【2026 Transmission Equipment】Negative temperature difference startup of steam turbines and how to avoid it

II. Why should we try to avoid it as much as possible? The rotor and cylinders cool down rapidly, generating significant thermal stress. When cold steam comes into contact with hot metal, the metal contracts sharply, which leads to substantial thermal shock and tensile stress, thereby reducing the rotor’s lifespan.

The differential expansion shifts sharply in the negative direction (negative differential expansion): the rotor cools faster than the cylinder → the rotor contracts more rapidly → the value of the differential expansion becomes excessively negative → in severe cases, this leads to significant static and dynamic friction as well as increased vibration in the unit.

Insufficient steam superheat makes it easy for water to be carried in the steam, resulting in low temperature and high humidity due to water hammer → water in the steam → water entering the turbine → blade erosion, vibration, and shaft bending.

III. How to avoid negative temperature difference start-up? (Standard procedure) Strict control before starting up: The main steam temperature must be at least 50°C higher than the highest metal temperature of the cylinder; this is a mandatory requirement specified in the regulations. Ensure that the superheat of main steam and reheat steam is ≥56°C to prevent water carryover and wet steam erosion. After ignition, the temperature is raised first, followed by an increase in pressure; then rotation is initiated to raise the steam temperature, and rotation is started only after a positive temperature difference is achieved. Starting in hot/extra-hot conditions requires strict adherence to temperature matching; low-temperature steam must not be fed into a hot turbine. Ensure thorough drainage before starting up to prevent water accumulation at low temperatures from entering the cylinder.

For reference: Under what conditions does negative temperature difference start-up generally occur?       Negative temperature difference start-up generally occurs when the unit needs to be started in a very hot or hot condition. This happens when the unit is shut down for a short period to address defects; at that time, the temperature of the turbine cylinders remains high, while the temperature of the main steam drops rapidly due to the boiler going out of service. At the beginning of ignition, the difference between the temperature of the main steam and that of the cylinders is negative, and it is under these conditions that negative temperature difference start-up occurs. Since the unit needs to resume operation quickly and the parameters of the main steam remain constant, it becomes possible to use negative temperature difference starting in such cases.

Parameters for sudden startup in the four startup states of turbines and the criteria for them: 1. The fundamental basis for classifying startup states. The classification of turbines into cold, warm, hot, and very hot states is based primarily and directly on the metal temperature of the inner wall of the first stage (regulating stage) of the high-pressure cylinder before startup; the shutdown time serves only as an empirical reference value. The insulation performance of different units, ambient temperature, and shutdown methods all significantly affect the rate at which metal temperatures drop; therefore, it is absolutely impossible to determine the startup status based solely on the shutdown time.

Common classification standards for mainstream domestic turbines (with slight variations among manufacturers): – Cold state: Metal temperature of the control stage < 150–200°C (downtime is usually > 72 hours) – Warm state: Metal temperature of the control stage 150–370°C (downtime is usually 10–72 hours) – Hot state: Metal temperature of the control stage 370–450°C (downtime is usually 1–10 hours) – Very hot state: Metal temperature of the control stage > 450°C (downtime is usually < 1 hour)

II. Detailed basis and principles for determining key parameters 1. Main steam temperature: The primary factor in controlling thermal stress. This is the parameter with the highest priority among all those related to startup, as it directly determines the safety of unit operation and its service life.

Core principle: Startup must occur under a strict positive temperature difference; startup under a negative temperature difference is absolutely prohibited. The main steam temperature must be 50–100°C higher than the metal temperature of the control stage. The superheat of the steam must be ≥50°C (to prevent water from being carried in with the steam after expansion, which could cause water hammer and blade erosion). The maximum allowable temperature difference must not exceed 150°C, as exceeding this value will result in irreversible plastic deformation

Temperature selection logic for different states: – Cold state: The metal temperature is low, allowing for a larger initial temperature difference; however, the rate of temperature increase must be strictly controlled at 1–1.5°C/minute. – Hot state/Very hot state: The metal temperature is high, so the initial temperature difference must be reduced to 30–50°C, as the allowable stress of the material decreases significantly at high temperatures, resulting in greater thermal stresses for the same temperature difference