Case History: Magnetic coupling reduces pump and motor problems at pulp and paper mills
April 16, 2001 -- Based on newly released results related to maintenance and energy savings at two U.S. pulp and pa...
April 16, 2001 — Based on newly released results related to maintenance and energy savings at two U.S. pulp and paper mills, new technology from MagnaDrive Corp. of Seattle, Wash., has been shown to generate significant benefits in the use of motor-driven systems.
At the Daishowa America mill in Port Angeles, Wash., the company’s adjustable speed drive technology cut energy demand in one application by 56 per cent. In another installation at the Ponderay Newsprint mill in Usk, Wash., it reduced energy use by 62 per cent.
Using the power of rare-earth magnets to precisely control the load speed, the MagnaDrive mechanical adjustable speed drive (ASD) significantly reduces energy use. Its vibration-free design also lowers equipment and maintenance costs, a benefit experienced at both the Daishowa and Ponderay facilities.
In a typical application, the ASD is installed between a motor and the load the motor is driving, generally a pump, fan or blower. The drive is a non-electronic, mechanical device. It consists of two independent components that have no physical contact. One component, a precision rotor assembly containing high-energy permanent magnets, is mounted on the load shaft. The second component, a copper conductor assembly, is connected to the motor shaft. Relative motion between the magnets and the copper creates a magnetic field that transmits torque through the air gap between the components.
Varying the width of the air gap changes the coupling force, so the amount of torque transmitted between the motor and load can be continuously adjusted. This allows precise and efficient speed control for optimum performance, including soft starts and stops. The result is a much more efficient use of energy. Because there is no direct mechanical connection between the motor and load, the MagnaDrive ASD also eliminates the costly wear-and-tear caused by vibration.
Daishowa America’s Port Angeles plant manufactures about 160,000 tons of paper annually, using wood and recycled pulp. Mill managers installed MagnaDrive ASDs on two 100-hp, 1175-rpm motors that power pumps in a wastewater treatment process. Prior to installation of the ASDs, the motors drove the pumps at a flow rate of 7,000 gpm, while the treatment process itself averaged a flow rate of 4,800 gpm. To achieve that rate, the system used bypass and discharge valves to restrict and regulate the flow. Consequently, a bypass valve returned 2,200 gpm to the pump station, wasting significant energy.
In addition, starting the motors while they were connected to the pumps caused locked rotor currents, leading to motor overheating. Starting and stopping the pumps also created flow surges and water hammer that damaged the pipes. And because the pumps always operated at top speed, the equipment suffered from vibration and cavitation.
The ASDs now control the speed of the pumps to maintain the required flow rate. This eliminates the use of the energy-wasting bypass and discharge valves, reducing energy demand from 142 kW to 62 kW, a 56 per cent savings. Moreover, the motors can be started uncoupled from the pumps, avoiding the flow surges, water hammer and overheating. The ASDs also eliminated system vibration. As a result, the mill is achieving maintenance cost savings estimated at $15,000 per year.
“We’re extremely pleased with the energy savings we’re getting with the MagnaDrive ASDs,” said Rex Springer, pulp and utilities manager at the Daishowa plant. “With the stability, reliability and reduced maintenance requirements of the system, we’re excited about moving forward with other applications in the mill.”
Ponderay’s mill has an annual capacity of 265,700 tons of newsprint. Plant managers installed a MagnaDrive ASD to increase the energy efficiency of a two-stage pulping and de-inking process. A 250-hp, 1800-rpm motor-driven system pumped water at full speed under constant pressure at 50 psi. However, during each hour-long cycle, only the de-inking stage of the process — lasting just 12 minutes — required the maximum flow of 3,200 gpm. For the other 48 minutes, the process required only 1,610 gpm.
To reduce the flow to 1,610 gpm while maintaining the necessary 50 psi pressure, the system used a bypass valve to redirect 1,590 gpm back to the pump. Because the pump always ran at full capacity, the system also suffered the damaging effects of vibration and cavitation.
Ponderay’s managers used the magnetic ASD to vary the speed of the pump while maintaining the constant 50-psi line pressure. This allowed them to close the wasteful bypass valve. The pump now runs at maximum speed for the 12-minute de-inking process and at a lower speed for the 48 minutes when flow demand is reduced. Energy demand dropped from 173 kW to 65 kW, a 62 per cent reduction. The ASD also dramatically reduced vibration and eliminated the cavitation problem, lowering maintenance costs.
“This new technology delivered real energy savings for us,” said Don Guenther, energy manager at Ponderay. “We’re also very pleased with the additional maintenance cost savings. We plan to add more MagnaDrive ASDs in our facility to take greater advantage of these economic benefits.”
At the Daishowa mill, variable frequency drive (VFD) technology was ruled out for these applications because of limited space and high infrastructure costs. At Ponderay, a VFD was not even considered due to significantly higher VFD costs associated with the 2,300-volt system.
Founded in 1999, privately held MagnaDrive Corp. is based in Seattle, Wash. Currently all the company’s installations are in U.S. facilities and it has no office in Canada. Its ASDs have been installed in 37 systems in several industries, including water/wastewater treatment, pulp and paper, HVAC, material processing, irrigation, food processing, and power generation. Its ASDs are available for motor sizes up to 500 hp. For more information, visit www.magnadrive.com.