Equipment and technology improvements can be sound investments.
by Dave DePasquale, Siemens Industry, Inc.

While electronic sensors of the AMB were located on the shaft at the bearing positions, the backup bearings were provided next to each of the two magnetic bearings, just in case the AMB closed-loop control was to unexpectedly fail. These backup bearings had an air gap of approximately 0.5 millimeters, while the air gap of the magnetic bearings and the sensors is about 2 millimeters. Further, the backup bearings were dry-lubricated, and the sleeve bearing shells had a coating of a special material and were split. This would make for easy replacement, if necessary, without disassembling the complete stator/rotor of the motor.

The corresponding sleeves of the backup bearings on the shaft featured a galvanic anti-adhesion coating, which helped prevent too much material from being transferred from the sleeve bearing shells to the sleeves when the backup bearings must be used.

Installing Drives for Reliability 

To further increase reliability and efficiency, the refinery opted to connect the CCU’s new high-speed motors to two medium-voltage drives. Variable frequency drives (VFDs) provide the motors with the ability to accommodate the changing demands of the CCUs, thereby providing additional energy savings.

The motor/drive combination did not require any complex gear unit, which facilitated a more compact drive train system and eliminated the costs associated with gear unit inspection and service.

Commissioned in spring 2010, the VFDs’ low-voltage cells’ topology could be scaled precisely for a wide range of voltage and output power. With the ability to bypass any one cell during operation, the VFDs could maintain the full output voltage necessary for the process to continue uninterrupted. This cell-based configuration also provided the refinery with easy access to drive components for scheduled maintenance. This reduced system repair time to minutes. 

The cell bypass ensures automatic bypass of a failed power cell in less than 500 milliseconds. Instead of shutting down the entire drive, a process-tolerant protection system (ProToPS) provides a hierarchical system of warnings. This control strategy allows time to evaluate the situation and respond appropriately.

“We had severe thunderstorms, resulting in a power failure at the refinery. Due to the power loss ride-through capability of both the motors and the drives, all continued to operate at full-rated power,” says the refinery team leader.

An integral transformer with phase-shifted secondaries provided 24-pulse or better input harmonic cancellation with a power factor above 0.95 under any operating conditions. This eliminated the need for input harmonic filters or power factor compensation. It completely removed any common-mode voltages from being imposed on the motor. The VFDs supplied an output voltage that was so close to a perfect sine-wave shape that the CCU motors could be operated without any additional stress or overheating that might result from excessive dV/dt or harmonic distortion.

Success Breeds More Success

With the proven success of the CCU’s motors and drives, in October 2010, the refinery decided to upgrading an existing gas flare recovery system. Like all flare stacks, the ones at the refinery are used to eliminate waste gas (and sometimes liquid), which otherwise cannot be used or transported. Most important, this recovery system is a safety stop-gap, reducing strain on equipment due to high pressure and protecting employees and facilities during an emergency situation.

A motor/drive combination, with VFDs, feature a cell bypass that ensures automatic bypass of a failed power cell, which provides uninterrupted operation, easy access to drive components and reduced repair time in the field.

The refinery installed four air-cooled drives and four medium-voltage motors to the flare gas recovery system. Working together, these motors and drives ensured that pressure relief valves on the equipment operated efficiently and vented during over-pressure situations. Because a small amount of gas was continuously released, similar to a pilot light on a stove, the VFD’s capabilities reduced costs by operating at low speeds or high speeds based on demand.

Operational Savings, Increased Production and No Shutdowns

With the new products on-board, the refinery has:

  • Increased production
  • Decreased operational and maintenance costs
  • Had no unscheduled shutdowns of the CCUs

The refinery will process higher sulfur content crude oils more cost effectively than sweeter, lighter oils. This helps to reduce costs. Because no new refineries have been built in the U.S. during the past 30 years and demand for petroleum products is rising at a steady rate, upgrades to equipment and technologies at existing refineries are sound investments.