and industrial drive systems, the safe, stable, and efficient operation of turbines is directly related to the economic efficiency and reliability of the entire power plant. In the operation and management of steam turbines, the \"coasting time\" during shutdown is an important parameter for condition monitoring. It not only reflects the mechanical characteristics of the unit but also serves as a key basis for assessing the health status of the equipment and the compliance of operating procedures. However, there is no fixed standard value for the normal coasting time of turbines; its duration is influenced by various factors, and a comprehensive assessment must be made taking into account the specific type of turbine, operating conditions, and historical data. I. Basic concepts and typical ranges of turbine coasting time. Coasting time refers to the time it takes for a turbine to continue rotating due to its own rotational inertia after the main steam supply is cut off, until it finally comes to a complete stop. This process is essentially one in which the rotor’s kinetic energy is gradually exhausted by frictional forces, blowing losses, and other factors. Due to differences in capacity, structure, materials, rotor mass, and other aspects among different units, their coasting times vary significantly. Generally, the approximate range for the coasting time is as follows: ● Medium and small-sized units (such as industrial drives or small CHP units): The normal coasting time is about 15 to 40 minutes ； ● Large generator sets (such as supercritical and ultra-supercritical units in the 300MW to 1000MW range): the coasting time is usually between 40 and 90 minutes, with some units having high inertia requiring even longer times. For example, the 9F-class gas-steam combined cycle units operated by Jiangsu Datang International Jintan Thermal Power Co., Ltd., thanks to their advanced design and high-efficiency turbines, have a large rotor inertia; as a result, their coasting time remains stable and long under standard operating conditions, indicating good equipment condition and system sealing. The 660,000-kilowatt ultra-supercritical coal-fired unit at Hubei Huadian Jiangling Power Plant features a design thermal efficiency as high as 46.34%, boasts a high level of system optimization, and exhibits excellent stability in its idle operation characteristics. II. Main factors affecting coasting time 1. Rotor inertia (GD²): This is the key physical parameter that determines the coasting time. The larger the rotor’s mass and diameter, the greater its moment of inertia, which gives it a stronger ability to maintain rotation; as a result, the coasting time is prolonged. Large-capacity units generally have higher GD² values. 2. Vacuum level control: Whether the condenser vacuum is maintained during shutdown has a significant impact on the coast-down time. Maintaining a vacuum during the high-speed phase reduces the blowby friction caused by air entering the cylinder, thereby extending the coasting time ； If the vacuum is broken too early, the coasting time will be significantly reduced. 3. Friction and energy loss: ○ Bearing oil film friction ； ○ Blowing friction between the impeller and residual steam (proportional to the cube of rotational speed) ； ○ Are there any slight frictions or sticking issues with the internal moving parts? 4. Operational standardization: Whether the steam supply to the shaft seal is stopped in a timely manner ； ○ Ensure thorough drainage from the main steam and reheat steam pipes to prevent backflow of steam into the cylinders ； ○ Ensure that the steam inlet valve is tightly closed to prevent steam leakage and resulting work, which could cause abnormal prolongation of the idling time. 5. Equipment health status: Issues such as sliding between moving and stationary parts inside the turbine, bearing wear, and blade fouling can all lead to a reduction in the coasting time or to abnormal bends in the curve. III. The key to determining “normality”: It is the coasting curve, rather than a single time value, that truly serves as the core criterion for assessing whether coasting is “normal”. It is not a specific time figure, but rather the shape of the coasting curve when compared to historical baseline curves. During each normal shutdown, the variation of speed over time should be recorded to draw a standard coastdown curve, which serves as the \"operational fingerprint\" of that unit. ● Characteristics of a normal curve: smooth, continuous, with an exponential decay trend, and without any abrupt changes or plateaus. ● Anomaly detection: ○ Significantly reduced idling time: may indicate mechanical friction inside, bearing failure, or premature vacuum loss ； ○ Abnormally prolonged idle time: This is relatively rare, but it may indicate that the main steam valve or control valve is not closing properly, allowing steam to leak into the cylinder and continue to do work ； ○ A bend or sharp drop in the curve may correspond to a certain critical speed range, or it may indicate imbalance or friction in a specific component such as the generator rotor. Therefore, operators should compare the measured coasting curve with the standard curve after each shutdown; if a deviation of more than ±20% is detected or if the shape of the curve is abnormal, a technical analysis must be conducted to identify any potential issues. IV. Three stages of the typical coastdown process: Taking large-scale generator sets as an example, the coastdown process can be divided into the following three stages: 1. High-speed stage (from rated speed to approximately 1500–2000 r/min): ○ Characteristics: The speed drops most rapidly, primarily due to the friction caused by the impeller’s airflow ； ○ Key operating points: Maintain vacuum and closely monitor vibration and noise. 2. Medium-speed stage: ○ Characteristics: Blowing loss decreases; bearing friction and air resistance become the main resistances, and the decline in speed is gradual ； ○ Key operating points: According to the procedures, the vacuum break valve can be opened when the rotation speed drops to around 1500 r/min. 3. Low-speed stage: ○ Characteristics: The speed decreases extremely slowly, mainly to overcome the mechanical friction of the bearings ； ○ Endpoint: Once the rotational speed returns to zero, start continuous turning of the rotor immediately to prevent thermal bending of the rotor due to temperature differences between the upper and lower cylinders. V. Practical Significance and Operational Recommendations for Idle Operation Management 1. Establish a standard database: For each unit, its standard idle operation curve should be determined and stored after the first startup and commissioning or after major repairs, to serve as a benchmark for subsequent operations. 2. Strengthen operational standardization: Strictly follow the operating procedures to ensure consistency in each shutdown procedure, thereby avoiding fluctuations in the idling characteristics due to variations in human operation. 3. As a condition diagnosis tool: The idle operation period is regarded as the “stethoscopy phase” of the turbine, and by combining parameters such as vibration, temperature, and sound, an overall assessment of its internal condition can be made. 4. Support for equipment management and technological upgrades: For example, Hebei Huadian Shijiazhuang Thermal Power Co., Ltd. achieved energy savings and reduced consumption by optimizing the start-up and shutdown procedures for its gas-fired units ； Jiangsu Datang Jintan Thermal Power has improved the efficiency of its turbines by addressing the problem of vibration, resulting in a 3.1% increase in cylinder efficiency and a reduction in startup time by 1.5 hours; these improvements are also reflected in more stable and longer idle operation periods. 5. Incorporation into the operational training system: Turbine operation supervisors should master the basic skills of coasting analysis, understand the physical mechanisms and engineering implications behind it, and improve their ability to predict accidents and handle emergencies. VI. Conclusion In summary, the “normal coasting time” of a turbine is not a static value, but rather a dynamic, comparable technical parameter that requires continuous monitoring. It is both a reflection of the mechanical characteristics of the unit and an indication of the level of operational management. Against the backdrop of current power systems striving for high reliability, high efficiency, and low carbon emissions, meticulous management of the idle operation process not only helps ensure the safety of units but also provides strong support for equipment condition assessment, energy-saving optimization, and fault prediction. Only by adhering to standardized operations, scientific analysis, and systematic management can the safe and economical operation of turbines throughout their entire life cycle be truly achieved.