44. What effects does an excessively high or low steam inlet pressure have on the operation of a turbine?

Answer: 1. When the main steam pressure increases, the total useful enthalpy drop increases as well, and the working capacity of the steam improves. Therefore, if the load remains unchanged, the steam flow rate can be reduced, which is beneficial for the efficient operation of the unit. However, the steam humidity will increase in the last few stages, which is particularly detrimental to the operation of the final stage blades. For the regulating stage, the most hazardous operating condition occurs when the first regulating valve is fully open; at this time the initial pressure increases, resulting in an increase in both the enthalpy drop and flow rate in the regulating stage, which is unfavorable for it. However, when operating at rated load, the enthalpy drop in the regulating stage is not at its maximum, so the risk is generally low. If the main steam pressure increases without exceeding the limits, the unit operates at its rated load; as long as the humidity of the exhaust steam from the last stage remains within the allowable range, the control stages can be considered to be safe. However, the main steam pressure cannot be increased arbitrarily. Excessively high main steam pressure leads to a large enthalpy drop in the regulation stages, which can damage the nozzles and blades over time. Additionally, when the main steam pressure rises beyond acceptable levels, the steam humidity at the last few stages of the turbine increases, resulting in erosion of those blades. An excessive increase in the new steam pressure can also lead to increased stress on pressure-bearing components such as steam ducts, steam chambers, and valves, posing a certain threat to the safe operation of the unit. 2. When the steam pressure decreases, if the temperature of the new steam and other operating conditions remain unchanged, a decrease in steam pressure leads to a decrease in load. If the load remains unchanged, the steam flow increases. When the steam pressure decreases, the enthalpy drop in the regulating stage reduces while the reaction degree increases; meanwhile, the enthalpy drop in the final stage increases and the reaction degree decreases. There is no significant change, or only a minor change, in the overall axial thrust of the unit. A decrease in steam pressure leads to an increase in the steam consumption of the unit, thereby reducing its efficiency. When the steam pressure drops significantly, in order to maintain the rated load, the flow rate exceeds the capacity of the final stage, which results in increased blade stress and axial thrust; therefore, the load should be limited.

Turbines are designed with the rated main steam pressure in mind when determining the strength of their various components; therefore, when the main steam pressure exceeds this rated value, it causes overload in the main steam pipes, as well as in the valves and steam chambers and blades of the control valves, which can even lead to damage to these components. Furthermore, when the steam pressure exceeds the rated value, it increases the operating temperature of the steam in the last stages of the turbine, thereby worsening the operating conditions of the blades in those final stages. When the steam pressure is below the design value, the efficiency of the turbine decreases; more steam is required at the same load, which leads to an increase in axial thrust. At the same time, it increases the stress on the subsequent blades, and in severe cases, it can cause the blades to deform. Furthermore, when the steam pressure is too low, the nozzle becomes clogged, preventing the turbine’s power from reaching the rated value.