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Axial vibration, thrust alignment and coupling selection are critical factors in the smooth operation of turbomachinery trains. Axial alignment should ensure that during all operating cases, when each set of operating temperatures and conditions have been reached and mechanical and thermal movements have taken place, the rotors remain in the proper position without axial vibrations or excessive axial movement. In complex turbomachinery trains, this is achieved through a combination of proper train design, correct coupling selection and providing couplings with defined pre-stretches.
Usually a coupling does not have a fixed axial clearance. However, the flexible elements have axial spring stiffness. Connected machines, therefore, will behave like springs in series. That is why there is a potential for axial vibration (and axial resonance) particularly in complex turbomachinery trains (containing several machine casings).
Axial movement of turbomachinery shafts (or rotors) has to be limited by locating proper axial (thrust) bearings. This is particularly important for turbomachinery connected to electrical machines (electrical generators or electrical motors). Suitable couplings should be used and proper coupling pre-stretches should be set.
The electrical machine has to be aligned magnetically. If the electrical machine rotor is pulled out of the magnetic center during operation, considerable axial forces may be generated which can cause a problem. There are also reports of trouble regarding “limited- end-float” particularly for multiple discpack type couplings.
After installation and during a shutdown, the rotors usually stay in the position, regardless of some minor temperature changes, because of the large axial friction forces in the bearings during standstill compared to the coupling spring forces. However, during a shutdown, the rotors could expand or contract due to potentially large temperature changes.
Each rotor, after all, is supported by two (sometimes three) radial bearings. The fixed point is in one side. Train rotor geometry is generally asymmetric and the bearing which offers higher load (a larger friction force) can be considered as the fixed point. As a result, the following four scenarios should be considered for turbomachinery:
• Ambient temperature scenario: occurs after installation or during a relatively long shutdown
• Start-up scenario: a moment when the shaft is floating on the oil film of the bearings (or floating on magnetic bearings) while the shaft is still at ambient temperature and no thermal movement has occurred. This causes the couplings to evenly distribute the force which leads to movements in the shaft
• Operating scenario: occurs during a normal operation
• Stand still after shutdown scenario: one bearing holds the shaft in place, so the shaft will shrink (or expand) in another direction
The preferred coupling type is almost always high-torsional, axial-stiffness couplings. The coupling type is selected mainly based on train loading and required misalignment.
A correctly selected coupling is capable of simultaneously transmitting the maximum steady-state torques, cyclic torques, and the maximum transient torques under all conditions of angular misalignment, axial displacement, speed and temperature variation to which the turbomachinery train will be subjected in service.
The maximum angular misalignment is specified with respect to the expected misalignment during start-up, normal operation and shutdown of the coupled machines. This is based on known effects from thermal, pressure and dynamic forces.
The coupling’s steady-state angular misalignment capability is usually not less than 0.2°. The maximum axial displacement depends on the amount and the direction of the relative movement of the shaft ends toward or away from each other in various operating conditions.
As a rough indication, the minimum steady-state axial deflection capability is estimated by the largest shaft diameter divided by 125. For example, if the largest shaft diameter in a turbomachinery train is around 500 mm, a steady-state axial deflection of around 4 mm could be expected.
In addition, the coupling service factor is important and allows for various modes of rated and off-design operations. For metallic flexible element couplings, the minimum recommended service factor is 1.5.
A coupling should withstand the cyclic torque associated with start-up or transient conditions. A sufficient spacer length (known as shaft separation) is required for removal of couplings (typically 400 mm or more).
The potential unbalance of a coupling is usually defined according to ANSI/AGMA 9000-C90. As a rule of thumb: for low speed (around 1,500 rpm) and high speed (above 6,000 rpm), coupling potential mass-center displacement of 50 μm and 15 μm could be considered, respectively.
Regarding acceptable tolerances, a coupling guard should withstand a 100 kg static load with a deflection of less than 0.0005 times the unsupported length. In this case, a proper radial clearance, say a minimum of 25 mm, is required. A useful tip is to ask the coupling manufacturer to study and indicate which component(s) should be inspected (or replaced) following the occurrence of torque greater than the peak torque rating.
Non-metallic-flexible couplings (using rubber elements) and some metallic-flexible couplings such as single-element convoluted diaphragm styles can exhibit a significant axial vibration. Multi-diaphragm and nonconvoluted single-element flexible couplings, though, typically do not exhibit such axial vibration behavior.
Amin Almasi is a Chartered Professional Engineer in Australia, Queensland and U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.