Understanding the Active and Inactive Sides of a Steam Turbine Thrust Bearing

Thrust bearings are crucial components in steam turbines, ensuring the rotors remain properly aligned and preventing excessive axial movement. These bearings consist of two sides: the active side and the inactive side. Understanding the roles of each side is essential for the proper functioning and maintenance of steam turbines.

The Active and Inactive Sides of a Steam Turbine Thrust Bearing

Thrust bearings are designed to support the axial load produced by the turbine during operation. They help maintain the rotor's position and prevent excessive axial movement.

Active Side of a Thrust Bearing

Function: The active side of the thrust bearing carries the axial load generated by the turbine during operation. This side is in direct contact with the rotating components.

Design: This side typically features a series of pads or shoes that are lubricated to minimize friction and wear. The design facilitates effective load distribution and cooling.

Operation: As the turbine operates, steam pressure pushes the rotor against the active side, absorbing the thrust force and keeping the rotor centered.

Inactive Side of a Thrust Bearing

Function: The inactive side does not carry the axial load during normal operation. Instead, it serves as a backup or support structure in case of an imbalance or failure in the active side.

Design: This side may include structural components but is generally not designed to handle the primary thrust loads. It may also contain features for alignment or support.

Operation: Under normal conditions, the inactive side does not engage significantly. However, it helps maintain the integrity of the bearing assembly and provides stability.

Complications in Thrust Bearing Operation

Whether a turbine's rotor is pushed toward the exhaust end or the inlet connection depends on the steam path configuration and the type of turbine (impulse or reaction design).

In a Straight Flow Impulse Turbine

The rotor in a straight flow impulse turbine is typically pushed toward the exhaust end due to the force imparted upon the moving blades by the axial component of the steam's velocity. The pressure drop in each stage of an impulse turbine is primarily achieved in the stationary nozzles, not the moving blades.

In a Reaction Turbine

In a reaction turbine, where up to 50% of the total pressure drop is taken across the moving blades, the rotor has a greater tendency to be pushed away from the inlet connection toward the exhaust end. This tendency is countered by the balance piston formed in the rotor forging at the high-pressure end gland, limiting the rotor thrust loads that must be absorbed by the thrust bearing.

Complex steam path arrangements can further complicate which thrust face is active or inactive. For example, in turbines with numerous directional changes in steam flow, identifying the active and inactive sides can be challenging.

The active thrust must be able to absorb significant loads to prevent issues. In cases where the active thrust becomes over-active, it can cause the Babbited surfaces to melt within seconds, presenting a serious operational problem. Proper operating practices and maintenance are crucial to avoid such issues.

Understanding the roles of the active and inactive sides of a thrust bearing is critical for the proper operation and maintenance of steam turbines. Ensuring the bears can handle these loads effectively can prevent costly downtime and inefficiencies.