INTRODUCTION: cascading impacts. An operator of many utility-scale steamturbines

INTRODUCTION: Steam turbine rotors bend during operation, but the bearing and supports are designed to keep the static and dynamic forces under control. However, bending can cause impact between stationary and rotating parts—often cascading impacts. An operator of many utility-scale steamturbines shares its extensive field experience identifying the root cause of failures as well as successful solutions.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. The problems often reduce plant availability by limiting generation and increase plant operation and maintenance cost. Extreme rotor bending problems often involve interaction between the turbine’s rotor and stationary parts. Rotor bending may becaused by a variety of static and dynamic factors, many of which will be explored in this article.We begin with mechanical factors related to the rotor, the largest rotating assembly in the turbine. 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. The discussion is based on incidenthappened at the six-unit, 1,890-MW , located in Ahwaz, Iran. The units were commissioned from 1980 through 1985.DISCUSSION TOPIC:1.Rotor Rubs: It almost goes without saying that rubbing in the labyrinths or diaphragms, caused by insufficient clearances, disrupts the end sealing of the rotor. 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. 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.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 thesurface of the rotor. The rub can cause elastic deformation of the rotor at the point of impact and temporary rotor shaft bending. The shaft bending willusually cause increased vibration levels (Figure 1).Fig 1: Rubbing in the sealing of a high-pressure rotor of a 300-MW unit caused bending of this rotor and blade tip rubsUneven cooling of the rotor, particularly after shutdown, also causes the rotor to contact stationary parts. 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. This situation can cause permanent shaft bending.2.Rotor Imbalance Increases Vibration: Shaft curvature also shifts the rotation axis of the shaft by moving the mass center of the rotor, creating vibration. This vibration affects blades in three significant ways.First, the vibration causes blade structural problems. The centrifugal forces encountered during operation are significant, causing an increase in the tensile forces in the blade cross-section and, if the center of mass is not on the radial line, bending stresses also occur. Additionally, bending stresses are created in blade joints under the pressure effects of the HP steamflowing axially through the turbine cylinder. The magnitude of these stresses is dependent on the flow rate of steam, the temperature drop across the blade stage, the rotational speed of the blades, and the blade weight. The temperature of the steam, superheated in the first stage and saturated in thefinal stages, will have an effect on the mechanical properties and corrosion of the blade materials (Figure 2). Fig 2: Failure of this high-pressure rotor control stage was caused by uneven distribution of steam due to corrosion.3.Casing Also an Important Factor in Rotor Vibration: There are many ways that casing temperature fluctuations can cause steam turbine vibration. Casing problems can cause misalignment in many different ways, mostly related to expansion and contraction due to temperature fluctuations.First, the turbine cylinder can have temperature stratification caused by insufficient thermal isolation from the casing and/or weak insulation in otherareas. Loss of thermal isolation can be caused by poor insulation at connections between joints and pipes to the casings, usually at the bottom of theturbine. Poor casing insulation at the bottom, for example, can cause a temperature gradient from the top to the bottom of the casing, which can lead to casing distortion and elastic rotor bending. The vendor will define the acceptable casing temperature gradient. In our experience, the gradient must be no more than 60C. New HP turbines are particularly sensitive to casing thermal gradients.Next, if the turbine is started from a hot condition before it has returned to within curvature limits, then rotating blades and stationary diaphragms may rub and cause damage to seals and diaphragm glands. As shaft weights increase, so do the dimensions of the turbine rotor, turbine cylinder, and the thermal inertia of the shaft. The effect is to require longer periods of time between startups (and on turning gear) so any curvature of the rotor is removed before the next startup.Finally, uncompensated movement of steam pipes connected to the casing can cause casing movements and rotor vibration. This is particularly true for large-diameter pipes with thick walls. When the turbine is operating, the rotor has axial movement, as does the turbine casing. Thermal expansionof the system is accounted for in the turbine design. The forces and moments that these large pipes apply to the casing are also considered duringturbine design. Excessive pipe connection loads may cause casing deformation, and the bending moments applied to the casing flanges may alsocause displacement and movement of the cylinder within the casing, either of which can increase rotor vibration.Acknowledgements:The Authors thank to department of Mechanical Engineering of Centennial College to provide technical support and also thank to Prof. Cristina Toma for Guidance and knowledge about topic of mechanical failure.The authors also thank to myPOWER publisher to provide this information about incident happened Ramin Power Plant.References:1. MyPOWER PUBLISHER’s research paper.2. Incident report of 890-MW , located in Ahwaz, Iran. The units were commissioned from 1980 through 1985.