Reasons for excessively high exhaust temperature due to prolonged low-speed warm-up time during the turbine startup process

I would like to ask the original poster: how long is considered an excessive time for low-speed engine warming up? To what level can the exhaust temperature rise if this time is too long, and what are the potential hazards?

1) The time required for low-speed warming up is determined based on the turbine’s speed-up curve; it is generally between 30 and 45 minutes. Another factor to consider is the exhaust temperature. If enough time has passed and the exhaust temperature has reached the specified value, then the process can proceed further; if only about 20 minutes have passed and the exhaust temperature already meets the specified level, it’s okay to continue warming up for a while longer. 2) The exhaust temperature must not exceed 120 degrees Celsius, as it is easy for this figure to be surpassed if the warming up process takes too long. 3): The hazard is uneven heating of the rotor, resulting in elastic deformation.

Are you referring to warming up the turbine when it is started up? You have already explained the reasons for the high exhaust temperature: the warm-up time at low speeds is too long; it should be better to reduce the opening of the main steam valve during warm-up.

I know that too long a warm-up time at low speeds can lead to excessively high exhaust temperatures, but why is that? I don’t know the specific reasons for this, so I hope everyone can explain it to me. Thank you.

It is mainly frictional blowdown loss. During low-speed warm-up, the first few stages perform more work while the later stages hardly do any work. At the same time, during low-speed warm-up, the vacuum level is not high, resulting in significant frictional blowing losses that cause the exhaust temperature to rise. After the exhaust temperature rises, during acceleration, as the subsequent stages begin to contribute to power generation, the exhaust temperature drops significantly, which can easily lead to deformation of the cylinder block and the formation of cracks.

If possible, please explain friction blowdown loss in detail. Thank you.

It is because the load during warm-up is too low, preventing the turbine steam from performing sufficient work for energy conversion. As for the hazards, the previous sea friends have already explained them clearly. Specific solution: 1. Find a way to reduce the steam temperature or pressure while ensuring the steam superheat remains intact ； 2 Increase the compressor load, so that the compressor operates under as high a load as possible during warming up ；

I think as long as one follows the specifications of the acceleration curve, it’s possible to avoid excessively high exhaust temperatures; just make sure the time taken isn’t too long.

After the turbine rotor is started, a warming-up and speed-raising process is required before the speed reaches the rated value. The purpose of warming up is also to ensure even heating of the turbine components, reduce temperature differences, and prevent excessive thermal deformation and thermal stress. The warm-up time and warm-up speed vary depending on the random group parameters, power, and structure. The low-speed warm-up time for medium-pressure units is approximately 20-30 minutes ； High-parameter units take longer, about 1-2 hours. In addition to reducing thermal deformation and thermal stress, the purpose of low-speed warm-up is mainly to provide operators with an opportunity to thoroughly inspect the performance of the unit after it has been started up. The low-speed warm-up time should not be too long, for the following reasons: 1. Since the turbine has no load, the amount of steam flowing into it is very small; as a result, the heat from the exhaust cylinder cannot be completely carried away by the steam, preventing the exhaust chamber from being adequately cooled, which in turn causes the exhaust temperature to rise. 2. Due to the small opening degree of the low-speed control valve, the fresh steam is severely throttled. According to the principles of throttling and the properties of steam, the enthalpy of the steam remains unchanged after throttling while its pressure decreases; as a result, the expansion line corresponding to the decrease in steam enthalpy shifts to the right on the enthalpy-entropy diagram ； With a constant exhaust pressure, the end temperature increases, possibly reaching the superheated region; as a result, the exhaust temperature rises. 3. During startup, the vacuum level is generally low while the exhaust pressure is high; as a result, the exhaust temperature also increases. 4. The size increases only near the last stage blades, resulting in blowdown friction losses that heat the steam and raise the exhaust temperature. Hazard: Excessively high exhaust temperature can cause uneven heating and expansion of the cylinders, leading to a shift in the unit’s center and resulting in vibrations. It can also cause cracks to appear in the flanges of the condenser copper tubes, allowing cooling water to leak out.