The steam turbine is one of the three main engines of thermal power plants: boilers, steam turbines, and generators. The main body is composed of a rotating part and a fixed part. The rotating part is composed of impeller, main shaft, coupling, and blades; the fixed part is composed of nozzles, cylinders, steam seals, partitions, fasteners, bearings, etc.
In the process of thermal power generation, the steam turbine converts thermal energy into mechanical energy. The steam turbine directly affects the efficiency of the power plant and the unit’s safety. When the steam turbine generator set is in operation, various impurities will be carried when the steam enters the steam turbine, resulting in fouling and corrosion. Primarily the last stage blades operate under extremely harsh conditions and are exposed to the combined effects of high temperature, high pressure, centrifugal force, steam force, steam exciting force, corrosion and vibration, and water droplet erosion in the wet steam zone, and face more severe damage risk. Moreover, as the load increases, the corrosion and salt accumulation of the steam turbine blades will become more serious, and the safe operation and work efficiency of the unit will also be affected more significantly.
The acid corrosion of steam turbine is caused by the acidic substances in the steam between the vapor-liquid phases in the initial condensation zone of the turbine low-pressure cylinder, mainly in the vapor phase, while the acidic substances are concentrated in the initial condensation water, which reduces the pH value, resulting in comparison with acidic substances Corrosion occurs on sensitive cast iron and steel parts.
After the high-pressure cylinder performs work, the main steam enters the low-pressure cylinder. Due to the expansion of the cylinder volume and the decrease in temperature and pressure, the main steam may become saturated steam, or even wet steam, and finally form condensed water, thereby causing impurities in the steam Redistribution in this area will increase the acidity of the condensate. These acidic condensed water will hit the inner wall with the movement of the leaves, which will cause acid corrosion. At this time, if there is a lot of air in the cylinder, such as air leakage into the steam turbine, excessive dissolved oxygen content, etc., this corrosion will be aggravated.
Oxygen corrosion is generally related to temperature, pH, dissolved oxygen, load, and flow rate. The most critical factors are dissolved oxygen and pH. A protective film is formed on the surface of the blade. The main component of this protective film is iron oxide. When this protective film is damaged due to various factors, a local battery will be formed between the water contacted by the surface and the surface of the protective film. The iron is precipitated from the anode to create corrosion.
When a steam turbine operates under low load conditions, as the unit power increases, the expansion angle of the flow path will gradually increase. When its volumetric flow rate decreases sharply relative to the volumetric flow rate under optimal design conditions, it will cause the parameters of the entire flow field to change a lot. The lower the load, the larger the recirculation zone. At the beginning of the start, the scope of the return flow will be expanded and even expanded to the entire exhaust cylinder. Moreover, for high-power steam turbines, the exhaust steam humidity of its final stage blades is generally relatively high. Therefore, after the final stage blades, the steam flow carries liquid water droplets to impact the blades and finally form water erosion.
In many power plants, the cobalt alloy strips(also called cobalt alloy plates) are induction brazed to SUS410Cb steam turbine blades to improve blades’ wear and erosion resistance.
