Ⅰ. Core: Mechanical impact caused by momentum difference (the most intuitive blade damage)
Under normal operating conditions, the working fluid of a steam turbine is superheated dry steam, which has a very low density (for example, the main steam density of subcritical units is about 40 kg/m³). After being accelerated through the nozzles, it strikes the blade working arc at a speed almost the same as the blade rotation, smoothly transferring kinetic energy to do work.When cold water/cold steam enters, the damage reverses completely:
1. Density difference brings thousand-fold impact force: Water has a density of 1000 kg/m³, more than 25 times that of superheated steam. For the same volume, water's momentum is over 25 times that of steam. When high-speed rotating blades (tip speed of last-stage blades can reach 600 m/s) hit water droplets or masses, it's like a high-speed car hitting a boulder, instantly generating impact forces far beyond design limits.
2. Reverse impact amplifies vibration damage: Water droplets have much greater inertia than steam and cannot be accelerated to steam speed in the nozzles, so they hit the blade's backside (non-working arc) at much lower speed than the steam, with a force direction completely opposite to normal conditions. This alternating impact can trigger severe blade vibrations, easily causing fatigue fractures at the blade root, and broken blades can damage subsequent stages of blades.
Ⅱ. Thermal Shock and Thermal Stress (Hiddenly Dangerous, Can Easily Cause Blade Cracks/Deformation)Turbine blades, rotors, and cylinders are usually under high-temperature steady conditions (the temperature of high-pressure cylinder blades is 400–560°C), with uniform internal metal temperature fields, resulting in minimal thermal stress.- When cold water (20–100°C) or low-temperature saturated steam (like around 200°C steam from a heater leak) enters, the metal surface cools and contracts rapidly in a few seconds, while the interior of the metal remains hot, contracting much slower than the surface.
- This huge internal-to-external temperature difference creates tensile stress (surface in tension, interior in compression). When the tensile stress exceeds the blade material's yield limit, macroscopic cracks occur directly; repeated thermal shocks make the cracks expand quickly, eventually causing blade fractures.
- At the same time, the differential expansion between the rotor and cylinder can suddenly exceed limits, causing friction between moving and stationary parts, further worsening blade damage.
Ⅲ. Sudden Surge in Axial Thrust (Chain Reaction Damage, Thrust Bearing Burns Out Leading to Total Machine Failure)This is the principle behind water hammer that's most often overlooked but has the worst consequences. It can indirectly cause catastrophic damage to the blades and the entire rotor:1. Normal axial thrust is generated by the pressure difference between steam stages and is balanced by the thrust bearing.
2. Water is incompressible. When it enters the flow path, it blocks the inter-stage steam passages, preventing the steam from the front stages from flowing backward smoothly. This causes the inter-stage pressure difference to spike 3-10 times, and axial thrust skyrockets accordingly.
3. The thrust bearing can't handle the overload and will burn out in seconds, causing the rotor to violently shift axially (even a movement over 1mm can cause severe dynamic and static friction). The blades then collide rigidly with the diaphragms and cylinders, leading to entire stages of blades being broken off, rotor bending, and cylinder deformation.
Additional Notes- Cold steam ≠ safe: When low-temperature saturated steam enters a turbine, a lot of water droplets form due to expansion and pressure drop, which can also cause water hammer. It's even sneakier than cold water (the steam can pass through the main and control valves easily, making it hard to detect in advance).
- Blade damage characteristics: Blades damaged by water hammer usually show brittle fractures with neat breaks, often with multiple blades damaged in a row. Cracks from thermal shock tend to spread radially along the blade, with stress concentration at the root being the most prone to cracking.
