Misalignment of the coupling between two shafts or between a shaft and bearing may cause bending in the system. The turbine vendor also defines the rotational speed of the turning gear. For example, the turbine should remain on turning gear until the high-pressure (HP) cylinder temperature is below 150C and the oil temperature is below 75C. To avoid rotor bending during cooling, turbine vendors provide very specific instructions on the allowable rate of cooling.

When on turning gearing, the limit is 0.05 mm. The allowed bending in 3,000-rpm turbines is up to 0.02–0.03 mm in each section. If the bending is allowed to continue, it is possible that the yield strength of the metal could be exceeded, causing a permanent deformation of the shaft. The effect cascades if allowed to continue. Again, the rubbing causes friction between stationary and rotating parts, localized heating of the rotor metal at the contact point, and shaft bending.įurthermore, uneven shaft warming caused by rubbing between rotating and stationary parts can cause further bending of the shaft in the same direction of the existing bow and cause additional contact with stationary parts, increasing temperatures and therefore causing more bending. Even if the clearance change is small, there may be significant rubbing along the rotor to cause damage. The vibration is caused by insufficient clearance between stationary and rotating parts, as well as a shaft located off-center in its bearing. The effect of a permanent shaft bend caused by uneven cooling will immediately appear as high rotor vibration at the next startup. This situation can cause permanent shaft bending. After a unit shuts down, the relatively high-temperature rotor may bend solely due to the mass of the rotor and the distance between bearing supports, if left in a stationary position to cool. Uneven cooling of the rotor, particularly after shutdown, also causes the rotor to contact stationary parts. Courtesy: Ahwaz Power Generation Management Co. Rubbing in the sealing of a high-pressure rotor of a 300-MW unit caused bending of this rotor and blade tip rubs. The shaft bending will usually cause increased vibration levels (Figure 1).ġ. The rub can cause elastic deformation of the rotor at the point of impact and temporary rotor shaft bending. The forces produced by the impact of the large rotating rotor mass with the poorly functioning stationary seals often impress a layer of metal on the surface of the rotor. Secondarily, there may be a localized temperature increase at the point of contact, causing increased metal temperatures at the point of contact due to friction. This situation commonly occurs when the high-mass rotor at operating speed comes in contact with a stationary surface, typically caused by a too-small clearance between the labyrinth or diaphragm gland seals and the rotor. It almost goes without saying that rubbing in the labyrinths or diaphragms, caused by insufficient clearances, disrupts the end sealing of the rotor. The units were commissioned from 1980 through 1985.

The discussion is based on the authors’ experiences at the six-unit, 1,890-MW Ramin Power Plant, located in Ahwaz, Iran. Working from the inside out, we next look at rotor balance issues, followed by rotor and casing misalignment problems, and problems caused by the casing. We begin with mechanical factors related to the rotor, the largest rotating assembly in the turbine. Rotor bending may be caused by a variety of static and dynamic factors, many of which will be explored in this article. Extreme rotor bending problems often involve interaction between the turbine’s rotor and stationary parts.

The problems often reduce plant availability by limiting generation and increase plant operation and maintenance cost. Rotor bending that results in premature failure of steam turbine blades and other internal components is one of the most serious problems experienced in power plant operations. An operator of many utility-scale steam turbines shares its extensive field experience identifying the root cause of failures as well as successful solutions. However, bending can cause impact between stationary and rotating parts-often cascading impacts. Steam turbine rotors bend during operation, but the bearing and supports are designed to keep the static and dynamic forces under control.

Steam ynglet