Machinery trains (such as pumps or compressor trains) are the backbone of many plants. These trains should meet the highest efficiency, best reliability, and the proper flexibility in the operation. Typically, various different checks and verifications are needed to assess and manage the risks of any mechanical, operational, control, and electrical issues, such as torsional vibration issues, rotor-dynamics problems, performance shortfalls, different dynamic instabilities, alternative operating scenarios or resonance cases.
Torsional problems have been reported for many machineries. They sometimes act like a silent killer. For instance, torsional resonances might destroy the shaft assembly of a machinery train by fatigue without significant traces, measured vibration or noises. Torsional vibration can significantly affect the reliability and safety of machineries. This article discusses torsional vibration for machineries and related key topics such as coupling selection, torsional excitations, and torsional monitoring.
For machinery trains, there are a wide variety of excitations and incidents related to electrical, mechanical, and aerodynamic forces and loads, which can excite torsional vibrations. The torsional response to these excitations will generally be multimodal with a slow decay rate because of the light damping. The damping of machinery torsional modes is very low, and it can lead to the generation of high torques in the machinery shaft assembly as result of resonances or transient excitations. The resulting effect on the amount of shaft fatigue expenditure may be much higher than the amount of torsional torque, because of the nonlinear nature of the fatigue process.
Torsional modal damping values may increase as result of the aerodynamic damping (inside machinery such as a compressor or a turbine), or by using some kind of flexible elements in the connections, particularly non-metallic flexible couplings. Some of the more significant damping mechanisms are:
- fluid forces on the impellers, blades, shafts, rotors, seals, and other components;
- shaft material hysteresis, particularly at high levels of oscillating strain;
- energy dissipation from coupling such as slippage (friction) during high torsional oscillation or damping in non-metallic elements;
- bearing oil losses or magnetic bearing controlled-damping effects;
- various mechanisms of electrical damping.
Modal damping is depending mainly on torsional mode, machinery train load, and train configuration. Particularly, the magnitude of the modal damping varies greatly with the mode. Most of the individual damping mechanisms are complex and could not be accurately predicted. Usually, a comprehensive sensitivity study should be performed to evaluate the damping effects.
How to evaluate torsional vibration?
For the check and evaluation of torsional vibration, an accurate torsional model of the entire machinery train is needed. An optimum number of torsional elements should be identified and used. The best guidelines are often gained through experiences. Key considerations in such checks are vibration response frequency range, locations that have distinctly different diameters, and geometric discontinuities.
Generally, the main body regions of an individual rotor/shaft have significant larger diameter sections than the rotor/shaft extension at each end. These shaft extensions often contain the seals and bearings and may terminate with connection to another shaft or coupling.
Torsional behavior of electrical machines
The torsional behaviors of electrical machines, such as electrical motor drivers, need great care. Particularly, in some rotors of electric machines (such as deeply slotted rotors), the cross-section does not remain planar during twisting (known as “wrapping effect”). This can affect the torsional behavior and torsional parameters.
In addition, for rotors that contain materials embedded in slots or cavities, the stiffness of the rotor may be a function of the rotor speed because of centrifugal stiffening effects. Rotating torsional shaker tests could provide useful information that qualifies this effect and help to properly assign parameters to better simulate the actual torsional behavior. These effects are difficult to determine analytically.
One of the forcing functions in the torsional vibration for electrical machineries is the air gap torque. Electrical excitations generated in the electrical network could be translated to torsional excitation torques, which should be quite accurately determined for proper simulations and checks. In general, the electrical torque on electrical machines could be assumed uncoupled from the torsional vibration of the train. However, for some excitations (such as sub-synchronous excitations), there is a cross-coupling between the machinery torsional response and current oscillations in the electrical network.
Couplings are important parts in machinery trains, and they play an important role in the torsional behavior of the machinery. They should be selected by considering many factors and parameters, such as torsional and rotor-dynamics behaviors of the machinery, operational requirements, and many more.
Particularly selected coupling(s) should accommodate operational misalignment. This is mainly thermal misalignment as the temperature changes. In fact, some machineries such as turbines will heat up during operation and some others (for example, compressors and pumps in low-temperature services) will cool down. Often a coupling is required to transmit (or isolate) thrust loads. In general, there are two main classes of coupling used on rotating machineries:
- High-torsional-stiffness couplings (often known as “quasi-rigid couplings,” sometimes referred to as “metallic-flexible couplings”): These are preferred couplings for many machineries. Most popular couplings for critical machineries fall into high-torsional-stiffness coupling category. These couplings can accommodate operational misalignment while they present the stiffness and rigidity needed to keep natural torsional frequencies at high levels outside of the operating speed range. They need minimum maintenance and they offer high reliability. However, for some special applications, suitable coupling in this category might not be available or cannot be used.
- Flexible couplings (often known as “non-metallic-element flexible couplings”): These couplings should only be used in special circumstances when other suitable options cannot be used. They require relatively high maintenance and sometimes they lead to operational problems and low reliability. For some machinery trains, it is necessary to specify a non-metallic flexible coupling with a relatively low stiffness and relatively high damping values.
High-torsional-stiffness couplings (metallic-flexible couplings)
Modern machinery trains have been using high-torsional-stiffness couplings, which are provided for infinite life and require relatively low maintenance. High-torsional-stiffness couplings (metallic-flexible couplings) can be used for a wide variety of applications, including pumps, axial compressors, centrifugal compressors, various electric motors, gas turbines, steam turbines, etc. Extreme temperatures can be accommodated by making allowances in the material strength and flexibility of the couplings. These couplings can also be customized with additional features such as a redundant drive that engages in the event of a flexible element failure, a torque monitoring system or an electrical insulation.
Different types and models of these couplings are available, including disc type coupling and diaphragm type coupling. Each of these can be further separated into different styles and sub-groups.
- Disc type couplings accommodate misalignment by using flexible discs that are connected by alternating bolts to opposing flanges. The thickness of discs and distance between bolts determine the amount of flexibility of the coupling. Disc type couplings can be circular, scalloped, or straight-sided, depending on the shape of the flexible discs.
- Diaphragm type couplings accommodate misalignment by allowing movement of their outer diameter relative to inner diameter (or vice versa). The diaphragm is plate shaped with a contoured profile machined into one or both sides. The amount of flexibility is determined by the thickness of the contoured profile and the difference between outer and inner diameters. Diaphragm type couplings can have straight or wavy flexible profiles, can be continuous or contain holes, can have single or multiple diaphragms, and can be made as a bolted or welded assembly.
Many types of couplings, such as disc-type or diaphragm-type, can be arranged in a reduced moment configuration. In this configuration, flexible elements are attached outboard of the shaft end, allowing one size of coupling to accommodate a large range of shaft sizes. Although, reduced moment configuration may not be popular from a reliability point of view, it can be used if other suitable options are not available or possible.
For many large machinery trains, due to large sizes, complications in torsional behavior or other issues, such as suitable couplings, cannot be found and the rigid connection such as the flange-to-flange connection without a coupling may be used. This is not a good solution as such a rigid connection cannot accommodate the operational misalignment. However, this has been used in some machinery trains.
Inaccuracies in torsional evaluations
Torsional studies and evaluations have been used to verify torsional behavior of machinery trains to assure smooth and trouble-free operation. However, the actual torsional behavior of the machinery train is often completely different from the torsional analytical predictions. Therefore, sometimes things go wrong despite inaccurate torsional calculations. One reason is inaccuracies of coupling torsional flexibility values. This is particularly concerning for non-metallic flexible couplings such as those couplings with elastic (non-metallic) elements. A good recommendation is to conduct a torsional verification test as the part of the machinery shop performance test or site performance test.
Another concern is the nonlinear behavior of coupling torsional characters with the torque (load) variation. Many torsional study methods and tools are linear, and cannot accommodate the complex highly nonlinear torsional behaviors of couplings and shaft assemblies, particularly non-metallic flexible couplings. Deterioration and degradation of load transition components in the coupling is another problem, and this is far more serious for non-metallic flexible elements. A sensitivity analysis is needed to assess all these effects such as nonlinearities, degradation due to different reasons, and others.
Variations of coupling torsional characters with the torque (load) need particular attention. In many couplings where the transmitted torque (i.e., machinery load) increases, the natural torsional frequencies shift slightly to different values. Often, torsional natural frequencies move to higher values because flexible couplings become stiffer with increasing the coupling torque load.
A major reliability problem arises once a shifted torsional natural frequency coincides with an excitation frequency. High levels of vibration can occur or even the coupling or other load transmission component may fail by fatigue or other damaging mechanisms. This failure may come with initial cracks in the metallic or non-metallic components. At the standstill coupling, too often these cracks are not visible. Typically, there are little opportunities to measure, monitor, and detect these types of problems.
There have been different models and types of torsional monitoring systems. One is based on measurement of a set of magnetic pick-up probes on the teeth gears or shaft encodes. Strain gauges were used in old-fashioned methods. Modern, advanced methods use laser optics for torsional monitoring. Torsional vibration measurements are important, and they can provide feedbacks into the torsional model and torsional studies.
Improvements and corrections may be implemented to refine torsional analytical studies to better match with actual measurements. These corrections should be based on up-to-date theoretical knowledge and measured trending. Particularly, measurements could provide feedback to verify coupling torsional stiffness and damping values.
Theoretically there is a possibility to dig-out the casing vibration, particularly the gear unit casing vibration (in machinery trains containing gear unit), to obtain some torsional vibration data, although this is often challenging and difficult, involving complicated mathematical formulations and advanced analytical methods. It is not easy, but it can be possible.