Reducing the minimum load at which a steam turbine can reliably operate is one way to increase revenue for marginal base-loaded units during periods of low electrical demand. For this reason, it is not unusual to see merchant plants operating at "super minimum" load levels that are well below the typical 25% rated full-load limits. However, such units are operating well outside the original equipment manufacturer (OEM) design basis, and owners may experience undesirable damage to their turbines for a number of reasons. That’s why it is important for owners to understand the trade-offs and risks that come with such operation.
The following is an overview of the main steam turbine and generator issues that must be considered before deciding to operate a steam turbine generator below OEM minimum load limits.
Anticipate Increased HP-IP Rotor Vibration
Units with partial arc admission, where the lower arc valves open first, are more susceptible to increased vibration at reduced minimum loads. This is due to unbalanced upward steam pressure forces that tend to lift the rotor and partially unload the high-pressure/intermediate-pressure (HP-IP) bearings.
Older units employing plain journal bearings may experience oil whip and related vibration at reduced bearing loads. Assuming that proper supervisory instrumentation exists, a load test can determine if this is a concern. The operator can perform a load test and perform bearing adjustments at the next outage to determine if minimum load can be reliably reduced. Proper bearing clearances and preloads may be sufficient to eliminate this concern.
If adjustments to the bearings alone do not address oil whip concerns, the operator has two options: change the admission sequence such that the cover valves open first and convert to full arc admission, or retrofit the unit with tilt-pad bearings. A tilt pad retrofit to maintain stability and acceptable bearing vibration level is often the best option.
Modern units usually already employ tilt pad bearings. However, even with tilt pads, maintaining correct clearances and preloads is important to ensure sufficient damping. Adding tilt pad bearing preloads (Figure 1) normally addresses damping and subsynchronous stability concerns.
1. Tilt pad bearing preload. Pad preload, m, is the amount of convergence and divergence that is built into the oil film through the pad geometry. If the pad surface is completely concentric with the shaft, the pad is said to have zero preload. Some shaft eccentricity is needed to create a converging oil film. By adding preload, the bearing load capacity and stiffness are usually increased, and the possibility of pad flutter is reduced because the top pads carry more load. Preload is accomplished by boring the arcs of the pads to a larger diameter than the clearance diameter. Typical preload values range from 0.0 to 0.5, with the most common being about 0.3. Source: TG Advisers Inc.
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