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Good vibrations attained in large mixer installations

Modal analysis increases reliability and efficiency

February 01, 2013
KEYWORDS mixing / spx
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By Joel Berg

Mixing is a vital part of production processes found in many industries, including food & beverage, chemical, pharmaceutical and petrochemical — to name only the largest. In many of these industries, a trend toward larger mixer installations is apparent. It is assumed these installations provide increased efficiency, added production capacity and, ultimately, increased profitability. This “scaling-up” of mixing applications naturally increases cost and complexity, but the rewards can make it well worthwhile.

Given this increased investment, it makes sense for companies to ensure the machinery and operations involved are best protected. Large capacities deliver big benefits, but also can mean increasing exposure to a single point of failure. If something bad happens, the consequences for production targets and profits may be dire.

As readers may know, modal analysis is the study of the dynamic properties of structures under vibrational excitation. Modal analysis has been used effectively as a diagnostic tool to explain mixer failures in the field. The technique reveals issues with unwanted vibrations and interactions amongst a mixer, vessel and supporting structures.

Since the mid-1990s, modal analysis has increasingly been applied to large, high-power structures with the aim of understanding, identifying and eliminating harmful vibrations within the total mixer installation. This proved to be a cost-saving tool and a way for customers to further protect their investment and production efficiency — prevention being much better than cure.

A mixing process sets in motion fluid forces that evoke a dynamic response within the mixing system. Although static design analysis identifies stresses within the mixer unit, modal vibration analysis takes into account system resonances, which may amplify these stresses beyond the installation’s design limits. Resonant conditions within the process can lead to mechanical failure of components such as the shaft, bearings, motor mountings or other supporting structure components of a mixing system.

Increasing opportunities

Increasingly available computing power has made detailed modal analysis more practical and available for many applications. The wide use of three-dimensional computer-aided design (3D CAD) packages for component design makes readily available much of the information required for analysis. Finite element analysis (FEA) with 3D modeling and advanced computational fluid dynamics (CFD) may be used to analyze the stresses and resonances a total installation design experiences. Analyses identify dangerous resonant frequencies or the potential for premature component failure. Excitation frequencies at the high end, for motor revolutions-per-minute, and at the low end, for the vessel wave surface, are likely to cause unwanted vibrations. The modal analysis focuses on natural frequencies that may fall close to these ranges.

Modal vibration analysis makes sense for large, high-power installations where there is a large amount of energy in the system, and high-value processes where product loss due to a mechanical outage can be expensive. Analysis is also extremely useful where multiple units are mounted on the same vessel. It allows verifying whether the dynamic interaction of adjacent units could cause any problems. In some instances, vibrations may move outside of acceptable limits when process frequencies approach or equal any of the natural frequencies of the mixer, vessel or support structure. The analysis identifies these risk areas and this can be used to increase reliability and process efficiency.

An increasingly common practice in many industrial processing plants is vibration level monitoring of rotating equipment. Modal analysis can be used to provide guidance on optimal locations and expected vibration levels for monitoring, as well as provide valuable information for troubleshooting, should any vibration levels increase over the life of the system.

Protecting high-cost investments

Large, high-power mixer installations represent a considerable investment in a process line. Modal analysis can prevent premature mechanical failures and preclude problematic process conditions. This in turn helps ensure maximum efficiency and run times are obtained — protecting both machinery and profitability. Often simple structural modifications such as a change to the number of impeller blades, the impeller RPM or the addition of bracing can eliminate the potential for unwanted system vibrations. Similarly, a restriction of process operating ranges may be a sufficient and practical way to avoid unnecessary failures. In either case, modal analysis can significantly improve the reliability of an installation.

In many industries the need for efficiency and increased capacity may be vital to staying competitive. Demand for large mixing installations is rising. If you’re in the market, be aware that modal vibration analysis can deliver a deeper understanding of how a process will work and thereby deliver increasing efficiencies.

Joel Berg is senior mechanical technologist at SPX Flow Technology with 25 years of experience in the area of mechanical design and analysis, and currently responsible for development, dissemination and control of global mixer mechanical design programs. Focus areas include finite element analysis (FEA), vibrations analysis, and identification, benchmarking and implementation of emerging technologies and toolsets. Berg spent five years at the National Center for Remanufacturing and Resource Recovery at Rochester Institute of Technology.

Based in Charlotte, N.C., SPX Corp. is a global Fortune 500 multi-industry manufacturing leader with over $5 billion in annual revenue, operations in more than 35 countries and with over 18,000 employees. The company’s highly-specialized, engineered products and technologies are concentrated in three areas: flow technology, infrastructure, and vehicle service solutions. The SPX Flow Technology segment designs, manufactures and installs highly engineered solutions used to process, blend, meter and transport fluids, in addition to solutions for air and gas filtration and dehydration.

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