A refinery operated by one of the largest integrated energy companies in the U.S. has become an example of efficiency, cost savings and streamlined operation.
Located in the Midwest, the refinery is a vital component to the company’s operations, with a gross crude oil processing capacity of 306 million barrels per day (MBD). The refinery processes a mix of light, low-sulfur and heavy, high-sulfur crude oil. It receives domestic and foreign crude oil by pipelines and produces a high percentage of transportation fuels—such as gasoline, diesel fuel and jet fuel—in addition to other products, such as petrochemical feedstocks and asphalt.
The 2,200-acre site has been in continuous operation since it was constructed in the early 1900s. In 2007, the company announced a $1 billion investment to upgrade production operations at the refinery.
Upon completion of the upgrades in 2015, the plant will become one of the top-five largest refineries in the U.S. The refinery will also produce natural gas and sulfur-rich coke, which will be used as an energy source for area power plants.
“The refinery has become central to our North American operations, as our downstream partnerships will bring crude oil from the fields northeast of Alberta, Canada, to our operations for refining,” says the company’s global electrical team leader.
The heavy crude found in the oil sands takes longer to refine using a repeated process of heating and cooling to separate its parts for different uses.
“As a 24/7 operation, the refinery must maintain a high level of reliability, productivity and safety to ensure that we maximize efficiencies,” the team leader says.
Catalytic Cracking Unit Upgrade
With 30-year-old equipment nearing the end of its life-cycle, upgrading the four existing 4,500-horsepower motors that power dual catalytic cracking units (CCUs) was necessary to increase reliability, lower maintenance costs, and most important, provide uninterrupted service for 5 years.
As part of the refining process, the CCUs accept chains of hydrocarbons and break each into smaller ones in a chemical process called cracking. This allows the facility to make more products, such as gasoline, asphalt and aviation fuel.
As crude oil contains a variety of hydrocarbons, the CCUs are able to separate these chains of hydrocarbons using an extremely hot catalyst such as Zeolite, bauxite, silica-alumina or aluminum hydrosilicate. Both oil and catalyst in the CCU are then moved to another distillation column where the catalyst will be reused.
High-speed motors with active magnetic bearing systems bring operational benefits—such as wide speed range, multiple start-stops, fast ramp-up, unmanned or remote operations and monitoring, and lower the environmental footprint.
Answering the CCU Challenge
The solution was a combination of technology, an in-depth understanding of the products and situation and expertise to deliver one of the first solutions of its kind to the refinery.
Each CCU would be supported by two 4,500-horsepower, 4.5-kV, 78- to 105-hertz (4,700- to 6,300-rpm) totally enclosed, water-to-air cooled, high-speed motors with complete active magnetic bearing (AMB) systems. These were the first of their kind installed in the U.S. and used in a refinery.
These high-voltage motors featured a unique rotor design. The rotor was made from a special, single-piece forging. The rotor slots were then machined. Once this process was complete, a proprietary fusion process shaped and bonded the copper bars into the slot. The result was a solid rotor that could withstand the centrifugal forces associated with high-speed operation. The overall process led to reproducible vibration behavior under all conditions and throughout the life cycle of the product.
Ensuring Continued Operation
“One of our key requirements was reliability, as an unscheduled shutdown of our CCUs is unacceptable, and has a significant impact on our production levels and operating costs,” says the refinery engineer. “We had a high confidence level in the motors and drives to help us deliver uninterrupted service of our compressor train for 5 years, but these products had to operate in tough conditions as well.”
The AMB system brought several other operational benefits, including operating at high speeds, a wide speed range (API 617 and API 546), multiple start-stops, fast ramp-up, unmanned or remote operations and monitoring, reduced life-cycle costs, controllable rotor dynamics, no lubrication system and an oil-free string (including compressor) that reduced the environmental footprint.
The motors levitated the shaft and permitted motion without friction or wear, unlike traditional oil-lubricated or grease-lubricated motor bearings. The AMB consisted of an electromagnet assembly, a set of power amplifiers that supplied current to the electromagnets, a controller and gap sensor with associated electronics to provide the feedback required to control the position of the rotor within the gap. Each AMB was equipped with a backup bearing for emergency coast-down in the event of a power failure to the AMB system.
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.
