This is part two of a two-part article on coupling maintenance from Regal Beloit.
Equipment and Operating Conditions
Simulation of the torsional vibration response of a drivetrain is a critical analysis technique because it allows for the evaluation of transmitted torques during the equipment design stage. Because some degree of uncertainty is present in the results of the model, API standards require a 10% separation margin from any excitation frequency. While better data and modeling techniques have increased the accuracy of the torsional systems, problems may persist, resulting in undamped torsional vibration which may damage the equipment. Additionally, if the equipment is subject to an irregular operating schedule, the coupling may experience a high magnitude transient torque with each startup. Improper tuning of VFD controlled motors may subject the coupling to damaging torsional oscillations which weren’t accounted for in the design phase, thereby shortening the usable life of the coupling and possibly the equipment.
The usable life of the coupling may also be reduced by variations in the process conditions. Has a compressor been subjected to known surge events? Perhaps the equipment train does not run as smoothly as expected due to the process flow pulsations or the unexpected excitation of a system natural frequency. These events are typically identified using Condition Monitoring techniques it is typically not possible to determine how they could affect the coupling load case.
Couplings are designed to be the weakest link and lowest cost component on rotating equipment. They are often overlooked because of this, but frequently provide insight into the equipment operating conditions. The cracked outer discs of a disc coupling may tell you whether the equipment is operating under excessive misalignment and the excitation of a resonant torsional frequency may cause fretting in the center of a disc pack. If these issues remain unknown and aren’t addressed, there is an increased risk of equipment failure as well as the associated unplanned downtime and lost production.
Consider this scenario, Plant A and Plant B are operating identical steam turbine driven compressor trains since they were commissioned seven years ago. In Plant A, the coupling has been operating continuously with low vibration since startup, while in Plant B, the train has a history of issues causing shutdowns and operates with higher vibration attributed to misalignment due to thermal expansion. While both plants should include the coupling in the applicable maintenance plan for its parent asset, there is a higher probability of failure for the coupling in Plant B due to the additional loading it’s been subjected to while in service. Assuming the equipment train in Plant B costs ~$20M, the cost of a coupling recertification during the second turnaround at 14 years, is a small price to pay for assured continual performance.
The previous sections cover a few possible failure modes which can be mitigated through Kop-Flex® Factory Recertification service. These items can provide a foundation of questions to ask about your equipment to determine the proper maintenance schedule for the equipment and application. As an initial step, using the Preventative Maintenance intervals of the other equipment on the train provides an ideal time to inspect the flexible coupling. Depending on the risk factors of a specific train, Factory Recertification may be the best practice to achieve maximum uptime of your equipment.