Discussion on the water injection cooling device for steam turbines

During the startup of the turbine, as well as at no-load and low-load conditions, the steam flow rate is very low; it is not sufficient to remove the heat generated by internal frictional blowing in the low-pressure cylinder. This leads to an increase in the exhaust temperature, and consequently, the temperature of the exhaust cylinder also rises. High temperatures in the exhaust cylinder can cause significant deformation of the cylinder, disrupting the alignment of the moving and stationary parts of the turbine. This affects the turbine’s relative expansion as well as the axial and radial clearances between the moving and stationary components; in severe cases, it can lead to vibration in the unit and other accidents. Therefore, to prevent the exhaust cylinder temperature from rising too high, some turbines are equipped with low-load water injection cooling systems on the exhaust cylinders.

I wonder about the idea of using water spray for cooling… Is this related to online impeller cleaning? I wonder how the equipment manufacturer will explain it.

This mainly takes into account the warm-up phase of the turbine: during this stage, less steam enters the turbine, resulting in poor fluidity; friction occurs between the last-stage impeller and the steam, which causes the exhaust temperature to rise continuously. To prevent the last stage of the turbine from overheating, water is sprayed to cool it down

So why are some turbines equipped with an exhaust spray system while others are not? This is determined by the size of the turbine; the larger the turbine, the larger the rotor volume, and thus the longer the warming-up time. As a result, the exhaust temperature rises more, which necessitates the use of an end-stage spray system. With small steam turbines, the warm-up time is short; acceleration begins before the exhaust temperature rises, so there is no need to install a final-stage spray system.

During the startup of the turbine, due to the low amount of steam used, the steam does not fully convert all of its enthalpy into work; as a result, the enthalpy value is higher at the exit pressure. This makes it easy for the exhaust gas to overheat. The surface cooler is unable to completely condense the steam, which leads to an increase in vacuum level. Eventually, this may prevent the turbine from rotating, causing it to stop.

As far as I can see, in typical fully-condensing or steam-extraction type low-pressure cylinders, a spraying device is installed at the rear section. Due to the fully-condensing design of the exhaust cylinder, the pressure is low (vacuum), and the material used for the cylinder body cannot withstand high temperatures. To prevent the cylinder body from deforming due to excessive temperatures in the exhaust cylinder during warm-up, a spraying device is used to cool it down. Back-pressure turbines do not have this device.