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Striking the Proper Balance PUBLIC ACCESS

Correcting Rotor Imbalances During Operation Reduces Costly Downtime and Maintenance at Industrial Sites.

[+] Author Notes

Senior Editor

Mechanical Engineering 121(12), 54-56 (Dec 01, 1999) (3 pages) doi:10.1115/1.1999-DEC-3

This article reviews about rotating machine shaft that whether it serves an air compressor or machine tool, it should be balanced to ensure smooth performance. Over many hours of service, rotors can become increasingly unbalanced due to material deposition, erosion, or shifting of components. This imbalance causes vibration that wears down the machinery and impairs its function. A leading manufacturer of active balancing systems is BalaDyne Corp. of Ann Arbor, Mich. The company’s EM-2000 system corrects imbalances in turbomachinery used in chemical processing, oil exploration, and power generation, while the BalaDyne IV system balances industrial fans and grinders. PCS Nitrogen, a chemical processor, has reduced the overhauls of turbomachinery at its Lima, Ohio, ammonia processing plant by installing the EM-2000 active balancing system on its main process air compressor train. This installation reduced the repair costs and production downtime.

Any rotating machine shaft, whether it serves an air compressor or machine tool, should be balanced to ensure smooth performance. Over many hours of service, rotors can become increasingly unbalanced due to material deposition, erosion, or shifting of components. This imbalance causes vibration that wears down the machinery and impairs its function. It is typically corrected by disassembling the machine and rebalancing the rotor, in a labor-intensive process that can take hours or, in the case of turbines, several days.

An alternative is incorporating active balancing systems into rotating machinery to correct rotor imbalances during operation in several seconds. This has reduced costly rebalancing, and costlier maintenance for industrial businesses.

Briefly, active balancing systems detect imbalances through vibration and proximity sensors mounted on the rotating equipment. Sensor data is sent to the system's microprocessor-based controller to determine the degree and location of the imbalance, and to send a signal to balance compensators. These are ring-shaped components permanently attached to the rotating shaft. The compensator rings redistribute weight in order to correct the imbalance.

A leading manufacturer of active balancing systems is BalaDyne Corp. of Ann Arbor, Mich. The company's EM-2000 system corrects imbalances in turbomachinery used in chemical processing, oil exploration, and power generation, while the BalaDyne IV system balances industrial fans and grinders.

The EM-2000 system consists of one or more balancer rings permanently mounted to the rotor shaft. Each ring contains two counterweighted rotor assemblies that are held in place by permanent magnets.

The controller automatically monitors vibration and proximity. An operator can use the controller to make manual adjustments, or the system can automatically start a balance cycle and move the counterweight rotors into positions that will minimize vibration.

During a balance cycle, the controller sends power pulses to stationary noncontacting coil assemblies mounted around the rotating balance ring. The power pulses create a magnetic field across an air gap, interrupting the field created by the permanent magnets. This causes the counterweights to move until the rotor is rebalanced. The process is accomplished within seconds.

Once the balance adjustments are completed, the magnets lock the rotor assemblies into their new position. No more power is required to maintain balance, reducing the system's overall energy consumption.

Most installations use more than one balancer ring. They are connected to a control room computer that tracks the interactions of multiple balancer rings and provides simultaneous control to institute multiple-plane balancing.

U.S. Steel has extended the life of bearings on industrial fans used to transport air, coal dust, and gases at its Gary, Ind., plant from one month to six by using balancing systems (center) to actively balance shafts.

Grahic Jump LocationU.S. Steel has extended the life of bearings on industrial fans used to transport air, coal dust, and gases at its Gary, Ind., plant from one month to six by using balancing systems (center) to actively balance shafts.

PCS Nitrogen, a chemical processor, has reduced the overhauls of turbomachinery at its Lima, Ohio, ammonia processing plant by installing the EM-2000 active balancing system on its main process air compressor train. This installation reduced the repair costs and production downtime. The plant is operated by BP Chemical, a division of British Petroleum plc in London, and produces about 1,500 tons of ammonia products per day.

The main process air compressor train at PCS Nitrogen consists of a GE Frame V, a natural gas-powered turbine rated at 17,000 horsepower, coupled with a Dresser-Rand 4M lowpressure and a Dresser-Rand 21\1 high-pressure compressor. The high-pressure air compressor ran in an unbalanced condition because dirt and rust accumulated on its rotor to the point where it significantly increased vibration, according to Harry Kim, who was project engineer at BP Chemical before he retired. The vibration damaged bearings and seals, and the machine needed to be shut down for service every two years.

The entire ammonia plant came to a halt when the compressors were overhauled. "We would lose our entire 1,500 tons of output per day, times three to four days of production. Overall, we would sacrifice about 4,500 to 6,000 tons of production. It all added up to a very serious loss of revenue and time for us," said Kim.

Field balancing the compressor rotor called for work crews to tear down the compressor and turbine train, then perform several spin and weight adjustments using an off-line balancing system. This system uses a monitor to determine how much weight must be added on the rotor, and at what points, to correct the imbalance.

"We know that some dirt and rust will always build up on the wheels and rotors, so the balancing problems would never go away entirely," Kim said. "We just needed to find a better way to deal with it rather than tearing down the compressor-turbine train every two years."

Kim approached BalaDyne and convinced his company to use the active balancing system. BP installed the first unit at a PCS Nitrogen chemical processing plant on the island of Trinidad in September two years ago. Based on its success, BalaDyne was directed to install another EM- 2000 at the Lima plant in September last year, during a routinely scheduled maintenance overhaul.

BalaDyne engineers mounted the system with two balancer rings on the high-pressure compressor. They installed one balancer ring on the outboard end and the other balancer on the coupling hub on the drive end.

The BalaDyne and PCS Nitrogen engineers fired up the gas turbine and gradually increased its rotation to its minimum operating speed of 5,100 rpm. They adjusted the balance to bring compressor vibration from two-thousandths of an inch, or 2 mils, to near zero within minutes.

PCS Nitrogen plant managers continuously monitor their compressor and keep vibration within 0.4 to 0.5 mil, by operating the system manually. "We can now pocket the hundreds of thousands of dollars it used to take to do the extra overhauling, plus the expense of burning off natural gas. There is no more lost revenue, either," said Kim.

According to Craig Gallaher, process specialist at PCS Nitrogen, "We only have to overhaul the machinery once every four years now, which is our normal maintenance schedule for this equipment." Gallaher said that balancing during operation minimizes a source of plantwide machine stress, the starting and stopping of other equipment during compressor overhauls.

PCS Nitrogen halved the overhauls of the compressor at its ammonia plant in Lima, Ohio, by installing an active balancing system to reduce vibration that damaged bearings.

Grahic Jump LocationPCS Nitrogen halved the overhauls of the compressor at its ammonia plant in Lima, Ohio, by installing an active balancing system to reduce vibration that damaged bearings.

The BalaDyne IV active balancing system is fluid-based. The controller receives data from position and vibration sensors, then sends commands to a balancer permanently mounted on the rotating shaft of industrial fans or grinders. The balancer uses four connecting chambers that contain a high-density liquid. All four chambers contain heaters.

When the controller receives excessive-vibration data, it will direct that power be sent to the heaters in the appropriate chambers. This vaporizes the fluid so that it enters another, cooler, chamber, where it condenses back into liquid to correct the imbalance.

The power is sent to the balancer by means of a rotary transformer developed and patented by BalaDyne. The transformer consists of a stationary primary coil and a rotating secondary coil that allows power and data to transfer across an air gap between the two coils.

U.S. Steel saves more than $200,000 per year in equipment and labor costs by using the balancing systems on the two induction fans at its Gary, Ind., steelworks. The plant set a world record in 1997 when it manufactured nearly 6.3 million tons of hot-rolled steel bands. The two 10-foot-diameter TL Babcock single-inlet fans at the Gary steel works' coke operations send a mixture of air, coal dust, and various gases into hundreds of feet of ductwork through the 11 stories of the plant. A 1,SOO-hp, 4,000-V electric motor turning at 1,165 rpm powers the fans, each weighing approximately 11 tons.

Because the fans are single-inlet, they tend to pull to one side, causing excessive heat and stress on the bearings, which in turn causes vibration. In addition, the coal dust sent through the fan grinds down the blades, exacerbating vibration. U.S. Steel tried to correct the problem by manually balancing the fan wheel shafts during maintenance outages, but the fans would continue to vibrate, typically at 3 to 4 mils, although sometimes at as much as S mils, recalled James Meinert, maintenance shift manager.

When excessive vibration caused the fan shaft bearings to fail, the repairs cost $8,000 in parts and labor. In the course of a year, fan balancing and upkeep, including the cost of manual balancing, cost U.S. Steel more than $200,000. The one or two weeks of fan maintenance downtime took a toll in lost production.

U.S. Steel installed the BalaDyne IV balancers on the fan shafts in July 1998. BalaDyne engineers provided technical support, including the installation of B4 for Windows software so that the balancing system could be used automatically or manually. "Before we got the BalaDyne system, we were happy to get vibration down to 2.5 mils with manual balancing," said Meinert, who acknowledged that U.S. Steel can now keep vibration down to 0.8 mil.

Bearing failures have also decreased since the BalaDyne IV was installed. "We are replacing the fan bearings only once every six months now, instead of every month. We are saving about $80,000 in parts and labor, plus the $6,000 per year we were spending for the manual balancing," Meinert said. The company is also ahead by thousands of dollars in regained production time.

Balancing systems at a Cleveland plant reduced grinder vibrations for a smoother finish to engine camshafts and less whine in some Ford vehicles.

Grahic Jump LocationBalancing systems at a Cleveland plant reduced grinder vibrations for a smoother finish to engine camshafts and less whine in some Ford vehicles.

Similarly, the BalaDyne IV was used to actively balance the Schaudt grinders that Ford Motor Co. uses to machine the camshaft lobes on its car engines at an engine plant in Cleveland. After grinding, the camshaft lobes had microscopic roughness, or chatter, that caused a noticeable whining sound to come from some Ford engines.

The grinders originally relied on active balancers using water as a counterbalancing agent, but over time, the water became contaminated with grinding swarf (particles of metal) and coolant, clogging the balancers. The sealed BalaDyne IV balancers were immune to the problem.

BalaDyne engineers worked with counterparts from Ford and from Lake Erie Abrasives to install their own active balancers directly to the grinders' machine spindles. Lake Erie Abrasives, based in Cleveland, supplies Ford with lapping films used to finish the camshaft lobes.

Ford used the B4Setup software to analyze spindle vibration and pinpoint imbalances. The units were able to correct vibrations caused solely by spindle imbalance, but only partly corrected vibration caused by machines near the grinders. In those cases, Ford let BalaDyne install vibration isolators between the grinding machines and the shop floor to eliminate vibration from adjacent equipment. Three days of testing bore out the efficacy of this arrangement in reducing vibration.

Continued testing with the BalaDyne IV identified another culprit in causing vibration: The balancing point on some of the grinding wheels was marked incorrectly by the manufacturer by as much as 180 degrees. When notified, the manufacturer instituted its own total vibration control program using the BalaDyne IV.

Copyright © 1999 by ASME
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