Coupling Recertification Increases Plant Reliability

Regularly scheduled inspections, preventative maintenance and condition monitoring can assist in the early identification of potential problems, preventing damage to the coupling and connected equipment. There are a number of maintenance strategies that can be used to identify and prevent coupling failures and each methodology comes with pros and cons. The best strategy for an auxiliary pump will differ significantly from that of a 50,000 HP steam turbine compressor train.

A coupling will achieve its maximum service life when operated within its designed alignment and torque ratings. It is impossible to determine the precise loading of a coupling, since the combination of stresses due to torque and misalignment cannot be accurately quantified. Detrimental operating conditions as a result of inaccurate thermal growth offsets, shifting equipment foundations or high torsional oscillations, will shorten a coupling’s effective service life if the load case exceeds the as-designed values.

Key Points

  • Visual coupling inspections should be performed during scheduled turnarounds.
  • Kop-Flex® recertification is the best policy to achieve maximum uptime for critical equipment.
  • Re-certifications can be performed during a plant turnaround, resulting in a “like new” coupling for 50-60% the cost.

 

                                  Image 1 : Comparison between Goodman Diagrams of "as designed" and possible real world load case (indicated by red dot)
where infinite life is no longer achievable.



Image 2

 

The coupling Safety Factor is applied to address uncertainties during the coupling design process. Industry trends have resulted in an increased interval between plant turnarounds and more operating hours on equipment between service. The following circumstances may contribute to an accelerated reduction of the coupling safety factor during extended operating periods.

  • Corrosion
  • Contamination
  • Operating in high temperatures
  • Damage from coupling handling
  • Fretting between disc packs

INSPECTION CRITERIA

For critical equipment, scheduled turnarounds provide the best opportunity to perform a visual inspection of a coupling in service. These inspections are inherently invasive and cracks in the disc pack may not be visible without the removal and disassembly of the coupling. Condition monitoring is therefore the most widely used strategy for critical equipment. For example, the typical failure mode of a disc pack due to excessive misalignment is “unraveling” from the outermost discs (where the stresses are highest), inward. This results in a decreased torque capacity given fewer remaining discs, and also introduces a coupling imbalance which can be identified in the 1X frequency. As vibration trends up over time, the issue can be identified and the equipment shut down safely, typically avoiding catastrophic failure. However, if a transient misalignment or torque is severe enough to cause the discs to fail, it can happen too quickly for the fault to be identified and equipment shut down safely.

For non-critical equipment, periodic visual inspections of the disc packs may be a sufficient strategy to keep your equipment running. These inspections can be done under a strobe while the equipment is running, but it is strongly recommended that the equipment be stopped and locked out to ensure safety. Kop-Flex should be contacted if any anomalies are found and the following criteria should be considered to determine if the coupling is fit for continued service:

  • Spreading at the disc pack bolt or in an individual link
  • Spreading in two consecutive disc pack links
  • Cracked discs
  • Loose disc pack bolts

For a diaphragm flex element, the following inspection criteria should be considered and Kop-Flex contacted in the presence of any anomalies:

  • Scratches, gouges or nicks on the diaphragm profiles.
  • Presence of rippling effect along the diaphragm profile, indicative of a torque overload (Image 4).
  • Excessive runout between diaphragm flanges indicating yielding.

Field replacement of flex elements or factory recertification are the primary maintenance options available to rotating equipment managers. For lower speed, non-API 671 applications requiring a quick turnaround, field replacement of the disc packs may be ideal. For critical applications, Kop-Flex recertification resets the damage accumulation the coupling may have endured while operating under severe loading conditions and returns the service factor to the original as designed value. By addressing these issues, greater rotating equipment reliability can be achieved and plant uptime maximized.



Image 3 : Unacceptable spreading in disc pack



Image 4 : Rippling on diaphragm profile due to torque overload

INSPECTION CRITERIA

For critical equipment, scheduled turnarounds provide the best opportunity to perform a visual inspection of a coupling in service. These inspections are inherently invasive and cracks in the disc pack may not be visible without the removal and disassembly of the coupling. Condition monitoring is therefore the most widely used strategy for critical equipment. For example, the typical failure mode of a disc pack due to excessive misalignment is “unraveling” from the outermost discs (where the stresses are highest), inward. This results in a decreased torque capacity given fewer remaining discs, and also introduces a coupling imbalance which can be identified in the 1X frequency. As vibration trends up over time, the issue can be identified and the equipment shut down safely, typically avoiding catastrophic failure. However, if a transient misalignment or torque is severe enough to cause the discs to fail, it can happen too quickly for the fault to be identified and equipment shut down safely.

For non-critical equipment, periodic visual inspections of the disc packs may be a sufficient strategy to keep your equipment running. These inspections can be done under a strobe while the equipment is running, but it is strongly recommended that the equipment be stopped and locked out to ensure safety. Kop-Flex should be contacted if any anomalies are found and the following criteria should be considered to determine if the coupling is fit for continued service:

  • Spreading at the disc pack bolt or in an individual link
  • Spreading in two consecutive disc pack links
  • Cracked discs
  • Loose disc pack bolts

For a diaphragm flex element, the following inspection criteria should be considered and Kop-Flex contacted in the presence of any anomalies:

  • Scratches, gouges or nicks on the diaphragm profiles.
  • Presence of rippling effect along the diaphragm profile, indicative of a torque overload (Image 4).
  • Excessive runout between diaphragm flanges indicating yielding.

Field replacement of flex elements or factory recertification are the primary maintenance options available to rotating equipment managers. For lower speed, non-API 671 applications requiring a quick turnaround, field replacement of the disc packs may be ideal. For critical applications, Kop-Flex recertification resets the damage accumulation the coupling may have endured while operating under severe loading conditions and returns the service factor to the original as designed value. By addressing these issues, greater rotating equipment reliability can be achieved and plant uptime maximized.


Image 5 : Spool piece undergoing MPI
Image 6 : Reparable bore damage
Image 7 : Coupling hub before and after bead blasting

 

Once the components have been inspected and cleaned, they are reassembled with new disc packs, disc pack hardware, and flange hardware and re-balanced to the original specifications of the coupling (Image 8). The balanced coupling is then treated with a corrosion preventative and packaged in a custom wooden crate for return shipping.

A coupling recertification can be performed during a plant turnaround, resulting in a “like new” coupling for 50-60% the cost. While numerous maintenance strategies are available, the Kop-Flex recertification program is the only way to guarantee the replacement coupling is as good or better than the original.

Image 8: Re-balancing coupling

Image 9: Re-certified coupling from old to new

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