Suppression of Subsynchronous Vibrations in a 11 MW Steam Turbine Using Integral Squeeze Film Damper Technology at the Exhaust Side Bearing

The presence of high subsynchronous vibrations and other rotordynamic instabilities in steam turbines can prevent operation at full speed and/or full load. The destabilizing forces generating subsynchronous vibrations can be derived from bearings, seals, impellers or other aerodynamic sources. The present paper describes the case of an 11 MW steam turbine, driving a syngas centrifugal compressor train, affected by subsynchronous vibrations at full load. After the occurrence of anomalous vibrations at high load and a machine trip due to the high vibrations, the analysis of data collected at the site confirmed instability of the first lateral mode. Further calculations identified that the labyrinth seal at the balance drum was the main source of destabilizing effects, due to the high pre-swirl and the relatively tight seal clearance. The particular layout of the turbine, a passing-through machine with a combined journal/double thrust bearing on the steam admission side, together with the need for a fast and reliable corrective action limited the possible solutions. Based on the analyses performed, adjusting the clearance and preload of the journal bearings could not have ensured stable operation at each operating condition. The use of swirl brakes to reduce the steam pre-swirl at the recovery seal entrance would have required a lengthy overhaul of the unit and significant labor to access and modify the parts. The final choice was a drop-in replacement of only the rear bearing (on the steam exhaust side) with a bearing featuring integral squeeze film damper (ISFD) technology. In addition to being a time efficient solution, the ISFD technology ensured an effective tuning of stiffness and damping, as proven by the field results. The analyses carried out to understand the source of the subsynchronous vibrations and to identify possible corrective actions, as well as the comparison of rotordynamic data before and after the application of the bearing with ISFD technology, are discussed.

Losses of Steam Admission in Industrial Steam Turbines Depending on Geometrical Parameters

Due to the range of applications, industrial steam turbines show a compact and modular design including several branches for the admission and/or extraction of process steam. In conjunction with a flexible operation and partial load conditions, it is important to estimate the losses appearing at those branches sufficiently. Therefore, the results of an extended parameterized numerical study of a T-junction with steam admission are discussed in the first part of the current paper. This study, carried out with a 3D RANS CFD-solver, is used to determine the additional secondary loss, which is caused by deflection of the admitted steam and mixing with the main flow. At this, the loss distribution depends on geometrical parameters of the T-junction such as the area ratio of branch to main pipe diameter and the curvature of the transition piece. The secondary loss, calculated as a function of total pressure loss and local wall shear, is compared with measurement data from literature. In the second part of the paper the loss calculation procedure is adapted from theoretical computations to two actual industrial steam turbine configurations. First, a 3-stage segment of a high speed turbine which includes a circumferential slot for steam admission is examined. Therefore, flow ratios from 0 to 50 % of admitted steam, compared to main flow, are numerical performed. Second, a 2.5-stage low speed turbine segment with two asymmetrical branches and a fix flow ratio of 40% for the first branch, respectively 80% for the second branch is considered. All invested configurations illustrate how geometrical parameters affect the secondary loss distribution as well as the mixing process within subsequent turbine stages.

Partial Arc Steam Admission Optimization in Order to Reduce Vibration of Steam Turbine With Tilting-Pad Journal Bearings

In the investigated high pressure steam turbine, with increasing steam flow rate the exciting aerodynamic forces rise and cause high, but limited vibration of turbine bearings. The tilting-pad journal bearing load and load angle are changing as the steam flow rate changes in turbine with partial arc admission, and accordingly the dynamic characteristics of bearings change. The field experiments results at fossil fuel power plant presented. The aim of experiments was the partial arc steam admission optimization to reducing the effect associated with change of bearing load/load angle. At first, conventional (consecutive) order of valve opening was investigated. In this case, the bearing vibrations are rising while steam flow and valve outlet pressure are increasing. At some “critical point” the vibration level rises stepwise. Situation repeats symmetrically during turbine unloading/loading. In the second part of experiments, the consecutive order of valve opening was changed to “diagonal” one. As a result, the bearing vibration weakly depends on the steam flow rate and its value is significantly lower. Long-term turbine operation shows that “diagonal” steam admission is optimal for this type of turbine. From the analysis of the forces vectors follows that the “critical point” corresponds to static force, which acts in the direction between neighbor tilting-pads. It is contrary to operation and idle modes where this force is acting on pad. The static steam force is bigger and bearing loading is lower at the “critical point”. Numerical investigation of rotor-bearing threshold stability was performed for different bearing loading conditions with exciting aerodynamic steam forces. Two configurations of bearings were included into the model: LBP and LOP — at the “critical point”. Consecutive and so-called “diagonal” orders of valve opening were modeled. Threshold capacity is higher for the “diagonal” valve opening order.

Influence of Shroud Cavity Jet and Steam Admission Through a Circumferential Slot on the Flow Field in a Steam Turbine

Steam turbines for industrial application are often constructed according to modular design concepts. This allows interchangeable combinations of modules including steam admission and extraction. Prior to field tests the flow in a typical stage configuration of such a steam turbine is predicted numerically. Focus of the current work is the axial gap between high pressure and intermediate pressure part containing a circumferential slot. Mass flow used for axial thrust balancing re-enters the blade channel through this slot. Another exceptional feature appears at the high pressure vane carrier: For manufacturing reasons the last rotor shroud next to and upstream of the gap is not fully enclosed by the vane carrier. This results in a turbulent jet at the exit of the rotor shroud cavity mixing with both the blade channel flow as well as the incoming flow from the slot. A commercial 3D RANS CFD-solver (ANSYS CFX 12) is used to predict the mixing of the different flow partitions within the stage gap. Therefore, the last stage of the high pressure part, the gap with the slot and the first stage of the intermediate pressure part are modeled and solved numerically. The amount of flow through the circumferential slot is varied to discern the influences of the specific flow partitions. Additionally, a modification of the vane carrier helps to analyze radial distribution of incoming flow for the downstream vane row as well as scoring global loss characteristics. As the simulation results indicate, flow parameters up- and downstream and also fluctuations crossing the gap are affected by flow through the slot. Furthermore, the computed flow field shows locations appropriate for a traversing probe system to be used in the test facility.

Modernization of Existing US Navy Steam Turbines: Efficiency, Reliability and Maintainability

Earlier steam plant design requirements for the US Navy steam turbines were focused on reliability and maintainability. Simplicity of design implied easy operation, maintenance and service. Efficiency was not a top priority. Now, in an atmosphere of operating budget cuts and skyrocketing fuel costs along with environmental responsibilities, efficiency improvements are expected, and in some cases demanded on main propulsion units. Additional load due to modern electronic combat systems places extra demand on SSTG sets. Modernization with improved efficiency, reliability and maintainability, while retaining design simplicity is the optimal solution. Meanwhile during the last 50 years, the turbomachinery industry has developed numerous innovative improvements through extensive R&D efforts, advanced computerized simulation of aerodynamics and flow field analysis along with finite element modeling, new manufacturing methods and improved metallurgy and surface treatments. These advances allow efficiency improvements without adding complexity to the design. Modern airfoil design, the optimized transition from partial-arc to full-arc steam admission, tangential leaning vanes, advanced seal designs, streamlined steam path configuration and improved moisture removal are major areas worthy of consideration. Mechanical reliability can be improved with Taumel (orbital) peened tenons for blade packets or integral shrouds which give a 360 degree connection to all of the blades in a row. Electron beam welded diaphragms with EDM cut horizontal joints help to minimize thermal distortions and flow irregularities, particularly at the split lines. Improved welding procedures for casing, diaphragm and rotor repairs can result in shorter repair cycles and lower costs to further promote extension of the turbine’s life cycle. Maintainability can be improved with advanced materials that no longer require regular service, such as anti-lube (greaseless) bushings and replaceable components that don’t require in place machining. Combinations of the above improvements have been used successfully within the industry in upgrading hundreds of turbines, compressors and pumps in various applications including power generation, petrochemical, oil and gas, commercial marine, and other fields. Typical examples of efficiency improvement are 8%–14% over current operating parameters. [1, 2, 3, 4, 5] This paper presents various proven advanced turbine components used to upgrade existing steam turbines, which can be successfully used in US Navy applications as well.

Unsteady Forces Acting on the Rotor Blades and Shaft in the Control Stage for Different Steam Admission Variants

This paper concerns the unsteady high- and low-frequency excitation forces acting on the rotor blades and shaft in the control stage of a 200 MW steam turbine. An ideal gas flow through mutually moving stator and rotor blades was described in the form of unsteady Euler conservation equations, which were integrated using the Godunov-Kolgan explicit monotonous finite-volume difference scheme and a hybrid H-H grid. The effect of rotor blade mistuning on the unsteady forces acting on both the blades and the shaft was examined. Four different control stage steam admission variants were analysed. The actual levels of the stationary components of particular forces were determined by changes in the operating conditions of individual nozzle segments. Different mistuning variants generated different distributions of unsteady rotor blade force harmonics. The presented results show that the first harmonic does not always dominate the spectrum. When considering forces acting on the rotor blades and shaft, there exists an optimal procedure of turbine start-up.

Steam Turbine’s Damage Assessment

A turbo generator unit at Al-Khobar power and desalination plant (7 years old) was unable to start due to high vibration. Vibration trend increasing was noticed in the beginning of 2006 and inflated in 2007 due to unbalance which came to acceptable limits after rotor low speed balancing. Fortunately, this inspection prevented a complete damage of turbine due to detached of casing’s top pressure sensor’s sleeve which was digging into nozzle box body and created a big hole and was few millimeters from the rotor. Site repair was done for the nozzle box as per OEM procedure and sensor was omitted since it is not related to control system. In addition, both journal bearings were getting discoloration and pitting in left side due to selected operation mode of steam admission (partial arc mode) which shifting axis of rotation to left side by 30 μm and increases vibration reading compare to full arc mode. Also, another unit found with similar problems which seems to be a design issue. So an assessment study was done to evaluate the design.

Distortion Compensation by Shape Modification of Complex Turbine Geometries in the Presence of High Temperature Gradients

It is often the case that the turbine casings have rather complex shapes, and are loaded by three dimensional, high temperature gradients. This is especially so with e.g. steam turbine casings’ requiring shapes such as spirals for steam admission, spherical bowls for steam extraction, and the usual rectangular shaped flanges for bolting the casing halves together. The combination of shape complexity in the presence of high temperature gradients creates significant distortion at the inside casing walls along the steam path. This leads to flow losses between the blade tips and labyrinth seals which results in a reduction in turbine efficiency. The aim in this paper is to present methods for modifying standard design casings’ that lead to significant reduction in two primary forms of distortion. The first is based on metal blanket designs that implement a variable added thickness to the casing outside casing wall. The shape and thickness of the metal blankets are shown to be very naturally scaled to the distortion of the turbine itself. The second method involves trimming and cutting modifications to the bolting flanges which in fact leads to a reduction of material. The methods presented are thought to be generally applicable to any casing that distorts due to temperature gradients. To the authors best knowledge, the approach implementing metal blankets is unique and has not previously been covered in the literature. In fact, there seems to be little, if any literature related to the topic. The reduction in distortion of a modified standard design casing and a brief summary of the subsequent improvement in steady state efficiency is described. A 420 MW, intermediate pressure steam turbine inner casing with 60 bar, 600° C steam at the inlet, and 6 bar, 300° C at the exhaust is presented as an example.