The Finish Line Vol. 33 • May 2024 [Q2 2024]

RMS Rotordynamic Analysis Capability Examples

Overview of RMS Rotordynamic Capabilities

Lateral Rotordyanmic Analysis

• Undamped Critical Speed Analysis
• Damped Synchronous Response
• Stability Analysis

Torsional Rotordynamic Analysis

• Steady-State Torsional Analysis
• Transient Torsional Analysis

Custom Component Design/Analysis

• Rotors, Seals, Dampers, and Supports

Additional Capabilities

• API Rotordynamic Audit
• Start-up Analysis
• CFD-based Dynamic Coefficients
• Deflection and Clearance Map Analysis
• Diagnostics

RMS Customer RDA Needs

As an RMS customer, whether you had a major rerate or changing a floating ring oil seal to dry gas seal, chances are you have reached out to RMS to have a look at your rotor dynamic state. It is always wise to keep the RDA report of your equipment up-to-date, as rotor failures are costly, and fixes require fundamental changes to rotor-bearing systems. RMS customers have retained our RDA services for a variety of reasons in the past, as shown in the chart above.

RMS Project Example: Turbine Rerate

One of the unique capabilities of RMS engineering is its multi-disciplinary approach to turbomachinery. RMS customers routinely contact RMS to perform feasibility studies on their equipment for their upcoming rerates. RMS checks their rotor for the new operating condition and offers remedial design upgrades which are in-sync with RDA findings. As an example, the following steam turbine rotor came close to the 2nd critical speed which was active and flexible on the thrust end of the shaft. RMS proposed a shaft end upgrade to the spare rotor that would solve this issue with minimal cost and time impact.

RMS Project Example: Coupling Retrofit

RMS customers continuously upgrade the design of their couplings to enhance reliability and reduce downtime. A coupling retrofit affects both the lateral dynamics of involved rotors and the torsional dynamics of the entire train. RMS is experienced when and how the couplings should be evaluated for this change. Recently RMS analyzed MOT-GB-CC train that upgraded both couplings to flexible convoluted diaphragm design. RMS identified 2 out of 3 initial modes were inside excitation margin and coupling designs had to be adjusted.

RMS Project Example: New Impeller

A new impeller often changes the load and inertia distribution on the rotor and shifts the rotordynamic modes. RMS examines the rotor with the new component to ensure responses are within API acceptance criteria. RMS has the capability to check other static and dynamic impacts such as static deflection, shear and bending moment distribution under centrifugal load with the new overhung weight.

RMS Project Example: Rotor Shaft End Failure

One of RMS's customers had a unique application that resulted in sudden load rejection and reapplication to their turbine generator set. The generator shaft end failed prematurely and resulted in downtime to replace the shaft. RMS performed a transient torsional analysis to simulate the dynamics of applied loads and endured stresses and found out that the first fundamental torsional mode excited the failed shaft end which suffered from an inadequate fillet radius at a shaft step. RMS made recommendations to change the shaft design to one that can endure the shock loading condition.

RMS Project Example: Stability Analysis

High speed, energy dense turbomachinery typically run well above their 1st critical speed and they need to endure destabilizing forces from impellers, stages, seals, etc. Above certain load and speed condition the rotor vibration grows without bound until the rotor rubs or fails catastrophically. RMS has the expertise and capabilities to model these excitation forces and offer solutions in case they exceed stability limits. It is an API requirement to systematically check the rotor stability with component level analysis. RMS offers a variety of stabilizing solutions such as damper bearings and damper seals, and helps its customers to solve their problems all in one place.

Example: Vibration & New Bearing Design

Older turbomachinery designs did not benefit from the analysis tools that we have today. Many existing machines in the field are marginally stable with chronic vibration issues. RMS consulted with a customer regarding a unsynchronous vibration problem they had, and after diagnosing their vibration signatures, identified the problem as a bearing susceptible to oil whirl. RMS redesigned the bearing to be suitable for the application which suppressed the chronic vibration issue.

RMS Aerodynamic Analysis Capability Examples

Rerates and Aerodynamic Upgrades for Single and Multistage Compressors and Turbines

By harnessing advanced aerodynamic principles and leveraging state-of-the-art technologies, RMS engineers meticulously engineer solutions that optimize the efficiency and functionality of rotating equipment. These upgrades not only enhance operational capabilities but also extend the lifespan of critical machinery, ensuring maximum uptime and minimizing maintenance costs for clients.

Flowpath Modelling from Scan Data

High accuracy scan data is loaded into CAD Software and measured for critical flowpath parameters. The data is used to create high-quality true to life models for CFD and FEA analysis.

Performance Benchmarking and Data Analysis

At RMS, our approach to performance data analysis transcends mere number crunching; it's a strategic endeavor aimed at uncovering insights that drive innovation and elevate operational excellence. By harnessing real-time data streams and employing cutting-edge analytical tools, we gain profound insights into the intricate interplay of fluid dynamics within turbomachinery.

Flange to Flange Analysis

At RMS, our engineers employ cutting-edge computational fluid dynamics (CFD) simulations and sophisticated analytical techniques to conduct flange-to-flange analyses with unparalleled precision. By dissecting every component and interface along the flow path, we gain profound insights into pressure differentials, velocity distributions, and turbulence profiles, enabling us to optimize performance across the entire system.

Stage Design Optimization

Stage design optimization represents a pivotal process in the quest for aerodynamic excellence in turbomachinery. Each stage within a turbine or compressor plays a crucial role in transforming energy and facilitating fluid flow, making its design paramount to overall system performance.

At RMS, our approach to stage design optimization combines decades of expertise with cutting-edge computational tools and advanced modeling techniques. By meticulously analyzing factors such as blade geometry, inlet and outlet conditions, and flow path curvature, our engineers craft stage designs that maximize efficiency while minimizing aerodynamic losses.

Diaphragm Retrofits

The CFD results for the bolt pattern with fairings show that the airfoil shapes lower drag, and significantly reduce the expected losses.

Diffuser retrofits represent a strategic avenue for enhancing aerodynamic performance in turbomachinery, particularly in compressors where diffusers play a critical role in converting kinetic energy into pressure. At RMS, we recognize the transformative potential of diffuser retrofits in unlocking efficiency gains and optimizing system performance.

Our approach to diffuser retrofits begins with a comprehensive analysis of existing system characteristics and performance metrics. By leveraging advanced computational fluid dynamics (CFD) simulations and detailed flow modeling, we identify inefficiencies and opportunities for improvement within the diffuser section.

One common objective of diffuser retrofits is to mitigate flow separation and minimize losses associated with boundary layer effects and shock formation. Through careful redesign and optimization of diffuser geometry, including vane angles, throat areas, and expansion ratios, we aim to achieve smoother, more uniform flow distribution and maximize pressure recovery.

Secondary Flowpath and Leakage Analysis

Accurate prediction of stage leakage losses is critical to performance prediction, especially for low flow coefficient compressor stages where frictional losses are more prevalent.

Manufacturing Deviation Analysis

Comparative analyses of “as-built” vs “as-drawn” components helps make good engineering judgement about viability of parts with manufactured deviations.

Adds analytical insight into engineered/manufactured parts disposition decision making.

Transient CFD Analysis

Transient Computational Fluid Dynamics (CFD) analysis stands as a groundbreaking methodology in the realm of turbomachinery aerodynamics, offering a dynamic perspective on fluid flow behaviors and system performance. At RMS, we harness the power of transient CFD analysis to gain unparalleled insights into the complex, time-varying phenomena that govern turbomachinery operation.

Unlike steady-state simulations, which provide a snapshot of flow conditions at a single moment in time, transient CFD analysis allows engineers to capture the evolution of flow patterns over time. This capability is particularly valuable in turbomachinery applications, where fluid flow dynamics are inherently unsteady due to factors such as blade passing, shock waves, and vortex shedding.

By simulating transient flow phenomena with high temporal resolution, RMS engineers can accurately predict time-dependent effects such as blade loading fluctuations, pressure pulsations, and flow instabilities. This level of detail is essential for understanding the dynamic performance characteristics of turbomachinery systems and optimizing their design for enhanced efficiency, stability, and reliability.

RMS Structural Analysis Capability Examples

Rotor Blade and Disc Stress Modeling

Centrifugal forces tend to pull the blades radially outwards. Depending on the blade’s angle of attack, these forces also cause an “un-twisting” of the blade (see animation above). This load tends to be “steady”.

The centrifugal forces create tension and bending in the blade to disc attachment section. Tensile stresses are experienced in both the blade attachment and the disc attachment grooves (See snapshot above).

In fir-tree type geometries like the ones shown below, it is common for the maximum stresses in the blade fir-tree to occur in the outermost fillet, while the maximum stresses in the disk fir-tree tend to occur in the innermost fillet (due to relative stiffness). These locations commonly need to be evaluated for fatigue vulnerabilities.

In fir-tree type geometries like the ones shown above, it is common for the maximum stresses in the blade fir tree to occur in the outermost fillet, while the maximum stresses in the disk fir-tree tend to occur in the innermost fillet (due to relative stiffness). These locations commonly need to be evaluated for fatigue vulnerabilities.

Aerodynamic forces due to the fluctuating pressure from the flow field acting on the blades are transient forces. There is a tangential force component and an axial force component due to he associated pressure differentials in the flow field. These loads are cyclic.

Goodman Diagram – Evaluation of High Cycle Fatigue

The Goodman Diagram is a “fixed life” application of the stress-life approach. It includes mean stress correction effects, fatigue strength correction factors, service factors, etc… Typically, the Goodman diagram is created at the endurance limit (usually ~10 to the 7th power cycles) to determine whether or not a design has infinite life. The RMS Modified Goodman Diagram adds additional safety factor for RMS designs.

Off-Design Loading Conditions

Thermal Expansion Animation: No Catalyst Buildup between blades | Speed & Temperature

Thermal Expansion Animation: Catalyst Buildup between blades | Speed & Temperature

In real-world applications, off-design loading conditions are often encountered. This is especially true for Root-Cause Failure Analysis (RCFA) investigations where the machine was operated outside of it’s intended design constraints.

Off-design loads can create additional modeling challenges. Some off-design load examples include “choke” or “surge” operation, as well as “cleaning” cycles.

Foreign material deposit is also a condition which can occur in a machine which can cause additional thermal restraint and significantly magnify the stresses.

Low Cycle Fatigue Life Evaluation

A strain-based approach is used for LCF evaluation. Elastic-plastic (non-linear) analysis is used to calculate peak equivalent total strains (elastic + plastic). A strain range is calculated and used for LCF evaluation (strain vs. cycles to failure).

Strain life is calculated to be the worst case scenario based on the combined Basquin (elastic portion of the curve) and Coffin-Manson (plastic portion of the curve) approach. The strain-life curve is also mean-stress corrected based on the calculated steady stresses.

Rotor Blade Vibration Modeling

Vibration modeling is an important factor and needs to be considered for new blade design, failure investigation, or changing machine operating conditions due to a re-rate or replacement (speed change, temperature change, material change, etc…). Natural frequencies are modeled and compared to excitations to determine if resonance points are being excited or are close to being excited (separation margins).

Resonance can lead to catastrophic failure of a machine. Many modeling techniques for frequency modeling are the same as those used in stress modeling, although some requirements change. Boundary conditions are critical to accuracy (mass and stiffness of restrains can contribute to predicted frequencies, recall ω=√(𝑘/𝑚))

Axial compressor and expander blades have their own natural frequencies that must be calculated. These frequencies tend to correspond to the following fundamental mode shapes. The higher natural frequencies require more energy input to excite.

Sometimes “discs” are used to contain the blades in a rotor assembly. The discs themselves also have natural frequencies in combination with the blade frequencies. The disc modes tend to be categorized based on “nodal diameters”

Campbell Diagrams

The Campbell diagram is a way to plot the natural frequencies (of the part of interest) as a function of the rotating speed (this captures the speed stiffening behavior on frequency). The excitations are also overlaid on this plot. An intersection of excitation with natural frequency that occurs at the operating speed (or in the operating speed range) signifies a point where resonance can occur.

A Campbell diagram can also be applied over a speed range and include temperature softening effects. It is application specific.

Modal Testing

Whenever possible, frequency testing is performed to validate calculated FE frequencies. Frequency testing can also be performed as a first-pass calculation for a machine when the model data is not yet available.

A typical frequency response function from a blade modal test is shown below:

The peaks in the frequency response function represent the natural frequencies (resonances) of the tested blade.

The “width” of these peaks give an indication of the “damping”. A curve-fit (the red line) can be mapped to the frequency response function to provide estimations of the natural frequencies and damping.

If a modal test is performed over multiple locations on a blade, a frequency response curve can be determined at each location. This is known as a “Roving Impact” test. The frequency response plots can then be compared at each test point and “residues” can be calculated.

Structural finite element analysis is a key tool in the design process that helps engineers reduce risk and make decisions. Structural analysis also provides valuable insights when investigating machine failures, and is a vital tool in learning the root cause of failures. Frequency analysis is performed alongside testing whenever possible to validate methods and make sure all tools and practices continue to evolve with technology. RMS employs industry tested methods as well as R&D tools on a case-by-case basis to develop competitive and reliable solutions.

Embracing the OST Philosophy at RMS

In the fast-paced realm of the rotating equipment industry, success hinges on efficient operations, accurate data management, and timely decision-making. Amidst this landscape, RMS is pioneering a transformative internal philosophy known as OST: One Source of Truth.

Unlike conventional products or solutions, OST is a fundamental mindset that permeates every aspect of RMS's operations. At its core, OST embodies a commitment to centralizing data, fostering collaboration, and prioritizing accuracy. It represents a departure from fragmented approaches to data management, advocating for a unified, holistic view of information.

Central to the OST philosophy is its ability to bring in data from multiple SQL servers, transcending the limitations of siloed databases. By aggregating data sources and establishing relational connections, OST ensures that employees have access to a comprehensive and cohesive dataset. This approach replaces the reliance on multiple Excel logs and eliminates the inefficiencies associated with disparate data sources.

OST also revolutionizes communication within the organization by replacing traditional email traffic with intuitive dashboards. These dashboards serve as dynamic hubs of information, providing real-time insights and fostering collaboration across teams. Whether employees are collaborating on projects or seeking critical information, dashboards powered by OST serve as centralized hubs for data-driven decision-making.

One of the hallmarks of the OST philosophy is its ability to transcend geographical boundaries. In an era where remote work and distributed teams are increasingly prevalent, OST enables seamless support for jobs across different states. This means that employees can access pertinent job-related information regardless of their physical location, facilitating prompt decision-making and problem-solving.

OST also serves as a catalyst for automation, driving efficiency and accuracy across processes. By standardizing workflows and automating routine tasks, OST minimizes human error and accelerates project timelines. This focus on starting tasks on time is not merely about meeting deadlines—it's about delivering exceptional customer experiences. Late starts lead to late tasks, which ultimately jeopardize project timelines and customer satisfaction. By embracing the OST philosophy, RMS ensures that operations are optimized for maximum efficiency and customer satisfaction.

OST represents more than just a philosophy—it's a guiding principle that shapes every facet of RMS's operations. By centralizing data, fostering collaboration, and prioritizing accuracy, OST empowers RMS to achieve operational excellence and drive sustainable growth. In an industry where precision and reliability are paramount, embracing the OST philosophy sets RMS apart as a leader in data management and operational efficiency.