What are the impacts of unstable steam temperature on the safe and economical operation of steam turbines?

Excessively high temperatures can shorten the service life of the superheaters and turbines; in cases of severe overheating, it can even lead to the bursting of the superheater tubes. Excessively low temperatures, increased steam humidity in the last stages of the turbine, as well as the corrosive effect of water droplets on the blades, lead to uneven cooling of the turbine components when temperatures drop, which in turn causes wear and vibration in the turbine. A low steam temperature reduces the working capacity of the steam, leading to increased steam and heat consumption, which in turn lowers the economic efficiency of power generation by ±5°C.

An increase in the temperature of the new steam can improve the economic efficiency of the unit; this is particularly advantageous from an economic perspective when the steam pressure also increases accordingly. However, when the temperature of the new steam rises above the allowable value, the mechanical strength of certain components of the turbine unit decreases, such as pipes, valves, cylinders, flanges, bolts, nozzles, blades, etc. These components may experience a reduced lifespan or get damaged. The decrease in the temperature of the new steam reduces the economic efficiency of the unit, mainly due to increased steam consumption. As the temperature of the fresh steam decreases, the total enthalpy drop in the turbine is reduced, resulting in a lower load. To maintain the rated load, it is necessary to increase the flow rate of fresh steam. For the regulation stage, as the enthalpy drop across the stage decreases, the stress on the blades is reduced. For the first few pressure stages near the regulating stage, this is due to the decrease in steam temperature. This leads to a decrease in the metal temperature, thereby increasing its allowable stress. However, for the last stage of the turbine, it may be possible that increasing the steam inflow to maintain the rated load causes overloading of the blades; at the same time, low temperatures lead to an increase in humidity, thereby increasing humidity losses. and leads to erosion of the blades. As the temperature of the fresh steam decreases, the enthalpy drop across each stage of the turbine decreases, except for the last stage; as a result, the reaction degree increases and the axial thrust on the rotor grows, which is not conducive to the safe operation of the unit. In such cases, it is possible to reduce the pressure of the fresh steam accordingly in order to lower the load. In this regard, particular attention is drawn to the sharp drop in the temperature of the new steam, which causes serious accidents such as water hammer in the turbine.