Why does the exhaust temperature of the turbine increase during warm-up, and what should be done if it becomes too high?

During warm-up, the steam seal of the turbine has not yet been established, which allows a large amount of air from the atmosphere to leak into the low-pressure cylinder of the turbine and then be discharged into the surface cooler. An excess of non-condensable gas in the surface cooler leads to a decrease in its vacuum level; as a result, the exhaust pressure rises, and consequently the exhaust temperature also increases. Once the turbine is properly started up and its steam seal is functioning properly, the amount of air leaking in from the atmosphere is reduced, as is the amount of non-condensable gas. This allows the vacuum level in the surface cooler to be maintained, resulting in lower exhaust pressure and, accordingly, a lower exhaust temperature. Generally, during warm-up, the bypass of the manual steam seal steam self-acting control valve is opened, or steam is introduced from the steam pipeline system to supply the steam seal steam.

The turbine load is low, resulting in less steam work, a reduced enthalpy drop, as well as blowdown friction. Generally, it’s fine at around 120°C

The low-speed warm-up time should not be too long, otherwise it will cause the temperature in the exhaust section to become too high, leading to increased thermal expansion.

It’s because the turbine load is low, resulting in less work done by the steam, a reduced enthalpy drop, plus the friction from forced air circulation

This statement is incorrect; a steam seal must be established before warming up to prevent air from leaking in.

Hehe, what you call establishing a steam seal before warming up is actually false; it’s just a temporary solution. What’s done is to introduce steam from the outside (as opposed to using the steam generated by the turbine itself), but this doesn’t work very well – a lot of air still gets in. The books talk about establishing a steam seal, but in reality, when you look at it on site, that piece of paper or cloth placed at the end of the low-pressure cylinder simply floats in whatever direction the wind blows. Blowing friction does have an impact; as far as the operation of the several turbines in our plant is concerned, it prevents them from operating below their minimum continuous speed, and when they run at that minimum speed, the vacuum level is very good.

Reason: ① During warm-up, the steam flow rate entering the turbine is low, resulting in a low rotational speed; the enthalpy drop of the steam within each stage of the moving blades is not as large as under normal conditions, and the steam still has a considerable degree of superheat by the time it reaches the last stage； ②The steam flow velocity is low, and the size of the final-stage blades is large; as a result of the stirring action of these blades, frictional blowdown losses and reheating occur. The heat generated by these losses in turn heats the steam. The treatment method is ① maintaining an appropriate vacuum level ； ②Control the appropriate warm-up time.

The reasons for the increase in exhaust temperature are: (1) a decrease in vacuum inside the condenser, with air not being completely removed, resulting in a mixture of steam and air. The poor thermal conductivity of air leads to an increase in exhaust pressure, as well as a higher saturation temperature. ⑵ A large amount of drain water from components such as the main steam pipes, reheat steam pipes, and the cylinder itself is directed to the expansion tank; the steam emerging from this expansion tank is then sent to the throat section of the condenser. The temperature of this drain water and steam is 4 to 5 times higher than the saturated temperature inside the condenser. ⑶ During the warm-up process, the steam flow is low and the flow velocity is slow, so the heat generated by friction in the blades cannot be removed in a timely manner.

Answer: ① During warm-up, the steam flow rate entering the turbine is low and its speed is low; therefore, the enthalpy drop of the steam within each stage of the rotor blades is not as large as under normal conditions. By the time it reaches the last stage, the steam still has a considerable degree of superheat； ②The steam flow velocity is low, and the size of the final-stage blades is large; as a result of the stirring action of these blades, frictional blowdown losses and reheating occur. The heat generated by these losses in turn heats the steam. The treatment method is ① maintaining an appropriate vacuum level ； ②Control the appropriate warm-up time.

Support the view from floor 8; disagree with that from floor 2.