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Petroleum coke production is an important source of revenue in the oil and gas industry. Coke drums are used to separate petroleum coke from lighter hydrocarbons. Many drums have reliability problems that can cause shutdowns and affect productivity. Components that can successfully run with severe misalignment without having impact on motor/gearbox vibration are crucial for these applications.

In this example, BP Cherry Point, located in Blaine, Washington, replaced a hydraulic coupling with a torque limiting magnetic coupling in a coke cooling drum.

BP installed the MagnaDrive (Model MGTL-16/200) after a failure occurred with a standard hydraulic coupling in November 2016.  Since installation, the magnetic coupling has demonstrated significant benefits including vibration reduction over the hydraulic coupling it replaced. 

Since the magnetic coupling provides an infinite life design with no wear, BP can only evaluate the new component based on operating time or in the event of a full load shutdown restart—something the company does not purposely plan to do.

Installation of the magnetic coupling was easier than expected. Engineers were able to make use of the same (existing) motor mount plate by adding 8 jacking bolts to maneuver the motor and lock it in position. By keeping the same mounting plate, they can easily convert back to the original hydraulic coupling should that be necessary.

To avoid an increase in coke cooler rotor cold end torsional stress, the gearbox acceleration time should be kept equal or longer than what was provided by the hydraulic coupling.  The acceleration rate of the MagnaDrive coupling is adjustable by changing the air gap between the coupling magnets and copper conductors.  The MagnaDrive coupling was installed with a 1/8” air gap.       

The MagnaDrive coupling has twice the slip of a hydraulic coupling (with 80% oil fill per procedure).  The additional slip will cause the coke cooler rotor to rotate at a speed 2% slower.  The slower operating speed should be negligible (on coke cooler capacity). Acceleration time is longer with the MagnaDrive coupling so the rotor torsional stresses should not increase.  A larger air gap can be provided to increase acceleration time, however, the slip will also increase.   

One significant performance advantage of the MagnaDrive Coupling is the ability to visually observe (with a Strobe light) if the gearbox position shifts during a high torque startup of the coke cooler rotor.  A gearbox shift was visible (with the MagnaDrive coupling) when the coke cooler was first started (this occurred because the gearbox had not been adequately locked in position with welded chock blocks).  This shift most likely has occurred (unnoticed) with hydraulic couplings in the past resulting in misalignment and increased vibration. The MagnaDrive coupling can run with severe misalignment without having impact on motor/gearbox vibration.

The cooler drive with the hydraulic coupling had considerable metal in the hydraulic oil. The gearbox input shaft (installed in November 2016) also exhibited fretting damage under the hydraulic coupling, mounting sleeve.  The gearbox shaft was hand dressed and a replacement (in-kind) coupling was installed (with a tighter shaft sleeve fit). In the second cooler drive, the hydraulic coupling was also found to have wear metal in the oil along with significant fretting damage to the gearbox input shaft. The gearbox was replaced and the MagnaDrive coupling installed.

Of the two hydraulic couplings that were removed from the application, one has been sent back to the OEM for repair while the other has been kept on site as an emergency spare (in the machine shop). The average cost of repairing the hydraulic coupling would be the same cost to have a spare MagnaDrive coupling on-hand and available in storage.  

The engineers recommended a one-year operating evaluation before changing the hydraulic couplings to magnetic couplings on other coke cooler drives. To learn more about the MagnaDrive Model MGTL-16/200) visit:

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