ROTOR DEFORMATION DURING SHUT DOWN

• When steam admission is stopped in a steam turbine, the turbine still retains a high temperature for a few days. This is because the turbine rotor and other components of the turbine retain heat due to their thermal mass. Even though steam is no longer being admitted, the residual heat continues to dissipate slowly over time.
• If the steam turbine rotor is left stationary at this elevated temperature and allowed to cool down, a temperature difference can develop between the top and bottom of the rotor. This temperature difference can lead to bending or warping of the rotor. The upper portion of the rotor tends to be hotter than the lower portion due to the thermal gradient within the casing.
• If the rotor remains stationary in this condition, problems can arise when it is rolled or started up again. The temperature-induced bending or warping can cause the rotor to be unbalanced, leading to high vibrations during operation. This can be detrimental to the overall performance and reliability of the turbine.
• To mitigate these issues, it is recommended to rotate the turbine rotor at a lower speed when it is brought to a standstill. The slow rotation helps in achieving a more uniform cooling of the rotor, allowing the temperature to decrease evenly throughout the rotor. Additionally, the slow rotation promotes the cooling of the bearing babbit metal, which is the material used for the bearing surfaces. The slow rotation aids in the formation of a thin oil film between the rotor and the bearing babbit metal, facilitating heat transfer and removal.
• Overall, by employing slow rotation and allowing for uniform cooling, the potential problems associated with thermal gradients and uneven cooling can be minimized, leading to smoother and more reliable operation of the steam turbine.

“Overall, the mitigation through slow rotation aims to minimize thermal stresses, differential expansion, and associated mechanical problems in steam turbines during startup, shutdown, or other operational transitions..”

1. Introduction:

• Steam Admission:
  • Steam admission refers to the process of introducing steam into a steam turbine.
  • Steam turbines use the high-pressure and high-temperature steam to generate rotational energy, which is then converted into mechanical work or electricity.
  • Steam admission can be controlled to regulate the turbine’s speed and power output.
• Retaining Heat
  • Retaining heat refers to the ability of a system or component to maintain or trap heat within itself.
  • In the context of steam turbines, retaining heat is important to ensure efficient operation and prevent thermal losses.
  • Various insulation materials and techniques are used to minimize heat loss and maximize the thermal efficiency of steam turbines.
• Thermal Mass
  • Thermal mass refers to the property of a material or system to store and retain heat.
  • In the context of steam turbines, thermal mass is utilized to absorb and store excess heat, which helps in maintaining stable operating temperatures.
  • Components with high thermal mass can absorb more heat before reaching critical temperatures, enhancing the overall performance and safety of the turbine.
• Residual Heat
  • Residual heat is the remaining heat energy present in a system or component after the heat source has been removed or turned off.
  • In the case of steam turbines, residual heat can be present in the rotor, casing, and other components even when the steam admission has been stopped.
  • Managing residual heat is crucial to prevent overheating, deformation, and other potential issues.

2. Consequences of Steam Admission Stopping:

• High Temperature
  • When steam admission is stopped abruptly, the turbine components may retain high temperatures due to the residual heat.
  • This can lead to increased stress levels and potential damage to the turbine components.
• Cooling Down
  • After steam admission is stopped, the turbine components start to cool down.
  • The rate of cooling can vary depending on the design and thermal characteristics of the turbine.
  • Rapid cooling can cause thermal stresses and different rates of contraction among the components, leading to potential deformation.
• Temperature Difference
  • The cooling process after steam admission stopping creates temperature differences within the turbine components.
  • This temperature difference can cause uneven contraction or expansion, leading to thermal stresses and potential mechanical problems such as bending or warping.
• Bending
  • The uneven contraction or expansion caused by temperature differences can result in bending of the turbine components.
  • Excessive bending can affect the alignment and balance of the rotor, potentially leading to issues such as rubbing against stationary blades or reduced turbine efficiency.
• Warping
  • Warping refers to the distortion or deformation of the turbine components due to uneven temperature distribution and thermal stresses.
  • Warping can cause misalignment of rotor and stationary blades, leading to reduced performance, increased vibrations, and potential damage.

3. Potential Issues with a Stationary Rotor:

• Unbalanced Rotor:
  • A stationary rotor refers to a rotor that is not rotating or is at rest.
  • If the rotor is unbalanced, meaning its mass distribution is uneven, it can result in high vibrations when the turbine is in operation.
  • Unbalanced rotors can lead to reduced turbine performance, increased wear on bearings, and potential damage to the turbine components.
• High Vibrations
  • High vibrations can occur in a steam turbine with a stationary rotor due to various factors, including unbalanced rotor, misalignment, mechanical issues, or resonance effects.
  • Excessive vibrations can result in fatigue failure of turbine components, reduced efficiency, and increased maintenance requirements.
• Performance
  • A stationary rotor can impact the overall performance of a steam turbine.
  • It can lead to reduced power output, decreased efficiency, and increased wear on components due to uneven loading or misalignment.
• Reliability
  • The reliability of a steam turbine can be compromised with a stationary rotor.
  • Mechanical issues, unbalanced rotor, or high vibrations can increase the likelihood of unexpected downtime, maintenance requirements, and potential failures, affecting the overall reliability and availability of the turbine.

4. Thermal Gradient within the Casing:

• Upper Casing
  • The upper casing of a steam turbine surrounds the upper portion of the rotor and houses the stationary blades.
  • It is exposed to high-temperature steam during operation.
  • The upper casing experiences a thermal gradient, with higher temperatures closer to the steam admission and lower temperatures towards the casing’s outer surface.
• Lower Casing
  • The lower casing of a steam turbine surrounds the lower portion of the rotor.
  • It also experiences a thermal gradient, with higher temperatures closer to the steam admission and lower temperatures towards the casing’s outer surface.
  • The lower casing is responsible for housing the moving blades and directing the flow of steam.
• Thermal Gradient
  • The thermal gradient refers to the variation in temperature within a component or system.
  • In the context of steam turbine casings, the thermal gradient arises due to the exposure to high-temperature steam and the dissipation of heat towards the casing’s outer surface.
  • Managing the thermal gradient is important to prevent differential expansion, warping, or distortion of the casing components.

5. Thermal-Induced Bowing of the Rotor:

• Thermal-Induced Bowing
  • Thermal-induced bowing refers to the phenomenon where a rotor of a steam turbine experiences bending or deformation due to thermal gradients and associated differential expansion.
  • When exposed to high-temperature steam, the rotor may expand unevenly, causing it to bend or bow.
• Stationary Rotor
  • In the context of thermal-induced bowing, the stationary rotor refers to a rotor at rest or not rotating.
  • When the rotor is exposed to high temperatures, the differential expansion across its length can lead to bowing, affecting the rotor’s alignment, balance, and potentially causing contact with stationary blades.

6. Mitigation through Slow Rotation:

• Slow Rotation
  • Slow rotation is a mitigation technique used to minimize the effects of thermal-induced bowing and other thermal-related issues.
  • By rotating the rotor at a slow speed during startup or shutdown, the differential expansion and associated stresses are reduced, reducing the likelihood of excessive bending or deformation.
• Uniform Cooling
  • Slow rotation allows for more uniform cooling of the rotor, preventing abrupt cooling and associated thermal stresses.
  • This helps to maintain better alignment and reduce the risk of warping or bending.
• Temperature Decrease
  • Slow rotation facilitates a controlled decrease in temperature throughout the turbine components, including the rotor.
  • Gradual cooling helps to mitigate thermal gradients and minimize the differential expansion that can lead to bowing or other mechanical issues.
• Bearing Babbit Metal
  • Babbit metal is a soft alloy often used in turbine bearings.
  • Slow rotation allows the babbit metal to gradually adapt to temperature changes, reducing the risk of damage or excessive wear caused by sudden temperature differences.
• Thin Oil Film
  • Slow rotation allows for the formation of a thin oil film between the rotor and stationary components, such as bearings or seals.
  • This film helps to reduce friction and dissipate heat, contributing to smoother operation and improved reliability.
• Heat Transfer
  • Slow rotation enhances heat transfer within the turbine components.
  • The gradual movement of the rotor promotes more effective heat dissipation, reducing the likelihood of thermal gradients, warping, or other thermal-related issues.

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