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High-speed balancing for rotors operating above their first critical speed isn’t always necessary; it depends on a detailed analysis of the rotor’s dynamic behavior and modal response at operational speeds, not a blanket rule.

Beyond the First Critical Speed: Do All Rotors Need High-Speed Balancing?

Managers from a gas-treating company investigated the effects of varnish on the vibration of bearing No.2 of a Siemens SGT 600 gas turbine.

Varnish’s Effect on Gas Turbine Vibration: A Case Study

In spring 2025, we surveyed our readers about the state of the industry and the dynamic factors shaping the turbomachinery sector. Based on the survey results, we gathered insights, identified trends, and asked about solutions to ongoing challenges, such as supply chain constraints, rising material costs, labor shortages, and increasing power demand. With more than 100 respondents, we collected comprehensive perspectives on five main areas: industry challenges, external factors affecting the industry, solutions, current and planned investments, and recent advancements.

With the boom of AI and data centers, surging power demand, a rollback of U.S. support for clean energy in favor of fossil fuels, and backlogged gas turbine orders, the need for comprehensive and complex energy solutions is greater than ever.

“The power landscape is shifting faster than we’ve seen in decades,” said Doug Machon, VP of Business Development at IHI Power Services Corp. (IPSC). “Electricity demand is surging, driven in large part by the exponential growth of data centers and AI infrastructure, as well as increased electrification across sectors like transportation and manufacturing. Our customers are being asked to maintain grid reliability, manage aging assets, integrate battery storage, and stay aligned with long-term decarbonization goals—all while navigating market volatility and evolving compliance demands. It’s one of the most dynamic and operationally demanding periods in the history of U.S. power generation.”

OEMs like Baker Hughes are securing contracts for equipment deliveries specifically for data center projects. The company signed an agreement for 16 NovaLT gas turbines to power Frontier Infrastructure Holdings’ data center projects in Wyoming and Texas. It’s also providing TURBINE-X Energy with its NovaLT gas turbine technology and auxiliary equipment for various data center projects across North America.

Another example is the joint venture between Engine No. 1 and Chevron, which aims to build a multi-GW-scale, co-located power plant and AI data center using GE Vernova’s gas turbine technology powered by electricity generated from domestic natural gas.

Environmental factors, such as record temperatures and natural disasters, grid instability, and increased wear and tear on power plant equipment, are also significantly impacting the industry.

“Events such as the widespread blackout in Spain and Portugal this spring have underscored how rapidly a disturbance can cascade across interconnected systems, reinforcing the need for fast-reacting, reliable assets that support grid resilience,” said Giuseppe Tilocca, Technical Lead at ETN Global.

"Many power plants have been in service for decades, and the shift to more frequent start/stop cycles—driven by variable load demand and renewable integration—is accelerating wear on key systems,” Machon said. “Mechanical fatigue, thermal stress, and corrosion are all increasing as units operate less predictably than in baseload conditions. Weather-related stress in certain geographic areas only adds to the challenge.” 

Survey respondents ranked several key challenges facing the turbomachinery industry. The top three are:

The energy sector faces a critical challenge in workforce skills and availability, driven by experienced personnel retiring, an ongoing skills gap, and the rapid integration of new technologies like AI and digitalization.

“The turbomachinery industry is facing a deepening workforce crisis, marked by declining vocational interest, an aging labor pool, and increasing skill mismatches,” Tilocca said. “Unlike in other energy roles, skills in gas turbine operations and retrofits are highly specialized and not easily transferable. Despite rising demand, most energy companies report difficulty hiring qualified candidates, particularly for manufacturing, installation, and repair.”

OEMs like Atlas Copco work with universities and participate in job fairs to attract talent early in their careers and offer opportunities within the company. “We utilize various social media channels to reach potential talent, and we also employ AI to enhance efficiency and address potential labor shortages,” said Ulrich Schmitz, the Vice President of Marketing in the Gas and Processing Division of Atlas Copco.

to listen to Schmitz's interview with Turbomachinery International, discussing pressing issues facing the turbomachinery industry. 

IPSC sees people as its most valuable asset. “In today’s market, recruiting and retaining top talent requires intention,” Machon said. “IPSC keeps the focus on building a strong internal culture and a clear path for professional development. That includes formal skills training, leadership development programs, and a performance structure that prepares employees for the role they have right now and for what’s next. We also collaborate closely with local communities in harder-to-staff regions to create workforce readiness programs. Whether it’s internships, apprenticeships, or community partnerships, we’re working to grow the next generation of technical talent from the ground up.”

Organizations across the industry, like ASME’s Workforce Development Group, ETN Global’s Young Engineers Committee, the Turbomachinery Lab at Texas A&M, and the Southwest Research Institute (SwRI), promote learning, training, and education in the turbomachinery industry.

“[To address the need for qualified professionals], education and training initiatives are expanding globally,” Tilocca said. “Vocational and higher education programs are being modernized to reflect current industry needs, especially in digital technologies, system integration, and energy efficiency. The whole industry needs a collaborative effort to bridge this gap, making workforce development a strategic pillar for the sector’s long-term resilience.”

SwRI’s mission is to benefit the government, industry, and the public through its research, “so we are very active in trying to enhance and enable technology learning across the industry,” said Tim Allison, Director of R&D in the Machinery Department of SwRI. The Institute runs a monthly webinar series where “we freely talk about turbomachinery or energy system technologies” and offers several on-site trainings.

The Turbomachinery Laboratory at Texas A&M trains graduate students in industry practices, ensuring they are well-prepared for their careers. And throughout the year, the team also organizes several short courses and designs the technical agenda at the Turbomachinery & Pump Symposium to support education and enhance the professional development of engineers.

Listen to industry leaders discuss strategies

—collaborating with academia, partners, and networking—to cultivate a skilled workforce and address labor challenges.

Rising material costs are posing significant challenges for manufacturers, as they struggle to maintain competitive pricing while ensuring the quality and performance of their products. Many manufacturers are working to expand their supplier base, localize their efforts, and diversify their options to buffer disruptions such as geopolitical events and natural disasters.

“Hanwha is actively working to secure stable international supply chains by expanding its network of raw materials and components and reducing dependency on any single region or supplier,” said Kihun Do, VP and General Manager of the Equipment Business Division at Hanwha Power Systems. “We are also planning to build packaging and service centers in the Middle East and the United States, and an after-service storage facility in Europe. Operating various overseas service centers helps provide more direct control over the supply chain and transportation costs and reduce the lead time to handle customers’ needs.”

Atlas Copco is also addressing rising material costs by leveraging its global footprint. “We have factories in various regions, including the United States, Europe, and Asia, that provide us with a broad pool of qualified vendors,” Schmitz said. “This approach also helps when tariffs are involved; we can produce more locally if we have suitable suppliers. That’s one of our key initiatives.”

Duke Energy has developed strategies to ensure materials are ready and available to respond to emergencies, including storms. “We are looking ahead to identify risks and partnering closely with other business units within the company to be proactive,” said Shawna Berger, APR/Enterprise Communications at Duke Energy. “We took the lessons learned from being reactive during COVID-19 and applied them to our proactive strategy today. We’re building a stronger supply chain that is data-driven, customer-centric, and resilient against dynamic supply markets.”

Matthias Grapow, VP of Business Development and Digitalization at Everllence, said in such a volatile and fast-changing political environment, providing fixed solutions is extremely challenging: “Local procurement is one approach we’ve used for years, and we continue to explore it where possible. However, expanding local sourcing depends heavily on the capacity and stability of suppliers, who are themselves navigating uncertainty. Given the long-term and global nature of our business, we focus on flexible strategies rather than rigid solutions. Devising solutions where the situation changes daily and for a business with long lead times is a loss of time and effort.”

More than 51% of respondents identified supply-chain constraints as their primary challenge. The industry is responding by utilizing local supply chains, collaborating closely with customers and suppliers, and modernizing or leveraging existing equipment.

“To address the various supply-chain challenges—many of which are industry-wide or arise from conditions beyond our control—we maintain strong supplier relationships and prioritize opportunities to localize our supply chain to serve our distinct geographies,” said Dave Ross, GE Vernova’s Gas Power business CEO and President for the Americas region. “At the same time, we maintain a globally diverse supply chain for operational resiliency.”

In March, GE Vernova acquired Woodward’s heavy-duty gas turbine combustion parts business based in Greenville, SC, to strengthen its domestic supply chain. “Combustion parts, like those manufactured by [Woodward], play an integral role in GE Vernova’s gas turbines, supporting customers’ ability to meet growing energy demands from load growth, data centers, and other areas,” he said.

51% of respondents identified supply-chain constraints 
as their primary challenge.

GE Vernova is also investing more than $160 million and hiring over 650 new employees at its Greenville facility to increase capacity to help meet market needs. Most notably, the facility is increasing its production capacity by more than 25% to 70 - 80 heavy-duty gas turbines per year (beginning in the second half of 2026) and will ship approximately 20 GW annually starting in 2027.

Mark Babyak, Vice President of New Apparatus at Ebara Elliott Energy (EEE), said EEE is challenging its suppliers to adapt their sourcing to minimize the impact of tariff costs and remains flexible in adjusting its sourcing strategies to manage market fluctuation successfully.

Many organizations are modernizing existing power plant equipment or retrofitting legacy or existing gas turbines to meet increasing power demands and supply-chain constraints. Investing in upgrades and modernizations also helps organizations stay competitive.

“By jointly defining schedules, anticipating potential bottlenecks, and streamlining procurement strategies, we reduce the risk of disruption and ensure the timely delivery of critical components and services,” said Luigi Coltelletti, SVP of Strategic Marketing at Ansaldo Energia. “A key part of our approach lies in offering upgrade packages that are specifically tailored to be minimally invasive. These solutions require only limited modifications to existing plant configurations, enabling fast implementation and shorter outage durations. This helps our customers maintain high availability while minimizing exposure to volatile material costs and logistical delays.”

ISCP is increasingly supporting its customers well beyond day-to-day plant operations by assisting with acquisition due diligence, plant optimization efforts, and lifecycle strategy development. “Clients are asking for deeper technical insight, especially during M&A activity and when evaluating capital upgrades or retrofit options,” Machon said. “We’ve expanded our capabilities accordingly, tapping into the robust power generation design and deployment expertise of our parent company, which spans a wide range of thermal and renewable technologies.”

Utilities like Duke Energy are partnering with OEMs to preemptively procure equipment, staying ahead of supply-chain bottlenecks and meeting the rising demands of advanced manufacturing, data centers, and population growth. It partnered with GE Vernova for the option to purchase up to 11 HE gas turbines, steam turbines, and other related equipment, including generators. “The agreement allows for turbine delivery to support our current integrated resource plans and to accommodate even the most robust growth scenarios,” Berger said. “Using these turbines at Duke Energy plants also allows us to leverage existing infrastructure, including transmission capabilities, significantly reducing costs and accelerating deployment.”

Respondents ranked external factors influencing strategic decisions on a scale from 1 (most impactful) to 7 (least impactful). Global economic conditions were considered the most impactful, with 40% of respondents ranking it first and an average score of 2.24. Next was energy market volatility (2.64), with 24% of respondents ranking it first, and decarbonization and sustainability goals (3.87). Factors considered less influential included advances in digitalization and Industry 4.0 (4.93) and the availability of investment or funding (5.05).

“GE Vernova’s Gas Power business engages in discussions daily with our customers about how to satisfy the growing demand of critical energy markets while maintaining a stable electric grid to support growth, prosperity, and energy security for their communities,” Ross said.

The conversations focus on three key areas: risk management, technological investments, and global supply-chain strategies. “Given the market realities of today—and likely for the foreseeable future—project timing becomes one of the toughest challenges for our customers’ ability to support the electricity demand of their customers.”

Ross highlighted four key considerations for project timing:

Equipment availability: There is an urgency to lock down manufacturing slots for the long lead items necessary to build a new power plant: gas turbines, generators, transformers, and other electrical gear.

Engineering, procurement, and construction (EPC) companies’ capacity: Many of the top-tier EPC firms are filling up, and access to skilled labor, technicians, and experienced project managers is a key pacing item.

Acquiring the required permits and satisfying all regulatory requirements.

The wait for the grid interconnection queues is, in some cases, pushing projects out.

In terms of technical logistics investments and supply-chain management, GE Vernova is investing nearly $600 million in its U.S. factories and facilities over the next two years to help meet the current growing demand, including a $300 million investment in its Gas Power business globally.

“Given the global nature of energy markets, GE Vernova is focusing on strengthening supply chains to ensure reliability and minimize disruptions caused by geopolitical tensions or other external factors,” Ross said.

Grapow said Everllence recognizes that much of the volatility in today’s energy markets and global economic environment stems from politically driven developments and policies from major global players. “While these external factors do influence the broader landscape, our focus remains on the project, the contract, and the long-term relationship,” he said. “We believe in the resilience of our industry and the critical role we play in the global energy transition. This is not a short-term market; we operate with a long-term perspective, confident that energy infrastructure and turbomachinery will remain vital for decades to come.”

Coltelletti made a similar point: “Despite current volatility and uncertainty, the heavy-duty rotating machinery market remains strong. These technologies will continue to play a critical role in ensuring energy security and enabling the energy transition. Efficiency, operational flexibility, and green-fuel capability are exactly what energy producers need to navigate today’s complex landscape, ensuring dispatchable power, grid stability, and long-
term sustainability.”

A total of 61.05% of respondents are opting for modular solutions to meet rising power demands and limited equipment supply. Fewer than one-third (29.47%) are using smaller gas turbines. Among "other" respondents (29.47%; respondents could select multiple options, so totals exceed 100%), notable trends include the accelerated adoption of renewable energy, particularly solar power, a strong emphasis on optimizing and upgrading existing energy infrastructure for better efficiency, and the evolving role of gas-fired power generation. Additionally, there is a recurring focus on improving supply chain dynamics and offering tailored solutions.

“There is a clear and growing global trend toward smaller, modular, and even mobile gas turbine technologies, especially in emerging markets and for decentralized applications,” Tilocca said. He named several factors that are driving this shift:

Fast start-up and ramp rates make small turbines ideally suited to support intermittent renewable generation and provide ancillary services.

Lower capital expenditure and shorter deployment timelines make micro-units attractive in rapidly changing or resource-constrained environments.

Operational flexibility is increasingly valued in power systems dealing with volatility, partial load operations, and localized reliability issues.

Mobile power solutions are increasingly being adopted to address temporary or remote energy needs.

“There’s a real sense of optimism in this space. Mobile power is no longer just a short-term solution; it is a strategic enabler for countries and companies looking to build energy systems that are faster, cleaner, and more resilient for the future,” Midhat Mirabi, GE Vernova’s Managing Director of Aero New Units, said in an interview with 

 on how mobile solutions give islands like the Bahamas (where it shipped its trailer-mounted TM2500 mobile aeroderivative solution) a means to modernize island power generation systems and provide stable, affordable electricity, especially during summer peak times.

Ross said GE Vernova sees gas power playing a big role in providing dispatchable and affordable power and firming the grid to allow more wind and solar to be integrated. “We continue to see an increase in demand for flexible dispatchable generation where GE Vernova aeroderivative solutions have a strong fit,” he said.

Its trailer-mounted TM2500* mobile aeroderivative solution is what “we call a ‘power plant on wheels,’” Ross said. “We are proud to increase power generation capacity and work toward providing more reliable power for the growing communities and the critical tourism sector. Plus, our aeroderivative solutions help power-plant operators transition from heavy fuel oil to a solution that can burn less carbon-emitting fuels, like natural gas.”

Another example is Siemens Energy and Eaton: They developed a fast-track approach to modular data-center construction with integrated onsite power that provides reliable, grid-independent energy supplies and standardized modular systems. It allows the simultaneous construction of data centers and onsite power generation with grid connection and renewable integration when needed. The configuration offers customers the flexibility to choose the right mix of energy sources with quick construction and start-up times.

The standard configuration of Siemens Energy’s modular power plant concept generates 500 MW of electricity. The size of the plant can be scaled up and down depending on its modular approach. It also includes an optional emission-free clean air grid connection that can be installed either during construction or as a retrofit.

Although its turbomachinery business is not directly focused on mobile power, Grapow said Everllence is seeing its engines increasingly being used in modular and mobile power solutions: “These solutions are deployed in crisis regions around the world to provide urgently needed electricity. Moreover, we are seeing a marked increase in the demand for our emergency diesel generators. They are becoming a key part of future energy security strategies. The large-scale blackout in Spain underlined the importance of reliable backup systems, showing many operators that emergency power is now a necessity, not just a fallback.”

“From small- to micro-sized turbines, micro gas turbines remain an area of active innovation and deployment,” Tilocca said. “These compact systems are particularly promising for distributed energy and combined-heat-and-power (CHP) applications in decentralized and integrated settings.”

Baker Hughes, Hanwha Power Systems, and Hanwha Ocean signed an agreement to combine Baker Hughes’ small gas turbine technology with Hanwha’s ammonia combustion system. The goal is to develop dual-fuel small turbines—capable of 100% ammonia combustion with natural gas—for use in marine, onshore, offshore, electric generation, and mechanical drive applications.

Ansaldo Energia has also tapped into the micro-turbine market, as its group portfolio includes 100-kW fuel-flexible microturbines ideal for distributed electricity production and designed to offer high-efficiency solutions for cogeneration and trigeneration. “Modular installation allows us to cover a wide range of different power needs, with each module delivering 100 kW of electrical power and approximately 200 kW of thermal power,” Coltelletti said.

Its AE-T100 microturbines work with different fuels, including biogas, and are tested for burning up to 100% hydrogen in the fuel mix. They can also be powered by external sources, which complement biomass combustion and concentrated solar generation plants.

SwRI is also seeing increased interest in R&D for modular/small-scale distributed generation enabled by low-cost PV and anywhere-suitable energy storage technologies such as pumped thermal energy storage or liquid air energy storage. “With the interest in geothermal power along with the fact that every geothermal application has slightly different well and/or ambient conditions, we are seeing interest in developing modular systems that are easily adaptable to a wide range of sites,” Allison said.

To stay competitive, 59.80% of respondents said they are investing or planning to invest in energy-efficiency improvements; 42.16% are focusing on predictive maintenance and/or workforce training; and 36.27% are investing in emissions-reduction technologies.

“Every one of our clients is looking for ways to improve plant performance while reducing operational strain and resource consumption,” Machon said.

Tilocca named several trends in efficiency improvements:

Waste heat recovery systems, heat upgrade technologies, and heat pumps

Traditional gas turbine CHP with electric heating like boilers

Predictive maintenance is a recurring trend in this space. “One of the most important trends we’re watching is the rising demand for near-perfect uptime,” Machon said. “These users are energy-intensive, availability-sensitive, and increasingly central to grid demand. To meet these expectations, facilities will need more than just upgrades—they’ll need a more sophisticated design, highly optimized operations, and a greater reliance on predictive maintenance, automation, and workforce readiness.”

IPSC is already working with clients to prepare for this shift, supporting both new project development and long-term optimization of legacy sites. “As the industry navigates more frequent cycling, tighter margins, and aging infrastructure, asset owners are seeking smarter, more proactive ways to manage reliability and cost,” he said. “IPSC’s expanded fleet-wide training, enhanced CMMS deployment, and analytics-based maintenance planning are all designed to meet these needs.”

In terms of technological advancements, companies are primarily considering or investing in decarbonization technologies (51.55%), followed by AI and machine learning (ML) (44.33%), and fuel-focused advancements (29.90%). Advancements are also being made or considered in nuclear or other renewables that pair with turbomachines (25.77%) and auxiliaries and components (22.68%).

“AI stands to impact every part of the energy value chain, and our industry is seeing significant growth,” Berger said. “It has helped us automate processes, optimize decisions, and predict the likelihood of certain events occurring, among other use cases. Our journey has brought us to a place today where we are ready to scale our AI use cases in support of our energy modernization strategy—the investment in talent and planning for this technology is creating intentionality to deliver the best possible solutions to benefit our employees, customers, and communities.” 

IPSC is also actively exploring and integrating AI-driven capabilities into its operations. “While IPSC hasn’t fully deployed standalone AI platforms yet, we’re using advanced analytics to monitor equipment trends and predict potential failures before they occur,” Machon said. “There’s also growing curiosity about AI and ML applications in the O&M space. Most customers aren’t ready to deploy large-scale investments yet, but interest is high. We expect to see more adoption as the technology matures and ROI becomes easier to quantify. We see AI not as a replacement for expertise, but as a multiplier for the knowledge already embedded in our teams and systems.”

Allison said AI and ML’s most significant impacts are already being felt, and this was underscored at Turbo Expo 2025, as the conference explored how AI is changing the industry—from surging power demands to gas turbine technology, computational tools, and turbomachinery design.

“[AI and ML’s] high power demand is driving purchases of gas turbines across the United States, such that lead times are now stated as 7 to 8 years out!” he said. “Whether for power or cooling, data centers require large-scale technologies that align well with the scalability and power density of turbomachinery-based technologies.” 

In the United States, significant investment in decarbonization continues across the industry despite recent policy shifts that have reduced support for the Inflation Reduction Act and indicated a renewed backing for fossil fuels.

OEMs are actively integrating decarbonization into their advanced technologies—from gas turbines and mobile power solutions to key components.

The turbomachinery industry is navigating a transformative period marked by rising energy demand, supply chain disruptions, and a shift towards decarbonization. Key challenges include labor shortages, rising material costs, and supply-chain constraints. Companies are responding with modular solutions, investments in energy efficiency, and technological advancements like AI and decarbonization technologies. The industry is also focusing on workforce development to address skills gaps. External factors such as global economic conditions and energy market volatility significantly influence strategic decisions, emphasizing the need for resilience and adaptability.

Turbomachinery faces challenges like labor shortages and supply chain issues, but innovative solutions help the industry meet growing energy demand.



Energy Industry Outlook: Strategies to Tackle Supply Chain Issues, Power Demand, and Decarbonization 

In high-speed rotating machinery, a common misconception exists: Every rotor that operates above its first critical speed requires high-speed balancing (HSB). Although this assumption may seem intuitive, it overlooks the dynamic complexities and oversimplifies the matter. The truth is more nuanced. This paper reviews international standards and expert literature, combining them with practical engineering experience to evaluate when high-speed balancing is truly required. The analysis reveals that the decision should be based not merely on crossing a critical speed, but rather on detailed rotordynamic behavior and modal response at operational speeds.

Rotors in turbomachinery are typically classified as either rigid or flexible. Rigid rotors do not show significant bending at operating speeds, while flexible rotors do, often operating above their critical speeds. Balancing is essential to minimize imbalance-induced vibration, especially in high-speed systems. A common question in the industry arises: If a rotor operates above its critical speed, is high-speed balancing always required?

To answer this question, let’s review industry standards, influential literature, and practical case studies that provide clarity for designers, maintenance professionals, and vibration analysts.

REVIEW OF RELEVANT STANDARDS AND LITERATURE

This standard emphasizes the importance of conducting detailed rotordynamic analysis when dealing with flexible rotors, particularly those with operating speeds that are close to their critical speeds. It clarifies that the proximity of a rotor’s operating speed to its critical speed does not automatically necessitate high-speed balancing. Rather, the analysis should focus on understanding the rotor’s dynamic behavior, especially at its operating speed, to facilitate informed decisions about balancing methods.

The standard specifies that the behavior of flexible rotors can vary significantly based on their operating conditions and the complexity of their dynamic responses. Therefore, a more comprehensive approach involving rotordynamic modeling is necessary to ensure that the rotor’s vibration characteristics, including resonance effects and critical speed behavior, are accurately evaluated.

Furthermore, API 684 highlights the critical role of the bearing length-to-diameter (L/D) ratio in selecting appropriate dynamic coefficients for journal bearings. This factor is essential for accurately modeling the bearing’s influence on the rotor’s dynamic behavior. The L/D ratio affects the bearing’s stiffness and damping properties, which are vital for predicting the rotor’s response under various operating conditions. Proper consideration of this ratio ensures that the rotor-bearing interaction is modeled precisely, leading to improved predictions of rotor performance and more effective balancing strategies.

This repair standard outlines the conditions under which high-speed balancing (HSB) should be considered for rotors. While HSB is generally not necessary for compressors and turbine rotors, there are specific circumstances where it is recommended, including, but not limited to, rotors that:

Exhibit high vibration as they pass through critical speeds.

Accelerate slowly through critical speeds.

Installed in inaccessible locations, such as offshore.

Cannot be tested in their intended casing before installation.

Previously high-speed balanced and not disassembled.

This also includes rotors that are sensitive to imbalance, long and flexible, and used in extremely critical services.

Additionally, it states that a rotordynamic analysis of the rotor and its support system should be conducted before attempting a HSB. This analysis assists in determining the rotor mode shape and the optimal location for balance weights. The standard emphasizes that due to potential differences in the stiffness of the balancing machine bearing pedestals compared to actual field conditions, a revised balancing speed may be needed to account for variations in the critical speed observed in the field.

In ISO 21940-12, determining whether a rotor behaves as rigid or flexible is not solely based on the ratio of the first critical speed to the maximum operational speed. Instead, several factors and methods must be evaluated to decide on the appropriate balancing method accurately.

The ratio of the first bending resonance to the maximum operating speed.

Stiffness of the bearing system and temperature effects.

Rotor deformation due to speed and centrifugal force.

Mass distribution, balancing locations, and support methods.

Suggested Methods for Determining Rotor Behavior:

Consult the manufacturer or operator to gather information about the rotor’s characteristics and the recommended balancing method.

Analyze the ratio of the first resonance to the operating speed; if the first resonance is at least 1.5 times the operating speed, the rotor is generally considered rigid.

Experimental testing of vibration changes with speed variations helps identify sudden shifts in vibration.

Flexibility testing compares vibration responses at different positions and determines the rotor’s flexibility behavior.

Dynamic modeling of the rotor if precise stiffness and damping data are available.

By combining these methods with empirical data, the rotor’s behavior can be accurately determined, and the appropriate balancing method can be selected.

Victor Wowk emphasizes the importance of modal response and suggests that, rather than solely focusing on crossing the critical speed, the vibration behavior of the rotor at operating speeds should be the primary focus during the balancing process. He states that the critical speed is merely an initial indicator, and changes in vibration at operating speeds, along with the dynamic behavior of the rotor under these conditions, serve as the true indicators of the balance state and the need for further adjustments.

In this context, Wowk noted that the dynamic behavior of the rotor at operating speeds, which usually involves crossing the critical speed, can provide more precise information about balance issues and rotor dynamics under real-world conditions. This approach, by examining the rotor’s vibration response at operating speeds and taking into account factors such as mass distribution, centrifugal force, bearing stiffness, and the effect of temperature, can help identify more complex problems that are not observable at critical speed.

In other words, while crossing the critical speed may indicate some issues, evaluating the rotor’s vibration behavior at operating speeds is essential for a more accurate analysis and decision about the need for balancing, as these speeds reflect the system’s actual performance under operational conditions.

Drawing on the points mentioned and practical experience in this field, several factors affect whether HSB should be performed:

Dynamic amplification: If the amplification factor at operating speed is low, even flexible rotors may not require HSB.

L/D ratio: Higher Rotor length-to-diameter ratio values (

) suggest flexibility but are not alone sufficient (

Modal shape: If the dominant mode at operating speed does not significantly excite unbalance response, HSB may be unnecessary.

Bearing stiffness and support structure: Softer supports increase sensitivity to imbalance.

Criticality of the machine: Machinery in critical services (e.g., gas compressors in petrochemical plants) may justify conservative approaches.

Based on field experience and practical evidence, certain rotating machines—such as turboexpanders and centrifugal compressors— may operate above their first critical speed without requiring HSB due to favorable modal shapes and stable dynamic behavior at actual operating speeds. Conversely, there have also been instances where rotors operating below their first critical speed (particularly when the operating speed is 10 - 15% less than the first critical speed) exhibited vibration patterns that necessitated high-speed balancing due to unfavorable dynamic characteristics at operating speeds 

The author’s involvement in rotordynamic analysis during the commissioning and maintenance of large gas turbines and compressors demonstrates that decisions regarding HSB must be made on a case-by-case basis, based on actual vibration responses rather than a generalized threshold.

Avoiding unnecessary HSB in suitable conditions has resulted in reduced costs and downtime while maintaining operational reliability.

The necessity of HSB cannot be determined solely by general criteria, such as whether a rotor operates above its first critical speed. Instead, this decision should be based on a thorough assessment of the rotor’s dynamic behavior under actual operating conditions. Factors such as modal shape, amplification factor, bearing stiffness, support conditions, and the criticality of the equipment application all play a crucial role in this decision.

Field evidence has shown that some rotors, despite operating above their critical speed, do not require high-speed balancing due to their favorable dynamic characteristics. Conversely, there are cases where rotors operating below the critical speed have exhibited unfavorable vibration responses that necessitated HSB. Therefore, adopting a one-size-fits-all approach may lead to unnecessary maintenance actions or overlook hidden reliability risks.

Conduct thorough rotordynamic analysis during the design, commissioning, and troubleshooting phases.

High-speed balancing should be conducted only when the vibration behavior at operating speed warrants it, rather than relying solely on critical speed comparisons.

Utilize experimental testing, such as vibration tests and flexibility analysis, to confirm or rule out the necessity for HSB.

Consult with the equipment manufacturer to obtain expert opinions on the necessity of HSB and consider design adjustments or specific recommendations.

In high-criticality applications (e.g., gas turbines and petrochemical compressors), take a cautious yet evidence-based approach.

Document and analyze each case individually to enhance organizational knowledge and improve future balancing strategies.

Ultimately, the decision regarding HSB should be a case-specific engineering decision, rather than adhering to rigid, generalized rules. 

Mohammadreza Galeban is a Condition Monitoring Supervisor at the Persian Gulf Bidboland Gas Treating Co.

1. API 684:2019 “ Rotordynamic Tutorial: Lateral Critical Speeds, Unbalance Response, Stability, Train Torsionals, and Rotor Balancing.”

2. API Standard 687:2001, “Rotor Repair,” 3rd Edition.

3. ISO 21940-12:2016, “Mechanical vibration — Rotor balancing — Part 12: Procedures and Tolerances for Rotors With Flexible Behavior.”

4. Wowk, Victor. “Machinery Vibration: Balancing”, McGraw-Hill,1st Edition, 1998.

High-speed balancing (HSB) for rotors operating above their first critical speed is not always necessary. The decision should be based on a comprehensive assessment of the rotor's dynamic behavior at operational speeds, considering factors such as modal response, amplification factor, and bearing stiffness. Industry standards like API 684 and ISO 21940-12 emphasize detailed rotordynamic analysis over simplistic criteria. Practical experience shows that some rotors above critical speed may not need HSB, while others below it might. A case-specific approach, rather than a generalized rule, is recommended for HSB decisions.

Until the 1950s, turbomachinery safety and risk assessments were often seen as non-technical. However, following serious accidents, specialists were appointed as safety advisers and risk analysts. Additionally, HAZOP (hazard and operability study) reviews became standard practice. As turbomachinery packages have become larger and more powerful, often operating at higher power ratings, temperatures, and pressures, they manage larger quantities of hazardous fluids, posing significant risks. Thus, safety considerations are crucial, particularly for medium- and large-sized machinery packages. 

Years of experience with turbomachinery have demonstrated that, alongside standard engineering reviews and safety assessments, a HAZOP review is crucial for critical plants. Safety concerns and risks should be addressed in a dedicated HAZOP meeting, with results documented and followed up. However, the literature on HAZOP is fragmented, with limited specific information regarding its application to turbomachinery.

The objective of a HAZOP review is to identify potentially unusual occurrences within the various systems of turbomachinery packages and to anticipate their possible consequences. If any provisions, protections, or safety measures are found to be inadequate, recommendations for improvements will be made. 

In a turbomachinery package, it is essential to divide the system into subsystems, which include components known as “nodes” in HAZOP terminology. When selecting nodes, the process flow—whether it involves the main turbomachinery flow, lubrication oil flow, or cooling flow—should ideally be followed, with nodes identified sequentially. Each node should represent an isolated subsystem that performs a specific task, such as increasing pressure, changing temperature, or removing liquid. For example, the knock-out drum at the suction of a turbocompressor can be considered a node, as well as each turbocompressor casing and each cooler or heat exchanger. 

Before the HAZOP review begins, the nodes to be examined should be identified, highlighted, and listed. The HAZOP chairman must confirm this selection with team leads or managers before starting the review. While the list of nodes may be adjusted during the review process, it is essential to prepare a foundational list beforehand. 

Experience has shown that many HAZOP review teams waste valuable time trying to clarify the exact nature of a recommendation or required action. Future evaluations of that recommendation could completely change the proposed solution. If a solution or recommendation cannot be finalized within a reasonable timeframe (typically 5 to 10 minutes), it is advisable to request further study or evaluation of the problem and then move on to other areas of the review. The final assessment of any major recommendation should include a cost-benefit analysis, which is completed after the HAZOP review. 

A common mistake in many HAZOP reviews is beginning the analysis without a clear understanding or agreement on how the turbomachinery package and surrounding facilities are intended to operate. Before discussing hazards and consequences, the team should agree on the basic operational philosophy of the turbomachinery package under review.

Amin Almasi is a Chartered Professional Engineer in Australia and the U.K. He is a senior consultant specializing in rotating equipment, condition monitoring, and reliability.

Turbomachinery safety assessments have evolved significantly since the 1950s, with HAZOP reviews becoming a standard practice to address the risks associated with larger, more powerful machinery packages. These reviews are essential for identifying potential hazards and ensuring adequate safety measures are in place. The process involves dividing the system into subsystems, or "nodes," to systematically evaluate each component's function and potential risks. Challenges in HAZOP reviews include clarifying recommendations and ensuring a clear understanding of the machinery's operational philosophy. Cost-benefit analyses are crucial for finalizing major recommendations.

HAZOP reviews are a key process for identifying potential hazards and ensuring appropriate safety measures.

Mohammadreza Galeban

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