Digital Exclusive: Improving rotor performance with the high-speed balancing bunker
Centrifugal compressors and steam turbines operate at incredibly high speeds under immense loads every day. These units are used in manufacturing, construction, oil and gas, mining, chemicals, petrochemicals and power generation industries. Even though high-speed rotors are designed to be flexible, stress can, overtime, cause them to naturally warp and bend, creating imbalances. If these imbalances go unchecked for too long, they can lead to expensive field repairs, delayed plant production, unplanned downtime costing millions of dollars and, in worst cases, catastrophic damage.
To prevent this, rotors are routinely high-speed balanced once every 4 yrs–5 yrs.. A rotor must be low-speed balanced after it is manufactured, repaired or has conducted long-term service before it is put back in service. In balancing, rotors are spun at operating speed, and the effects of the leftover unbalance are studied to improve its behavior. However, to truly minimize potential failures caused by rotor vibration, rotors should undergo high-speed balancing.
The author's company's state-of-the-art high-speed balance bunkera is designed to simulate rotor responses to unbalance at operational speed. Being only seven years old, it is the newest of the three bunkers currently in use in the U.S. Gulf Coast. The bunker has variable input torques, ramp rates and specialized software designed to handle the most difficult of balancing challenges when other methods are insufficient.
High-speed balance bunkera features. The bunker’s varied ramp rates allow it to adjust during critical phases, so balancing occurs at the appropriate speed. The bunker’s high stiffness and sensitivity allow it to perform both low and high-speed precision balancing using tilt pad bearings. These bearings ensure maximum possible stability and closer simulation of field performance. The bunker’s rigid, non-deflecting balance pedestals are designed to measure pedestal velocities per API standards and test rotors under field-like operating conditions.
The author's company's high-speed balancing service is enhanced with its influence coefficient method software program that compiles data from sensors along several vector planes on the rotor and calculates balancing solutions. More precise calculations typically result in longer uptime and greater profit margins.
The custom software also reports on test results in real time. This way, clients can make faster, more careful data-driven decisions. With their control room’s PI server access, clients can view results as they are being reported, either from their convenient, on-site viewing lounge or through the remote viewing interface. This remote witness is cost-efficient, because of the computer system’s networking abilities.
In total, the balancing process takes about eight hours. Once testing and balancing is completed, clients are welcome to store their rotors inside the company's vertical storage facility in Houston, Texas, where they store a host of rotors, including the largest rotor in North America.
Rapid growth in high-speed balancing. High-speed balancing has a history dating back to the 1990s with analog technology. Today, digital data collection has improved its use immensely. Now, with databases of past work, referencing projects has expedited the balancing process much further and made it more streamlined. Since the bunker’s installation in 2018, the author's company has increased balances by over 205%.
Recent high-speed balance bunker case studies. At the end of 2019, one oil refinery’s compressor developed a high-speed imbalance. Their particular rotors had come from a manufacturer overseas, which previously had to do their balancing in-house, costing the customer tens of thousands of dollars in shipping expenses and extended downtime resulting in millions of dollars in opportunity production loss. Local balancing companies declined to service the rotor due to historical issues with this particular original equipment manufacturer's (OEM’s) design and prior attempts at high-speed balancing.
The client brought the rotor to the author's company, because they had firsthand knowledge of the high-speed balancing bunker’s capabilities. The team was able to quickly accelerate the rotor through its first natural frequency and achieve operational speed. At the first critical speed, the rotor exhibited acceptable absolute and relative vibration; however, upon reaching the second natural frequency, it began to show the signs of imbalance previously observed on site. The high-speed balance team utilized their advanced influence coefficient software to develop a solution for the residual unbalance within hours of starting.
Their state-of-the-art balancing software is designed to handle these advanced computational methods, to ensure accuracy and consistency for every client. By using these tools, the team was able to successfully balance the rotor quickly, saving the client time and money.
Analyzing problems and devising solutions. A customer brought their 38-yr-old extraction-condensing steam turbine to the author's company. There, they performed a visual inspection, dimensional inspection, runout inspection, non-destructive evaluation (NDE) inspection and low-speed balance check. They found no relevant damage or indicative flaws within the turbine. Then, they balanced the rotor with the high-speed balance bunkera. The rotor received a high-speed balance per API 687 requirements.
After the rotor was balanced, they found a balance repeatability issue. The non-repeatable balance behavior revealed itself after passing the first critical speed at above 7,000 rpm. The team assessed the possible cause(s) of the issue and determined the cause was a changing mass unbalance because of loose rotor parts. To remedy this, the assembled blades had to undergo a dimensional inspection. They conducted a gap measurement between the interlocking integral shroud bands (ISB) and found gaps up to 0.008 in. The acceptable gap size is smaller at only 0 in.–0.002 in., as seen in FIG. 1. They determined the excessive gaps were caused by wearing on the shroud contact surfaces leading to material removal.
FIG. 1. Interlocking, integrally-shrouded blades.
The team disassembled the blades from the rotor and performed another high-speed balance check. This confirmed repeated rotor balance behavior. They concluded that a gap check should be a standard inspection for affected turbine rotors. In this case, the high-speed balance test was extremely useful in assessing the blade assembly condition and helped the client avoid a potential catastrophic blade failure event.
Assessing conditions and avoiding failure. Another client brought in an 8-yr-old steam turbine rotor used for driving an ethylene gas compressor to the author's company. The team ran several high-speed balancing checks and found a similar vibration repeatability issue caused by snubber blades, as shown in FIG. 2. The team discovered excessive interlocking blade snubber gaps on stage six blades up to 0.011 in. They conducted a finite element analysis (FEA) of the blade snubber using actual turbine service data to see how the blades would respond under different amounts of physical force. From the analysis, the team determined the steam’s dynamic force exceeded the contact force between the blade snubbers. This was due to excessive snubber wear and fretting damage. Without high-speed balance testing, obscure rotor issues, such as these, would go unnoticed and negatively affect turbine performance.
FIG. 2. Snubber type blades.
Takeaways. The high-speed balance bunkera serves as a critical resource for maintaining the performance and reliability of high-speed rotors in turbomachinery. Key features, including its variable ramp rates, tilt pad bearings and influence coefficient software, make the bunker ideal for assessing complex rotor dynamics. By simulating real-world operating conditions, it allows users to find targeted solutions for vibration and imbalance issues. With these capabilities, the bunker minimizes risks tied to imbalances, reduces downtime and improves the operational stability of essential equipment.
The bunker’s recent case studies illustrate the value of high-speed balancing in diagnosing subtle mechanical issues, such as excessive blade gaps and resolving inconsistencies in rotor performance. This process not only prevents costly failures and unplanned shutdowns but also provides critical insights into rotor health and structural integrity. With detailed testing and data, the high-speed balance bunker supports the safe and sustainable operation of turbomachinery in oil and gas, refining and petrochemicals.
NOTE
a Mitsubishi Heavy Industries' high-speed balancing bunker
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