Damage and treatment of the last stage blades of low pressure steam turbine

1. Characteristics of the working environment of turbine blades

The working environment of steam turbine blades is very complex and harsh. Specifically, they can be divided into three parts: high, medium and low pressure sections. Compared with the blades in the high and medium pressure sections, the working conditions of the last blades in the low pressure section of the low pressure steam turbine have the following characteristics: the steam pressure at the last stage of the low pressure section is lower than the atmospheric pressure, the steam volume flow rate is significantly increased, and the flow is complex; the steam at the last stage of the low pressure section has a high humidity content, and the water droplets in the steam have a significant impact on the blades; when the steam turbine is running under variable conditions, the working state of the last blade of the low pressure section changes the most, which seriously affects its strength and vibration; the last blade of the low pressure section is longer than other blades, and the strength conditions are more stringent.

These characteristics require that the design of the low-pressure section last-stage blades must be considered more comprehensively and carefully during the design and manufacturing process of low-pressure steam turbines. Generally speaking, the design of the low-pressure section last-stage blades requires more advanced analysis programs, more calculations, and more complex structural designs than the design of other blades. The manufacturing is more difficult, such as: electric spark and flame quenching and high-frequency quenching strengthening of blades, thermal spraying, laser cladding, local laser surface quenching, peripheral inlaying, etc. Despite this, damage to the last-stage blades still occurs from time to time.

2 Damage forms and causes of the last stage blades in the low pressure section

There are many forms and causes of damage to the last-stage blades in the low-pressure section, the main ones are: forms and causes of mechanical damage; forms and causes of non-mechanical damage.

Mechanical damage and causes: For example, foreign hard particles enter the turbine and damage the blades, fixed parts inside the turbine fall off and damage the blades, the rotor and cylinder are not aligned well or the cylinder is deformed, causing the blades to rub against the steam seal, and grooves are worn on the blade shroud, etc. However, most of the damage is caused by reasons other than the design factors of the final blades, which are mechanical damage. This type of damage can be handled by different measures depending on its severity and the impact on operation.

Non-mechanical damage and causes: damage caused by corrosion of blades due to poor steam quality; damage caused by water erosion caused by the impact of liquid water in wet steam. This article mainly discusses the two non-mechanical damage causes and treatment methods of the low-pressure section blades: analysis of the damage causes caused by corrosion of blades due to poor steam quality and treatment methods.

Cause analysis: Usually, the low-pressure turbine blades are made of heat-resistant stainless steel. This material has good corrosion resistance because a dense and stable oxide protective film is formed on its surface. However, if the steam contains C02, S02, especially chloride ions, the protective film on the surface of the blade will be corroded and quickly develop in depth, causing corrosion to the blade, and the blade strength will be greatly reduced. Taking 2Cr13 stainless steel as an example, the bending fatigue strength in air at room temperature is 390 N/mm2 (unnotched specimen, stress cycle number n=5x107, the same below), and the bending fatigue strength in clean condensate water is still 275~315N/mm2. However, in an oxide solution with a NaCl content of >1%, the bending fatigue strength drops sharply to 115~135 N/mm2. Reduced fatigue strength means a shortened service life. Through instrument inspection of the final blades, it was found that corrosion of the low-pressure final blades mostly occurred at each stage in the wet steam zone, and local corrosion often occurred on the blade surface under the scale layer, which then expanded to form cracks. Continuing operation will cause blade breakage due to corrosion fatigue. Inspection and analysis of the broken blades by instruments showed that the fracture sediment layer contained chlorides.